76344263 Manual Sediment Transport Measurements in Rivers Estuaries and Coastal Seas

CHAPTER 5: INSTRUMENTS SEDIMENT TRANSPORT February 2006 Manual Sediment Transport Measurements Page: 5.107

5.6.3.5 Pump-Bottle sampler

Principle

This simple method is based on the continuous pumping (propeller type pump) of a water-sediment mixture.

On board of the survey vessel a small part of the pump discharge is used to fill a 1 liter-bottle or 2 liter-bottle

in 3 to 5 minutes by using a small siphon tube (Fig. 1A). Using this method, a relatively long sampling

period and hence a (statistically) reliable concentration measurement can be obtained.

When a peristaltic pump is used (discharge = 0.5-1 1/min), the bottle can be filled directly.

An optical sensor can be used to determine the silt concentration in the bottle after settling of the sand

particles.

Practical operation

1. lower intake nozzle to sampling position (use echo sounder),

2. adjust intake velocity (discharge meter and valve A),

3. wait one minute to flush the pump hose,

4. fill bottle by opening tap B (2 liters in 4 minutes),

5. remove bottle,

6. note data on measuring sheet (Figure 2).

Laboratory analysis

See Paragraphs 5.6.2.2 and 8.1.

Results and accuracy

The silt and sand concentration can be determined as:

csilt=Gsilt/V and csand=Gsand/V

in which:

Gs = dry mass of sediment (mg),

V = volume of water sample (l).

To determine the sampling efficiency of the pump-bottle method, laboratory measurements were carried out

in a flume using sand with D10 = 150 m, D50= 220 m and D90= 330 m. A siphon sampler was used to

determine the actual concentration (co) at the position of the intake nozzle. The sand concentration in the

flume was varied from 30 to 1700 mg/1. The intake velocity of the pump-bottle system was equal to the local

flow velocity in the flume (iso-kinetic sampling). The tap discharge Qs was varied from 0.2 to 2 liters per

minute. Figure 1B presents the average error in the concentration as a function of the discharge Qs showing

an average error smaller than 20% for a discharge Qs in the range 0.2 to 1 1/min.

For each concentration the largest and smallest deviation are also indicated.

Figure 1B shows a trend from a positive error for a small discharge to a negative error for a large discharge,

which can be explained by means of the hydraulic coefficient of the bottle filling process. For Qs = 2 l/min

the ratio of the tap velocity and pump velocity (= hydraulic coefficient) is 1.5 resulting in a negative

sampling error (see also Paragraph 5.6.2.1). For Qs smaller than 1 1/min, the hydraulic coefficient is smaller

than 1 resulting in a positive sampling error. Optimal sampling requires a discharge of about 0.5 1/min (2

liter bottle in 1 minutes). Grain-size analysis of the sediment particles collected in the bottle showed a D50 =

200 m which is about 10% smaller as the original sediment (D50 = 220 m).

Technical specifications

dimensions: length of 0.25 m; hose diameter 0.016 m; siphon diameter of 0.004 m

weight: 1 kg

measuring range: > 1 mg/l

cycle period: 5 min (minimum period between two measurements)


Advantages

1. simple and reliable instrument for silt and sand particles

2. relatively large sampling period (3 to 5 minutes)

3. small cycle period (5 minutes)

4. usable in wave conditions

Disadvantages

1. electricity and pump required

2. many bottles for laboratory analysis

3. small sediment samples


Figure 1

Pump Bottle sampler


Figure 2

Measuring Sheet Pump Bottle sampler


5.6.4 Optical and Acoustical sampling methods

5.6.4.1 General principles

Optical and acoustical sampling methods enable the continuous and contactless measurement of sediment

concentrations, which is an important advantage compared to the mechanical sampling methods. Although

based on different physical phenomena, optical and acoustical sampling methods are very similar in a

macroscopic sense. For both methods the measuring principles can be classified in (see Figure 1):

transmission,

scattering,

transmission-scattering.

Transmission

The source and detector are placed in an opposite direction of each other at a distance 1. The

sediment particles in the measuring volume reduce the beam intensity resulting in a reduced detector

signal. The relationship between the detector signal (It) and the sediment concentration (c) is:

It=k1 e-k2 c

in which:

k1 = calibration constant depending on instrument characteristics, fluid properties and travel distance (l),

k2 = calibration constant depending on particle properties (size, shape), wave length and travel distance (l).

Scattering

The source and detector are placed at an angle ( ) relative to each other (see Figure 1B). The detector

receives a part of the radiation scattered by the sediment particles in the measuring volume. The relationship

between detector signal (Is) and sediment concentration (c) is:

Is=k3 c e-k2 c

in which:

k3 = calibration constant depending on instrument characteristics, fluid and particle properties (size, shape),

wave length and travel distance (l).

An important disadvantage of the scattering method is the strong non-linearity of the relation between the

detector signal and sediment concentration for large concentrations.

Transmission-scattering

This method is based on the combination of transmission and scattering, as shown in Figure 1C. If the travel

distance for transmission and scattering is equal, a linear relationship for the ratio of both signals is obtained

I=Is/It=k4 c

in which:

k4 = calibration constant depending on instrument characteristics and particle properties.

Important advantages are the absolute linearity between the output signal (I) and the sediment concentration,

the independence of water colour and the reduced influence of fouling.

Calibration

For all measuring principles an in-situ calibration for determining the constants is necessary, if possible

under representative flow conditions covering the whole range of flow velocities and measuring positions

(close to bed and water-surface). Regular calibration is required because the constants may change in time

due to variations in temperature, salinity and pollution.


In practice, the optical and acoustical sampling methods can only be used in combination with a mechanical

sampling method to collect water-sediment samples for calibration. Usually, about 10% of the measurements

should be used for calibration.

The inaccuracy of field measurements may sometimes be rather large because of calibration problems

(Kirby et al, 1981), particularly for optical samplers. The main problem is the lack of synchronity between

the optical and mechanical sample collection. To minimize synchronity errors, the optical samplers should be

calibrated bij measuring the silt concentration on board of the ship using a pre-collected water-silt sample.

Measuring range

For an optimal sampling resolution the wave length and particle size must be of the same order of magnitude.

Therefore the optical method is most suitable for silt particles (> 50 m). Laboratory experiments using the

optical sampler, have shown that the addition of sand particles with a concentration equal to the silt

concentration increased the output signal with about 10% (Der Kinderen, 1981). The upper concentration

limit for optical samplers is about 25000 mg/1 (Kirby et al., 1981).

The acoustic method is most suitable for sand particles (>50 um). The upper concentration limit is about

10000 mg/1.

Advantages

An important advantage of optical and acoustical samplers is the continuous measurement of the suspended

sediment concentration. In combination with a chart recorder for data collection a relatively long period (one

month) can be sampled continuously and automatically. When there is very little variation of the silt

concentrations in lateral direction of the cross-section, measurements at one point can be considered as

representative for the whole cross-section. In that case the sensor can be fixed to a bridge pier or river side

installation. The measuring location must be easily accessible for regular cleaning of the sensor and changing

of batteries and chart records. Energy consumption and recorder maintenance can be minimized by using a

switch system activating the sensor and recorder only for short periods (5 min) at preset intervals (1 hour) as

reported by Brabben (1981). Another advantage of the continuous signal is the possibility of determining

continuous concentration profiles by raising the optical or acoustical sensor from the bed to the watersurface

(rapid profile method, Kirby et al 1981). Using this latter method a complete concentration profile can be

determined in one minute. To check the representativeness of these profiles, occasionally the concentration

profile should also be determined by means of a number of point-integrated measurements. The horizontal

variability can be determined by towing the sensor at a (monitored) depth below the water surface.

Finally, it is remarked that both sampling methods can also be used to measure the instantaneous sediment

concentration under wave conditions, provided the respons period is small enough.

References

Brabben, T.E., 1981. Use of Turbidity Monitor to assess Sediment Yields in East Java. Proc.Symp. Erosion and Sediment Transport Measurements, Florence, Italy

Der Kinderen, W.J.G.J., 1980. Silt Concentration Meters; Evaluation (in Dutch). Delft Hydraulics Laboratory, Report S453 I, The Netherlands

Der Kinderen, W.J.G.J., 1982. Silt Concentration Meters (in Dutch). Delft Hydraulics Laboratory, Report M1799 I, Delft, The Netherlands

Kirby, R. and Parker, W.R., 1981. The Behaviour of Cohesive Sediment in the inner Bristol Channel and Severn Estuary. Institute Oceanographic Sciences, Report No. 117, Taunton, England


Figure 1

General principles optical and acoustic sampling


5.6.4.2 Optical backscatter point sensor (OBS)

Principle

The OBS is an optical sensor for measuring turbidity and suspended solids concentrations by detecting

infrared light scattered from suspended matter (see Figures 1A and 1B). The response of the OBS sensors

strongly depends on the size, composition and shape of the suspended particles (see Figure 2). Battisto et al.

(1999) show that the OBS response to clay of 2 m is 50 times greater than to sand of 100 m of the same

concentration. Hence, each sensor has to be calibrated using sediment from the site of interest (see Figures 1

to 5). The measurement range for sand particles (in water free of silt and mud) is about 1 to 100 kg/m3. The

sampling frequency generally is 2 Hz.

The OBS sensors consist of a high intensity infrared emitting diode (IRED), a detector (four photodiodes),

and a linear, solid state temperature transducer (Downing et al., 1981). The (Optical Back Scatter) sensor

measures infrared radiation scattered by particles in the water at angles ranging from 140° to 165°. Infrared

radiation from the sensor is strongly attenuated in clear water (more than 98% after traveling just 0.2 m),

(D&A instruments, 1989). Therefore, even bright sunlight does not interfere with measurements made in

shallow water.

The diameter of the sensor is about 0.02 m (see Figure 1); the length is about 0.05 m (see Photographs 1, 2

and 3 below). The IRED produces a beam with half power points at 50 in the axial plane of the sensor and

30 in the radial plane. The detector integrates IR-light scattered between 140 and 160 . Visible light

incident on the sensor is absorbed by a filter. Sensor components are potted in glass-filled polycarbonate

with optical-grade epoxy.

The sensor gain of the OBS has to be adjusted in order to match the highest output voltage expected from the

OBS during the measurements with the input span of the data logger. Undesirable results will be obtained if

the gain is not correctly adjusted. When the gain is too high, data will be lost because the sensor output is

limited by the supply voltage and will “saturate” before peaks in sediment concentration are detected. If the

gain is too low, the full resolution of the data logger will not be utilized.

The performance of the OBS-sensor is claimed to be superior to most other in-situ turbidity sensors, because

of: small size and sample volume, linear response and wide dynamic range, insensitivity to bubbles and

phytoplankton, ambient light rejection and low temperature coefficient and low cost.

The OBS sensors are about the same size (or larger) as the length of gradients in the sand concentration

being measured. This may cause hydrodynamic noise in the output signal because the turbulent flow around

the sensor redistributes the particles in the water and increases the variation of sediment concentration above

natural levels. Furthermore, the volume sampled by the OBS sensors depends on how far the IR beam

penetrates into the water. This decreases as sediment concentration increases and so the sample volume is

constantly varying with concentration which may also cause random noise in the output signal. From limited

tests performed by the manufacturer it appeared unlikely that the random noise would exceed 30% of the

mean signal in situations with high concentrations of coarse sediment. The manufacturer recommends post-

processing the data with a low-pass filter to reduce the random noise in the output signal.

Other noise in the output signal may be caused by electronic noise or environmental conditions. According to

specifications, the electronic noise is insignificant for most applications. Some causes for environmental

noise are: biofouling, excess in suspended sediment resulting from scour around instrument structures and

cables moving in front of the OBS sensor with the currents.

Experiments have shown that the sensor gain varies with particle size. Ranging from mud (< 10 m) to sand

(> 200 m) the gain decreases approximately by a factor 10.

Hatcher et al. (2000) have used OBS sensors measuring at wavelengths of 442, 470, 510, 589, 620 and 671

nm with source beams originating from colour LED’s (six channel OBS; multi-spectral OBS) which can be

used to measure concentrations of sediment mixtures (multiple grain sizes). This makes it possible to

measure spectral responses of suspended particle concentrations across the optical range of wave lengths.

Using the differential response of the backscatter coefficient of the suspended constituents at six wave

lengths, an accurate estimation of concentration of mixtures can be obtained. This method is based on the

simultaneous solution of linear equations that relate output of optical backscatter sensors to concentrations of


various constituents of suspended sediments (see Green and Boon, 1993). The basic requirements are: 1)

linear sensor response to concentration of a particular sediment size, 2) different sensor response to different

sediment sizes and 3) grain shielding and multiple scattering should be negligible.

Calibration results from Utrecht University

A detailed description of the calibration of OBS sensors is given by Van de Meene (1994). The OBS sensors

were calibrated in a calibration tank of the Physical Geographic Laboratory at Utrecht University. Water is

circulated in a closed circuit by a strong slurry pump. The sediment is added from above in a large perspex

cylinder. The circulating water-sediment mixture is jetted into the cylinder, where the flow expands and

decelerates. A flow straightener is present to make the flow as smooth as possible. The water sediment

mixture flows undisturbed along the sensors with a velocity of approximately 0.25 m/s, which is large

enough to suppress inhomogeneities due to settling and small enough to prevent inhomogeneities due to

turbulence. Two OBS sensors can be calibrated simultaneously. A suction tube is present near the sensors to

draw concentration samples. The calibrations were carried out using cinput (=mass of sand in system divided

by volume of water) as the actual concentration. According to Van de Meene (1994) the sediment

distribution across the horizontal plane in the measurement region appeared reasonably homogeneous.

Variations were of the order of 5 to 10% of the mean concentration.

Figure 3 shows examples of the calibration curves for the OBS sensors used for the experiments carried out

in the large wave flume (GWK) in Hannover, Germany (grain size characteristics are d10 = 0.14 mm, d50 =

0.23 mm, d90 = 0.34 mm).

Figures 4 to 6 show calibration results using the bed material from tests in the wave tunnel (LOWT) of Delft

Hydraulics (two types of sand: d50 = 0.12-0.13 mm and 0.19-0.21 mm; d50 varied slightly based on samples

before and after the tests). The different response of the OBS sensors to the two different grain sizes is

reflected by the different slopes of the calibration curves.

Figure 5 shows this influence of the grain size on the calibration factor (slope of calibration curve). It can be

observed that the calibration coefficient is 2 to 3 times smaller when the grain size decreases with 30%.

Figure 6 shows the OBS concentrations measured in the calibration tank compared to the sand concentrations

from a pump sampler. It can be seen that the OBS concentrations show favourable comparison to pump

concentrations larger than 1 kg/m3. OBS values significantly deviate from pump concentrations smaller than

1 kg/m3. A systematic overestimation of the measured values can be observed for concentrations below 1

kg/m3.

The OBS sensors often show a reasonably steady offset concentration, which is related to the background

concentration of relatively fine sediments (silt and mud). It is common practice to subtract this offset value

from the original time series data. The offset can be defined as the minimum value of the data record (burst)

or as the 1% to 5% lowest value of the signal. For example, Battisto et al (1999) found that the most

appropriate cut-off voltage at the Duck site (USA) was 1% to 5% of the signal values.

Figure 7 shows time series values of two OBS sensors and one acoustic backscatter point sensor (ASTM) for

an experiment (M2) carried out in the large scale wave tank of Delft Hydraulics (Chung and Grasmeijer,

1999). The time-averaged ASTM-concentrations were about 1.3 kg/m3 at 0.115 m above the bed and 0.6

kg/m3 at 0.215 m above the bed. The OBS signal shows a background voltage of about 50 mV, which is

equivalent to a concentration of about 0.5 to 1 kg/m3. Hence, the background concentration to be subtracted

from the record is of the same order of magnitude as the sand concentration, which makes the application of

the OBS sensors rather dubious in the sand concentration range below 1 kg/m3. The acoustical ASTM sensor

does not show a background cocncentration due to fine sediments. This instrument is not sensitive for fine

sediments (<0.05 mm; smaller than the sand range).

Calibration results from Duck site, USA

Battisto et al. (1999) have made a comparison between OBS and pump sampler concentrations measured in

the surf zone at the Duck site (USA) during October 1997. For this study, OBS sensors were calibrated

separately using sand and mud collected at the Duck site. OBS voltage gain associated with mud was found

to be an order of magnitude larger than that for sand. Based on this calibration, Battisto et al. show that the

concentration of particles smaller than 63 m pumped at the Duck site during October 1997 correspond to the

lowest 1% to 5% of the output voltage recorded by the OBS sensors (background turbidity). The intake tubes

of the pump sampler were positioned approximately 0.1 to 0.2 m above the bed.


Calibrated OBS response above this background turbidity level was consistent with pumped sand

concentration as long as corrections were made for 1) varying size of suspended sand, 2) the precise time of

pump sampling, 3) apparent noise in the OBS records. Corrections for the smaller size of the suspended sand

relative to that used during calibration resulted in a decrease of the OBS sand concentration by about 50%.

Accounting for signal noise resulted in a decrease of the OBS sand concentration by about 0.05 to 0.2 kg/m3.

Despite these corrections the OBS concentrations are considerably larger (factor 2 to 5) than the pump

concentrations for sand concentrations smaller than 1 kg/m3. Hence, OBS data are unreliable for c<1 kg/m3.

OBS sensors are supplied by D&A instruments (www.d-a-instruments.com) and by Seapoint-

instruments (www.seapoint.com).

Technical specifications D&A sensor

dimensions sensor 0.018 x 0.05 m

housing 0.06 x 0.23 m

weight 1.3 kg

power 8-35 V/70 mA

measuring range mud 5 - 5000 mg/1

sand 100 - 100 000 mg/1

response period 10 Hz

temperature drift 0.05% per °C

Technical specifications Seapoint sensor

dimensions sensor 0.025 x 0.12 m

power 7-20 VDC, 3.5mA, 6 mA pk

measuring range mud 5 - 5000 mg/1

sand 100 - 100 000 mg/1

response period 10 Hz

temperature drift < 0.05% per °C; 0 to 65 oC

output 0-5 VDC

RMS noise < 1 mV

light source wavelength 880 nm

sensing distance < 5 cm

linearity <2% deviation 0-750 FTU

material ABS plastic, epoxy


Practical operation (on-line point measurements)

1. lower or raise sensor to sampling position (use echo sounder)

2. select sampling period (3 to 5 min)

3. read output signal (time-averaged)

4. collect water sample simultaneously for calibration (if necessary)

5. note data on measuring sheet (see Figure 8)

Advantages

1. small size and small sample volume

2. linear response; high-frequency response

3. insensitive to air bubbles and ambient light

4. large measuring range

5. long-term, stand-alone deployments (field-proven reliability)

6. low cost

Disadvantages

1. strongly dependent on particle size; regular calibration required

2. not usable in conditions with combined clay, silt and sand particles

3. not accurate for sand concentrations below 1000 mg/l (=1 kg/m3)

References

Battisto, G.M., Friedrichs, C.T., Miller, H.C. and Resio, D.T., 1999. Response of OBS to mixed grain size suspensions during Sandy Duck’97. Coastal Sediment Conference 99, ASCE, New York. pp. 297-312.

Chung, D.H. and Grasmeijer, B.T., 1999. Analysis of sand transport under regular and irregular waves in large-scale wave flume. Report R99-05, Department of Physical Geography, University of Utrecht.

Connor, C.S. and De Visser, A.M., 1992. A laboratory investigation of particle size effects of an optical backscatterance sensor. Marine Geology, Vol. 108, p. 151-159

D and A Instruments, 1989. Optical Backscatterance Turbidity Monitor. Instruction Manual Tech. Note 3, 2428, 39th Street, N.W., Washington, D.C., 20007, USA

Downing, J.P., Sternberg, R.W. and Lister, C.R.B., 1981. New Instrument for the Investigation of Sediment Suspension Processes in the Shallow Marine Environment. Marine Geology, 42, p. 19-34

Green, M.O. and Boon, J.D., 1993. The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors. Marine Geology, Vol. 110, p. 73-81

Hatcher, A., Hill, P., Grant, J. and Macpherson, P., 2000. Spectral optical backscatter of sand in suspension: effects of particle size, composition and colour. Marine Geology, Vol. 168, p. 115-128

Van de Meene, J.W.H., 1994. The shoreface connected ridges along the central Dutch coast, The Netherlands, Doctoral Thesis, Utrecht University, Department of Physical Geography, The Netherlands.


Photographs 1, 2, 3, and 4

OBS sampler in wave tunnel of Delft Hydraulics


Figure 1A

OBS Sensor (D&A instruments)

Figure 1B

OBS Sensor (Seapoint instruments)


Figure 2

OBS Calibration curves for sediments between 37 and 121 m (Connor and De Visser, 1992)

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30 35

concentration (kg/m3)

ou

ptu

t O

BS

(m

V)

OBS 358

OBS 355

OBS 408

Linear (OBS 358)

Linear (OBS 355)

Linear (OBS 408)

1

10

100

1000

10000

0.01 0.1 1 10 100

concentration (kg/m3)

ou

ptu

t O

BS

(m

V)

OBS 358

OBS 355

OBS 408

Figure 3

Calibration of OBS sensors used during GWK Hannover experiments


0

10

20

30

40

50

60

70

80

0 500 1000 1500 2000 2500

output (mV)

co

ncen

trati

on

(kg

/m3)

OBS 350

OBS 355

OBS 408

Linear (OBS 350)

Linear (OBS 355)

Linear (OBS 408)

0

10

20

30

40

50

60

70

80

0 500 1000 1500 2000 2500

output (mV)

co

ncen

trati

on

(kg

/m3)

OBS 350

OBS 355

OBS 408

Linear (OBS 350)

Linear (OBS 355)

Linear (OBS 408)

D50 = 0.19 mm

D50 = 0.12 mm

Figure 4

Calibration of OBS for LOWT experiments


0 .000

0 .010

0 .020

0 .030

0 .040

0 .050

0 .060

0 .070

0.05 0.07 0 .0 9 0.11 0.13 0.15 0 .17 0.19 0.21 0.23

gra in s ize D50 (m m)

ca

lib

rati

on

fa

cto

r (-

)

O BS 350

O BS 355

O BS 408

extrapo lation

Figure 5

Influence of sediment grain size on slope of OBS calibration curve, Oscillating Water Tunnel Experiments; calibration factor is slope of calibration curves (in kg/m3 per millivolt)

0.1

1

10

100

0.1 1 10 100

measured concentration (g/l)

pre

dic

ted

co

nc

en

tra

tio

n (

g/l)

OBS 350

0.1

1

10

100

0.1 1 10 100


pre

dic

ted

co

ncen

trati

on

(g

/l) OBS 355

0.1

1

10

100

0.1 1 10 100


pre

dic

ted

co

nc

en

tra

tio

n (

g/l)

OBS 408

Figure 6

OBS concentration based on calibration curves as a function of pump concentration in the calibration tank; two calibration curves with different sediment sizes (d50 = 0.12-0.13 and 0.19-0.21 mm) were used.


0

20

40

60

80

100

120

10:46 10:48 10:50 10:52 10:54 10:56

time (hh:mm)

ou

tpu

t A

ST

M (

mV

) uncalibrated ASTM at z = 0.125 m

0

100

200

300

400

500

10:46 10:48 10:50 10:52 10:54 10:56

time (hh:mm)

ou

tpu

t O

BS

(m

V)

uncalibrated OBS at z = 0.115 m

0

100

200

300

400

500

10:46 10:48 10:50 10:52 10:54 10:56

time (hh:mm)

ou

tpu

t O

BS

(m

V)

uncalibrated OBS at z = 0.215 m

Figure 7

Comparison of uncalibrated ASTM (or USTM) and OBS signals. Delta flume M2, Hm0 = 1.5 m, Tp = 5.0 s, h = 4.55 m, D50 = 0.16 mm.


Figure 8

Measuring Sheet for Optical sampler


5.6.4.3 Optical Laser diffraction point sensors (LISST)

Principle

Various Laser diffraction instruments are commercially available to measure the particle size and

concentration of suspended sediments. The LISST instruments (Laser In-Situ Scattering and

Transmissometery) are manufactured by Sequoia Inc, USA (www.sequoiasci.com), (Agrawal and

Pottsmith, 2000, 2002). Detailed information is given in Chapter 6.

LISST-100: This instrument is the most widely used Laser diffraction instrument, which delivers the size

distribution by inversion of the 32-angle scattering measurements.

LISST-ST; This instrument has been designed to obtain the settling velocity distribution of sediments of

different sizes. In this case, a sample of water is trapped and particles are allowed to settle in a 30 cm tall

settling column at the end of the instrument-housing. Movable doors are present on both ends of the tube,

which are programmed to open at regular intervals. Using a motorised propeller, a water sample is drawn

into the tube through 8 openings of 20 mm diameter. Throughout, the size distribution is monitored near

the bottom of the settling tube. After sampling, a few seconds are allowed for turbulence to break down

before the doors are closed and the sample is allowed to settle for several hours. During settlement of 12

and 24 hours runs, respectively 72 and 83 Laser scans are made in logarithmically scheduled time intervals.

Over time, the size distribution shows zero concentration in sizes that have settled out. The time for settling

is used to estimate settling velocity. From knowledge of the size versus settling velocity, mass density can

be estimated. This instrument obtains the settling velocity and particle density for 8 size classes in the 5 to

500 micron range. The assumption that all particles settle independently in a complete stagnant fluid is

often violated. As a result, the calculated particle density ditribution often becomes unrealistically wide to

compensate for effects such as convection and particle interaction.

LISST-25A and 25X; This instrument is a simpler, less expensive version of the LISST-100. Replacing

the multi-ring detector of the LISST-100, a special shape for a focal plane detector was invented. This

shape (comet-detector) is the result of solving the mathematical problem: does there exist a detector shape

that would measure light scattering in a manner that it holds calibration for all sizes? Indeed, the LISST-25

holds calibration for spheres over a 200 to 1 size range, where earlier sensors would vary in calibration by a

factor of 200! The LISST-25 instrument is a superior sensor to the LISST-100 when only concentration

measurement is required. The LISST-100 obtains sediment concentration by first inverting the 32 multi-

angle scattering data to construct the size distribution and then summing the concentrations in the 32 size

classes. When small numbers of particles are present, as can happen with coarse particles, the inversion can

miss them due to noise. In contrast, since the comet-detector directly estimates concentration from the

weighted sum of angular scattering, it misses nothing. A second attribute of the LISST-25 is that this

device obtains particle area concentration from the optical transmission. The ratio of the volume

concentration and area concentration is called the Sauter Mean Diameter (SMD), first introduced in the

aerodynamics-droplet combustion literature. The two types of LISST-25 refer to an analog output only

version and a second version that is fully recording and presents a coarse fraction concentration in addition

to the total suspended load. The LISST-25X instrument has new comet shapes built in to separate between

wash load finer than 63 micron and the sand load larger than 63 micron. The two new comet shapes

deliver the total concentration and SMD in the entire size range and concentration and SMD in the coarse

sand range. The comet shapes assume nothing regarding the underlying size distribution of sediments. The

only requirement is spherical shape for particles. Inaccuracies of perhaps as much as 100% may occur if the

particle composition changes from mineral to biogenic.

LISST-SL: This instrument is a streamlined body that draws a sediment-laden stream into it for Laser

measurements. It incorporates a Laser, optics, multi-ring detector identical to the LISST-100 and

electronics for signal amplification and data scheduling and transmission. A pump is also built-in to ensure

isokinetic withdrawal rates. The pump is controlled by a microprocessor, which is fed information about

the river velocity by a propeller type current meter to ensure isokinetic velocity sampling. The propeller is

mounted above the body itself and a sensor is employed to count the number of its rotations in a short

period of time. This device includes pressure transducers to record the depth of sampling. The LISST-SL


has been designed to provide real-time data on sediment concentrations and particle-size distributions. The

velocity and concentration data are used to compute fluxes (on-line) for up to 32 particle size classes at

points, verticals or in the entire stream cross-section (Gray, 2004).

The LISST-SL offers a very powerfull instrument for on-line measuring of particle size, concentration per

size class (32 classes) and hence transport using a separate sensor for velocity measurements in rivers and

estuaries. A severe limitation is the relatively small concentration range (up to 500 mg/l) due to

insufficient light penetration of the optical sensor in conditions with concentrations larger than 500 mg/l.


see Section 6.5.5 and website: www.sequoiasci.com

Advantages

1. simultaneous measurement of particle size, concentration and fall velocity

2. simulataneous measurement of mud, silt and sand particles

3. instrument can measure in stand-alone mode

Disadvantages

1. limited concentration range for silty and muddy sediments (up to 500 mg/l)

2. not close to bed (relatively large sampler size)

3. fragile instrument in coastal conditions with surface waves

4. calibration required for non-spherical particles

References

Agrawal, Y.C. and Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observations in sediment transport. Marine Geology, Vol. 168, p. 89-114

Agrawal, Y.C. and Pottsmith, H.C. 2002. Laser Diffraction Method: two new sediment sensors. Sequoia Inc., USA (www.sequoiasci.com)

Gray, J.R., 2004. The LISST-SL streamlined isokinetic suspended-sediment profiler. Proc. 19th Int. Symp. on River Sedimentation, Yichang, China.


5.6.4.4 Various other Optical point sensors

Various types of optical samplers were and are commercially available. Herein, the following types of

optical instruments are discussed:

Eur Control Mex 2,

Partech Twin-Gap,

Metrawatt GTU 702,

Monitek 230/134.

Submersible Optical sensor Eur Control Mex 2

The instrument is based on the transmission of light waves using two light paths with different lengths, as

shown in Figure 1A. The (submersible) sensor consists of two light sources (A,B) and two photodiodes

(C,D). The light sources are used in turn for a period of one second. Firstly, only source A is off. Diode C

receives light over a distance x1 , while diode D receives light over a distance x2. The diode currents are

converted by a logarithmic amplifier and stored. Secondly, the process is repeated using source B. The result

of both phases are added.

The influence of (uniform) pollution is negligible small. An important disadvantage of this instrument is

the sensitivity to ambient light due to unequal exposure of the photodiodes.


light paths: 0.122 and 0.027 m

dimensions: sensor 0.3x0.1x0.05 m

weight: 1 to 10 kg

energy: 220 volt or 24 volt

measuring range: 100-10000 mg/l

response period: 5 s


References

Der Kinderen, W.J.G.J., 1980. Silt Concentration Meters; Evaluation (in Dutch). Delft Hydraulics Laboratory, Research Report S453 I.

Der Kinderen, W.J.G.J., 1981. Silt Concentration Meters; Experimental Comparison (in Dutch). Delft Hydraulics Laboratory, Research Report S453 II.

Der Kinderen, W.J.G.J., 1982. Silt Concentration Meters (in Dutch). Delft Hydraulics Laboratory, Report M1799 I, Delft, The Netherlands.

Jansen, R.H.J., 1980. Methods for Measuring Velocity and Sediment Concentration in the Breaker zone (in Dutch). Delft Hydraulics Laboratory, Report R971, The Netherlands.


Figure 1

Eur Control Mex 2

Figure 2

Eur Control Mex 2


Submersible Optical sensor Partech Twin Gap

The instrument is based on light-transmission, using two light paths with different length as shown

schematically in Figure 3A. The (submersible) sensor contains a light source with on both sides a light

sensitive resistor being part of a wheatstone bridge. The influence of ambient light is relatively small due to a

small aperture angle of the detector. The influence of uniform pollution is negligible small.


light paths: 0.0127 and 0.0064 m


weight: 1 to 10 kg

energy: 220 volt or 24 volt


response period: <1 s


Figure 3/4

Partech Twin Gap


Pump-Optical sensor Metrawatt GTU 702

This instrument is based on the continuous pumping of a water-sediment mixture through the measuring

volume of the optical sensor (with light source and detector) on board of the survey vessel (Figure 5A). The

optical determination of the silt concentrations is based on the transmission-scattering principle using infra-

red light, as shown schematically in Figure 5B. Therefore, the influence of the electronic characteristics of

the instrument and also pollution is negligible small.

An important advantage of the pump-optical method is the relative simple calibration facility.


light paths: 0.032 m


weight: 1 to 10 kg

energy: 220 volt




Figure 5

Metrawatt


Pump-Optical sensor Monitek 230/134

This instrument is based on the pump-optical sampling method. The fluid sediment mixture is pumped

through the instrument (above water). The optical system is based on the transmission-scattering principle, as

shown schematically in Figure 6B. The light source is a simple electronic lamp that produces a narrow light

beam (2-3 mm) using a system of lenses. Two photodiodes are used to detect the directly transmitted light

and the forward scattered light. In an electronic circuit the ratio of both detector signals is determined

resulting in a linear output signal. An advantage of this instrument is the relatively large measuring range.

Another important advantage of the pump-optical method is the relative simple calibration facility.


light paths: 0.04 m


weight: 10 to 20 kg

energy: 220 volt




Figure 6

Monitek 230/134


5.6.4.5 Acoustic point sensors (ASTM, UHCM, ADV)

Principle

Delft Hydraulics (1994) has developed an instrument (ASTM or USTM; Acoustic or Ultrasonic Sand

Transport Meter; in Dutch: Acoustische Zand Transport Meter) for measuring the velocity and sand

concentration in a point. The USTM or ASTM (Figures 1 and 2) is an acoustic instrument for measuring the

flow velocity in 1 or 2 horizontal dimensions and the sand concentration.

The Acoustic Sand Transport Monitor (ASTM) is based on the transmission and scattering of ultrasound

waves by the suspended sand particles in the measuring volume, as shown schematically in Figure 1A. Using

the amplitude and frequency shift of the scattered signal, the concentration and velocity and hence the

transport of the sand particles can be determined simultaneously and continuously. The ASTM consists of a

sensor with a pre-amplifier unit mounted on a submersible carrier and a separate converter with panel

instruments and switches (Figure 2). The velocity measurement if mounted on a carrier is one-dimensional

and related to the carrier orientation, which is measured by means of a magnetic compass. The vertical

position is measured by a pressure gauge (height beneath water surface) and an echosounder (height above

bed) mounted on the carrier.

A transmitting frequency of 4.5 Mhz has been chosen to minimize the particle size dependency and to make

the instrument insensitive to silt particles (< 50 m). The influence of temperature and salinity variations is

also negligible.

Delft Hydraulics has designed a five-fold two-dimensional ASTM consisting of five identical sensors

(Figures 1B and 1C) for use as stand-alone instrument mounted in a tripod. Each sensor consists of one

transmitter and two receivers in a horizontal arrangement (measurement volume is about 0.2 m from

transmitter). The transducer heads are connected by a cable of 5 m to the electronics container (diameter of

0.27 m and length of 0.6 m). The transmitter produces a 4.4 MHz signal, which is scattered by the sediment

in suspension in front of the transmitter. This signal is subsequently sampled with a frequency of 2 Hz. The

backscattered signals are analysed to obtain the signal intensity and the frequency shift (Doppler effect). The

velocity of the sand particles can be derived (without calibration) from the frequency shift. The signal

intensity is a measure of the sand concentration and also depends on local sediment characteristics such as

the texture, the angularity and the density of the sediment (calibration curve). The measuring range for the

sand concentration of the ASTM (linear response) is about 0.1 to 10 kg/m3. According to the manufacturer

(Delft Hydraulics) the maximum error amounts to about 3% of the measured velocity value and 30% of the

measured concentration value. The five-fold ASTM was successfully used during long-term field

deployments in the North Sea to measure the sand transport process under combined current and wave

conditions (Grasmeijer et al., 2005).

The UHCM-instrument (only concentration) is a small-sized instrument (Figure 1B right) which has been

developed for the high concentration range of 1 to 100 kg/m3 near the bed (see Figure 1A). This instrument is

based on the measurement of the attenuation of ultra-sound by the sediment particles. The transducer heads

are close together at a distance of about 10 to 20 mm (depending on application; user-specified). Figures 4

and 5 show results of the acoustic sensor UHCM in a wave tunnel experiment (Van der Werf, 2006).

Rijkswaterstaat (2005) has made an attempt to use the backscattered signal from the point-measuring

Acoustic Doppler Velocitymeter (ADV-ocean) from SONTEK-instruments. This instrument was tested in a

laboratory flume and in field conditions. The laboratory tests showed that there is a positive correlation

between sediment concentration and backscattered signal. The field tests in the Western Scheldt estuary

(near Hansweert) consisted of concentration-measurements using the point-measuring ADV and another

point-measuring acoustic instrument (ASTM from Rijkswaterstaat). This latter instrument mounted in a

streamlined body suspended at a cable from the survey boat measures velocity and concentration

simultaneously in one point above the bed. The ADV was operated with a time-averaging interval of 0.5 s.

The sampling frequency was in the range of 10 to 20 Hz. The measured concentrations were in the range of

0.1 to 4 gr/l. Analysis of the measured concentrations using both instruments shows that the ADV cannot

measure concentrations larger than about 0.5 gr/l (500 mg/l) due to saturation effects. The concentrations

show a weak non-linear behaviour in the lower range between 0.1 and 0.5 gr/l. The velocities of the ADV


and ASTM show close agreement. The overall conclusion is that the ADV is potentially usable as sand

concentration and transportmeter, but its working range is insufficient at present (maximum concentration

of about 0.5 gr/l) to cover the near-bed region of the water column where the concentrations are largest and

most of the sand transport takes place.

Fugate and Friedrichs (2002) have successfully used the ADV backscattered signal (Sontek ADV) to

determine the concentrations in the range of 20 to 100 mg/l at a site in Chesapeake bay (USA). The ADV

proved to be a versatile instrument for characterizing the suspended sediment dynamics. Acoustic

backscatter from the ADV was relatively insensitive to grain size differences, thus producing good estimates

of mass concentrations. In addition, the ability of the ADV to measure Reynolds bed stresses (<u/w/>) and

vertical Reynolds fluxes (<w/c/>) of sediment allows estimation of the critical shear velocities and the

indirect analytical estimation of settling velocity of the sediments involved.

Calibration curve

The ASTM has been calibrated using pump sampling concentrations obtained during experiments in the

Deltaflume (Chung and Grasmeijer, 1999). The five intake openings of the pump sampling equipment

were positionned close to the acoustic sensors.

The time-averaged sand concentrations measured by the five intake tubes of the pump sampler near the

acoustic sensors have been used to determine the calibration curve of the ASTM. The time-averaged (over

about 15 min) sand concentrations are between 0 and 3.7 kg/m3 for the coarse sediment (0.33 mm sand) and

between 0 and 2.7 kg/m3 for the finer sediment (0.16 mm sand). The results are shown in Figure 3. It can be

observed that for concentrations larger than 0.05 g/l the output of the ASTM varies linearly with the

sediment concentration. For smaller concentrations (<0.05 g/l) the ASTM output is larger than may be

expected for a linear relationship. This is in agreement with the measuring range of the ASTM according to

technical specifications (0.1< c <10 kg/m3).

The calibration curve can be represented by: ConcentrationASTM = 0.257*OutputASTM. The effects of particle

size in sand size range of 0.16 to 0.33 mm (d50= 0.16 mm and d50= 0.33 mm) is negligibly small.

Practical operation for on-line measurements

1. lower (or raise) sensor to sampling position (use echo sounder)

2. select sampling period (5 minutes)

3. read output signals


5. note data on measuring sheet (Figure 6)

Remarks

1. use (electronic) time integrator

2. clean sensor regularly.

Results and accuracy of ASTM

The relationship between the output signal (I) and the sand concentration (c) is as follows:

I = k c

in which:

k = calibration constant depending on properties of the transducers and sand particles (size, shape).

The inaccuracy of the measured velocity is about 2% (of the full scale). The inaccuracy of the measured sand

concentration follows from the inaccuracy (scatter) of the calibration curve. Figure 1A shows an example for

field conditions (Eastern Scheldt, The Netherlands), where a pump filter sampler has been used for

calibration. The overall accuracy is about 20%.


Technical specifications (ASTM)

sound paths: 0.15 m; frequency 4.5 MHz


weight: 10 to 20 kg

energy: 220 volt, 50-60 Hz, 40 watt

output signal: 0-10 volt (analog)

outputs: 0 to 10 V for concentration (linear); -10 V to +10 V for velocity

RS-232C port of data transfer to PC, baud rate 9600

measuring range: 0.03-3 m/s; 10-10000 mg/l(particles 50 to 500 m)

response period: 0.1 s

cable: 5 m

conditions: fully immersible to 20 m; temperature range of 5 to 30 degrees


Advantages

1. rapid, simultaneous measurement of velocity, concentration and transport

2. linear response between 10 and 10,000 mg/l (concentration range)

3. small response period

4. weakly sensitive to sand particle size; not sensitive to silt

5. insensitive to temperature, salinity variations and sensor pollution

6. usable in wave conditions

7. long term, stand-alone deployments (field proven reliability)

Disadvantages

1. calibration required for concentration

2. fragile electronic equipment

3. large dimensions and weight (carrier)

4. intrusive transducer head arrangement; large dimensions of electronics container

5. not suitable for silt

References

Chung, D.H. and Grasmeijer, B.T., 1999. Analysis of sand transport under regular and irregular waves in large-scale wave flume. Report R99-05, Department of Physical Geography, University of Utrecht.

Delft Hydraulics, 1980. Methods for measuring velocity and sediment concentrations in breaker zone (in Dutch). Report R971. Delft, The Netherlands

Delft Hydraulics, 1994. Manual ASTM (in Dutch). Report B329, Delft. The Netherlands Fugate, D.C. and Friedrichs, C.T., 2002. Determining concentration and fall velocity of estuarine particle

populations using ADV, OBS and LISST. Continental Shelf Research, Vol. 22, p. 1867-1886 Grasmeijer, B.T. et al. 2005. Suspended sand concentrations and transports in tidal flow with and without

waves (Paper U). In: Sand transport and morphology of offshore sand mining pits edited by Van Rijn et al., ISBN90-800356-7-x. Aqua Publications, The Netherlands (www.aquapublications.nl)

Jansen, R.H.J., 1978. The in-situ Measurement of Sediment Transport by Means of Ultrasound Scattering. Delft Hydraulics Laboratory, Publication No. 203, Delft, The Netherlands

Jansen, R.H.J., 1979. An Ultrasonic Doppler Scatterometer for Measuring Suspended Sand Transport. Ultrasonics Internal Conference, Graz, Austria

Jansen, R.H.J., 1981. Combined Scattering and Attenuation of Ultrasound. IAHR-Workshop on Particle Motion and Sediment Transport, Rapperswil, Switzerland

Rijkswaterstaat, 2005. Acoustic Doppler Velocitymeter as sand transportmeter (in Dutch). Document RIKZ/KW/2005.110W. RIKZ, The Haque, The Netherlands

Schaafsma, A.S. and Der Kinderen, W., 1985. Ultrasonic Instruments for the Continuous Measurement of Suspended Sand Transport. IAHR-symposium Measuring Techniques, Delft, The Netherlands

Van der Werf, J.J., 2006. Sand transport over rippled beds in oscillatory flow. Doctoral Thesis, Department of Civil Engineering, University of Twente, The Netherlands


Figure 1A

Acoustic ASTM


Figure 1B

Left: ASTM (or USTM); five-fold and two-dimensional Right: UHCM transducer heads

Figure 1C

The five-fold two-dimensional ASTM (or USTM); side view (left) and topview (right)


Figure 2

Acoustic AZTM


0.001

0.01

0.1

1

10

0.001 0.01 0.1 1 10

output USTM (V2)

me

as

ure

d c

on

ce

ntr

ati

on

(g

/l)

Figure 3

Time-averaged output of ASTM sensors plotted against the sediment concentration measured with the pump sampling system (Chung and Grasmeijer, 1999)

Figure 4

Time-averaged and ripple-averaged sand concentration in wave tunnel experiment of UHCM compared with results of OPCON and Pumps samples (TSS)


Figure 5

Time-series of sand concentrations above ripple crest in wave tunnel experiment _________ Acoustic sensor (UHCM); ------- Optical sensor (OPCON)


Figure 6

Measuring Sheet acoustic sampler AZTM


5.6.4.6 Acoustic backscatter profiling sensor (ABS and ADCP)

Principle

Acoustic backscatter (ABS) measurement is a non-intrusive technique for the monitoring of suspended

sediment particles in the water column and changing sea bed characteristics. An acoustic backscatter

instrumentation package comprises acoustic sensors, data acquisition, storage and control electronics, and

data extraction and reduction software. An overview of the ABS-technique is given by Smerdon, Rees

and Vincent (www.aquatecgroup.com). Hereafter, a summary of this is given.

The basic principle of the acoustic backscatter approach is as follows. A short pulse (10 s) of acoustic

energy is emitted by a sonar transducer (1 to 5 MHz). As the sound pulse spreads away from the

transducer it insonifies any suspended material in the water column. This scatters the sound energy,

reflecting some of it back towards the sonar transducer, which also acts as a sound receptor. With

knowledge of the speed of sound in water, the scattering strength of the suspended material and the sound

propagation characteristics, a relationship may be developed between the intensity of the received echoes

and the characteristics of the suspended material. With typical acoustic ranges in excess of 1 metre, the

acoustic head remains outside the area of study and therefore makes the instrument non-intrusive. The

magnitude of the backscattered signal can be related to the sediment concentration, particle size and the

time delay between transmission and reception. The acoustic backscatter intensity from a uniform field of

particles of constant concentration is assumed to be an inverse function of the distance from the source

with corrections for attenuation due to water and particles. Calibration in uniform suspensions is required

to find this relationship. The theoretical background of the acoustics is described in detail by Thorne and

Hanes (2002). Early work was done by Hay (1983).

The sensor comprises acoustic transducers that emit pulses of sound, which are incident on the sea bed.

They receive sound reflected by the sea bed and suspended sediment in the intervening water mass. The

instrument records the amplitude of reflected sound at gated intervals, thus building a reflected sound profile.

With low angles of incidence, the technique may be used to monitor the formation and progress of sea bed

ripples. Perpendicular incidence angles will yield information on sediment suspension between the sensor

head and the sea bed, and on the erosion or accretion of the bed level. The vertical resolution is limited by

the length of the acoustic pulse and by the speed at which the signal is digitized and recorded. A vertical

resolution of about 1 cm is feasible. Temporal resolution depends on the pulse repetition rate and on the

number of pulses which must be averaged to produce statistically meaningfull backscatter profiles. Vincent

et al. (1991) used a pulse repetition rate of 10 Hz and four profiles were averaged before storing the data on

disc. On average, a profile was recorded every 0.58 s; 1250 average profiles were recorded during each burst

(12 min).

Libicki et al. (1989) identified two difficulties in estimating the suspended load from the backscatter

signals. First, their instrument did not measure in situ the attenuation of sound caused by the suspended

load, which was introducing errors when sediment concentrations were high. Second, their instrument was

unable to distinguish between changes in particle size distribution and sediment concentration, although

in their experiments they showed that the assumption of a time-invariant particle size distribution did not

introduce substantial errors. They felt that to decouple particle size distribution from sediment

concentration would require multiple frequency devices with impractically high upper frequency limits.

The acoustic method is most appropriate for particle size distributions on the order of tens to hundreds of

microns (say 10 to 500 microns).

Libicki et al. (1989) presented a detailed analysis of the sources of interference in the ABS signal. Besides

interference from non-sedimentary targets including fish, plankton and other marine biota, they analysed

the effect of bubbles, whose target strength can be highly significant with respect to that of sediment. They

concluded that the expected lifetime of 3MHz resonant bubbles was sufficiently short to be insignificant

for their system at their deployment depths. However, they concluded that there may be significant

interference from superresonant bubbles entrained by wave activity at depths of less than 3 m and that

deployment in or very near the surf zone would not yield accurate sediment concentration measurements.They also identified various noise sources and suggested means of mitigating them. Thermal noise is

generated in the electronics of the receive circuitry, while ambient noise arises from the marine

environment. Neither is coherent and both may be minimised by reducing the input signal bandwidth

through effective filtering. It may be quantified by periodically turning off the transmitter and recording


sound profiles without any echo return. This is normally done at the beginning of an ABS data gathering

cycle, when the first group of data recorded in exactly this manner.

Experimental and theoretical work by Thorne, Holdaway et al. (1995) has addressed the problem of

sound attenuation by suspended sediment by measuring the signal strength of the bottom echo. Hay and

Sheng (1992) carried out an analysis of three-frequency acoustic backscatter signals in which they

developed a procedure to extract not only suspended sediment concentration, but also particle size

distribution with encouraging results. By simultaneously measuring both parameters, it is now possible,

in principle, to estimate directly the vertical mass flux by settling.

The uncertainties associated with measurements from earlier single frequency instruments have now been

overcome. A detailed theoretical understanding of the acoustic response of suspended sediment, backed

up with laboratory and field studies, has established the multi-frequency acoustic backscatter instrument as a

useful tool for the study of near-bed sediment processes.

Usually, a multi-frequency acoustic instrument (ABS) is used to determine the sand concentrations in the

near-bed region (lowest 1 meter of the water column).

Attempts have been made to use the backscattered signal of the (ship-mounted or bottom-mounted) Acoustic

Doppler Current Profiler (ADCP) for determining the suspended sediment concentrations (Hoitink and

Hoekstra, 2005). Similarly, the ADV was used to determine the suspended sediment concentrations (Visser,

1997).

Instrument description (ABS)

To minimise effects on water flow in the vicinity of the measurement area, the small acoustic head is

mounted remotely from the main instrument housing and attached to it by an umbilical cable. Most of the

study of near-bed sediment processes is carried out in shallow coastal waters, to which a light plastic

housing of PVC or similar material is well suited. The ABS-instrument supplied by Aquatecgroup

(www.aquatecgroup.com) is based on three frequency ranges. The three-frequency acoustic head

contains three separate transducers, each tuned to a different operating frequency. Each transducer

operates both as a transmitter and as a receiver. The transducers are each mounted on separate acoustic

backings but placed closely together to minimise the type of inaccuracies ascribed to high spatial

separation by Hay and Sheng (1992). Figure 1 shows the acoustic head. The choice of frequencies and

the transducer characteristics are essential in determining how the system will operate. The following

guidelines may be used.

a) Higher frequencies are used to resolve smaller particles. Thorne, Waters et al. (1995) have obtained

through empirical methods, backed up with theoretical analysis, a simplified form function for a

suspension of irregularly shaped scatterers, which can be applied to suspended sediment. To determine

particle size, the choice of frequencies should be such that the particle sizes of interest span the regions of

variable response.

b) Higher frequencies are attenuated more. The range of interest for sediment studies is normally limited

to less than 2 metres. From Coates (1990), the two-way attenuation due to sound absorption by water

(frequencies in excess of 1MHz) is found to depend on temperature and frequency. The percentages of

attenuation of sound power due to water absorption for different frequencies and temperatures are given in

the following table.

.

Temp/Frequency 1 MHz 2 MHz 3 MHz 5 MHz 10 MHz

5°C 7.9% 28.1% 52.4% 87.3% 99.97%15°C 5.4% 19.9% 39.3% 75.0% 99.61%

25°C 3.7% 14.1% 29.0% 61.4% 97.79%

c) The dimensions and shape of the transducer affect the horizontal spatial resolution of the system and the

characteristics of the acoustic response. The volume of water that is insonified can be roughly divided into

two regions. In the far field of the transducer, the sound source may be treated as if it were a point source.

For a circular transducer, the insonified volume may be approximated as a cone with its tip at the

transducer. In the near field of the transducer, the relationship between range and intensity becomes

complex. The standard suspended sediment monitoring configuration insonifies a narrow vertical column,


with each of the three frequencies incident on a compact spatial volume. However, the characteristics of the

transducer may be adjusted to achieve specific, non-conical beam patterns. For example, by using

transducers that are long and thin, a beam pattern is created that has a narrow beam in one axis and a wide

beam in the orthogonal axis. Such a pattern can be used to examine changes in bedform shape by aiming

the transducer at a low incidence angle to the seabed. A narrow strip of seabed along the transducer axis is

insonified. Using a principle similar to sidescan sonar, the changing characteristics of bedform ripples may

be observed.

The sounder transmit and receive electronics is controlled by a dedicated microcontroller. This controls the

generation of the three transmitter and three receiver local oscillator frequencies, which are monitored for

stability and corrected if necessary. Each channel has a tuned amplifier and transformer to match the

transducers to the drive electronics for maximum efficiency. The transmitter generally operates at

approximately 1W, with a pulse duration corresponding to a pulse length of just under 10 mm. In the

transducer head, there are receiver preamplifiers for each channel, which are multiplexed into a balanced

Schottky-diode mixer. The mixed down frequency is amplified with a linear time-varied gain amplifier.

This is used to compensate for the decrease in received signal intensity due to geometrical spreading

losses and also helps to overcome attenuation due to absorption. The received signal is detected and filtered

to eliminate aliasing when sampled by the controller.

The data logger and controller system is a two-board assembly with a hard disk. The first board is a general

purpose combined computer and data logger card, which is based on the IBM PC/XT architecture. The

second board is a specially designed interface board. The PC/XT architecture is chosen as being the most

practical approach to interface to a hard disk, since the system has a DOS compatible operating system. The

second board is designed to interface to the sounder control signals and output and to provide a trigger and

data transfer interface to the user. Opto-isolated trigger inputs allow the use of external events to trigger the

logging cycle and the type of logging. A second serial port is interfaced to the datalogger card. This is a

high speed port capable of sustained transmissions at 115200 baud, that is used to set up the logger initially

and then to recover data from it after a deployment. The serial interface standard chosen is RS422. This is

a four-wire interface that can operate over several hundred metres and therefore overcomes the difficulty

of attempting to communicate with the logger on a wet ship deck. A timer is also interfaced to the XT bus.

This is used to time the data acquisition and sounder triggering cycle. The board also includes power supply

and control circuitry, so that the system can operate efficiently from a variety of battery types and voltages,

and can switch into and out of low power modes according to internal or external triggers. Data buffers are

fitted to allow the hard disk to be safely turned off while other parts of the system remain powered up. A

long period watchdog timer is included, which can provide a system power up and reset, typically every

4096 seconds in the absence of normal logging activity, if required. This safeguards against

microprocessor crashes or a failure to receive external triggers. Finally, two Flash EPROMs are used in

place of that on the data logger card to hold not just the BIOS and DOS, but also the main data logger

application program. The hard disk now fitted is a 520MB AT-style IDE hard disk, of the type typically

found in notebook computers. The system originally operated with XT hard disks, available up to 80 MB.

However, these have now become obsolete, and a new disk BIOS has been developed to control the AT

disk from the XT electronics. This type of disk is extremely rugged and when not performing disk

accesses, can withstand shocks in excess of 100g, thus protecting it from the level of shock that may be

experienced during instrument deployment or recovery. The data logger is supplied with a software package

that allows the user to set up the various logging parameters for a deployment. These include parameters

governing the triggering of data acquisition, the duration of logging, the acquisition rate, the type of

averaging to be performed, how many frequencies to use and whether only selected bins of data should be

processed.

Instrument calibration

The early history of ABS use was characterised by concerns about interpretation of backscatter data. In

particular, Libicki et al. (1989) were concerned both about the increased likelihood of error caused by

signal attenuation by the suspended load itself, and the inability, without using many frequencies, to

distinguish particle size and to separate variation in particle size from variation in suspended load. In a

series of theoretical studies of backscatter characteristics from suspended glass spheres (Thorne and

Hardcastle, 1991; Thorne and Campbell, 1992; Thorne et al. 1992 and Thorne, Manley et al., 1993),

the general form function for a suspension of regular shaped scatterers was derived and tested against


laboratory experiments. The use of glass spheres was chosen because the main constituent of both glass

and marine sediments is quartz. The usefulness of these experiments was further enhanced by recent

work on irregularly shaped scatterers (Thorne et al., 1995), which show the general principles applied to

regularly shaped scatterers with moderate size distribution is similar to a first order approximation to that of

irregularly shaped scatterers, such as would be found in marine sediment.

When sediment concentrations are high, the technique for calculating the concentration profile becomes

inaccurate. This is because the attenuation term (a) includes a contribution from suspended load-related

attenuation. The iterative approach required to solve for load introduces errors that can cause divergence

from the solution. Recently, Thorne, Holdaway et al. (1995) examined how the attenuation due to load

may be estimated independently, thereby constraining the load solution. The principle behind their method

is to monitor the signal strength of the seabed echo during calm periods, when there is little or no sediment

in suspension. This may be used to calculate the attenuation due to water alone, or simply to act as a

reference signal strength. By monitoring the strength of the seabed return during periods of high suspended

load, the attenuation due to suspended load may be derived.

A method of particle size determination using multi-frequency acoustic backscatter was described and tested

by Hay and Sheng (1992). The principle is to use the frequency-dependent variation in backscatter form

function for particles where ka<2 with k=2 f/c=wave number, f=frequency, c=speed of sound, a=mean

equivalent radius. For particles that lie on this sloping region for at least one frequency and which return

sufficient backscatter to be detected, a ratio between two or more frequency responses may be calculated.

There must be sufficiently high concentration levels to calculate with reasonable accuracy. In the

analyses described, if the measured standard error of estimates of particle size exceeded a certain level, the

estimates were discounted, even though this may have resulted in some valid fluctuations in concentration

being rejected. Using frequencies of 1MHz, 2.25MHz and 5MHz, they were able to estimate particle sizes in

the range 50 m to 170 m to between 10% and 20%. They also note that once the relative sensitivities of

the three frequencies have been established, the calibration of the system is site independent.

The expression for the suspended load profile relates suspended load to acoustic backscatter pressure. In a

practical system it is necessary to calibrate the system response to known suspensions to take account of

variations in the transducer sensitivity, amplifier gain, TVG response and perhaps transducer radiation

characteristics. The latter was noted by Downing et al. (1995) to be significant in the correction for near-

field results, in which the measured radiating dimensions of their test transducers varied from the physical

dimensions by up to 15%. The standard method of calibration is either to use a suspended sediment jet, as

described by Hay and Sheng (1992) or a sediment tank in which a homogeneous sediment suspension is

circulated. The former method was used to calibrate the relative sensitivities of the three transceiver systems

to jets of known particle size distribution. The latter system is commonly used to calibrate the overall

system response to a uniform distribution, which may then be extrapolated using the previously described

equations for suspended load profiles. Where possible, suspended or seabed sediment samples are taken

from the deployment site to back up general calibration data and to analyse particle size distributions for a

given site.

Field and laboratory deployments

Early ABS-experiments were mainly conducted with single frequency systems. Green and Vincent

(1991) described experiments to evaluate a model of concentration profile in a combined wave and current

flow. They analysed the predictions of sediment reference concentration and vertical rate of decay of

concentration. Soulsby et al. (1991) described an experiment to monitor suspended sediment over sand

waves. The ABS-data were transferred by cable to the beach. Acoustic measurements were supported by

pump samplers at key elevations. Van Hardenberg et al. (1991) took measurements using a 3MHz ABS-

system. They reported on a method to remove the kinematic effects of wave motion from their results to

leave only the effects from physical forcing of suspension. Vincent and Green (1991) observed

substantial differences in suspension patterns for two instances of similar wave and current conditions,

leading them to suggest that the time scale of bedform processes may be much longer than the 8 minute

samples they took. Vincent et al. (1991) postulated that seabed roughness had a significant effect on the

level of resuspension. Their work also highlighted many of the early shortcomings of the single frequency

ABS-approach.

Green et al. (1995) present measurements of wave heights, near-bed currents, bed shear stresses and

suspended sediment concentrations and fluxes during a severe storm using multi-frequency ABS


systems. The acoustic backscatter data revealed wave resuspension of bed sediment, modulation of

sediment concentration by wave groups and advection of clouds by currents. Heyse et al. (1995) and

Vincent et al. (1998) present results of a study of sand bank mobility on the Middelkerke Bank in the

North Sea off the Belgian coast. This includes a brief description of the correlation between OBS and ABS

sensors in monitoring the transport of sand by resuspension. A new instrumentation platform for the

monitoring of seabed boundary layer processes, ALICE, was introduced by Green (1996). In addition to

the ABS- system, the platform includes four electromagnetic current meters, an acoustic Doppler

velocimeter for measuring near-bed three-dimensional turbulence, a precision pressure sensor for measuring

waves and tide state, five optical backscatter sensors, a time lapse video camera and an automated water

sampler. The instruments are controlled by a master data logger.

A typical example of ABS-concentration profiles as a function of time within a burst is shown in Figure 2

(River Tees, UK).

Osborne et al. (1994) have used a single-frequency profiling ABS (2.8 MHz) in combination with a point-

measuring optical backscater instrument (OBS) in the nearshore zone at Stanhope Lane Beach, Prince

Edward Island, Canada (water depth of about 1.5 m, sand bed with d50 of about 0.23 mm). The ABS was

calibrated in a laboratory recirculation tank using sand taken from the field deployment site. As the ABS is

sensitive to any change in the acoustic impedance and so will respond to bubbles and organic material in

suspension, the measurements were taken well outside the breaker zone (bubble contamination was small;

organic material content was also small). Figure 3 shows typical results (with relatively large scatter) of ABS

and OBS concentrations close to the bed averaged over the wave half-cycle.

Vincent and Green (1999) described a field arrangement on the Continental Shelf (Pacific East Coast of

New Zealand) with three transducers (F1= 1, F2= 2 and F4= 4 MHz) and a pulse-repetition rate of 80 Hz; each

profile recorded consisted of an average of 16 pulses (5 Hz). The vertical resolution is 1 cm. The

concentration range is about 0.1 to 20 kg/m3. The system was calibrated in a laboratory recirculating

suspension tank using sand from the deployment site (0.33 mm sand).

The mass concentration at range r from the acoustic transducer is estimated from a function, depending on

the voltage V measured at range r, the sediment density, the speed of sound in water and the attenuation of

sound by water and sediment of radius a. The attenuation is a complex form function of ka, which describes

the efficiency with which sediment of radius (a) backscatters sound of acoustic wavenumber (k). Three

different acoustic frequencies are used to simultaneously determine the size and concentration of the

suspended sediment involved.

The strongest acoustic echoes are used to identify the position of the sand bed. Close to the bed, the bed echo

dominates the backscattered signal. The concentration at 1cm above the bed is defined at the height at which

the first uncontaminated echo occurs, which is identified from a break-in-slope in the concentration profile

close to the maximum backscattered signal in the burst-averaged profile. The uncertainty in height is about

0.5 cm.

The concentration profiles measured by the three transducers should be identical, if the calibration conditions

are perfect, which means that the suspended sand has the same size distribution at all heights in the water

column at the field site and in the laboratory tank. Vincent and Green (1999) show examples of

concentration profiles based on the three frequencies, which have relatively large differences (factor 3, see

Figure 4) in concentrations. The concentration profiles were calculated using the results of the calibration

tank (based on 0.33 mm sand from the bed at the field site). The concentration profiles differ systematically

with F1-concentration < F2-concentration < F4-concentration. When the backscatter data are re-processed

using F1 simultaneously to obtain concentration and size, the sand concentrations are between those of F2 and

F4-concentration and the suspended sand size varies between 0.25 and 0.15 mm. It is assumed that the

suspended sand has a Gaussian distribution at every height and that the width of the distribution is constant.

When the F1 and F4-frequency pair is used, the suspended sand sizes become smaller and the concentrations

become larger; the latter show a discontinuity due to the shape of the form function yielding ambiguous

results for F1-F4 pair. This latter combination of frequencies is very sensitive to small errors in backscatter

intensity. These analysis results suggest that the sizes of the suspended sand at the field site differ

significantly from that of the bed material used in the calibration procedure. The concentrations derived from

the F1-F2 pair were found to be the most reliable. Vincent and Green concluded that the applied form

function is not quite right and should be reconsidered.


Another problem is the elimination of the effects of air bubbles in the water column, if the ABS-system

(highly sensitive to air bubbles) is used in the surf zone with breaking waves (Huck et al., 1999). This can be

done by analysis of the time-averaged concentration profiles, which should show a decreasing concentration

with increasing height above the bed. If large amounts of bubbles are present, the concentration profiles

derived from the ABS will show an increase of concentration at higher levels. These data records should be

excluded from the analysis.

The optimum conditions for the ABS-system are: rather uniform fine sand (0.1 to 0.3 mm) in non-breaking

wave conditions.

Thorne et al. (2002) have used an ABS in the large-scale Deltaflume of Delft Hydraulics to measure the

near-bed sand concentration profiles under regular and irregular waves over a sand bed with d50 of 0.33 mm

(see Figures 5 and 6). A pump sampler was operated very close to the sampling profile of the ABS at

nominally 0.05, 0.07, 0.10, 0.13, 0.18, 0.25, 0.65, 1.05 and 1.55 m above the bed. Ten liter samples were

collected and dry weighed. Size analysis (by sieving) was performed to a subset of samples to determine the

sediment size profile in the near-bed region. The ABS was operated at three frequencies of 1, 2 and 4 MHz,

collecting backscatter profiles at each frequency at a sampling rate of 128 Hz, with a spatial resolution of

0.01 m, over an operating range of 1.28 m. The transducers were mounted at nominally 1.24 m above the

sediment bed. To evaluate the acoustic concentration profiles an explicit inversion scheme was applied to the

data (Thorne and Hardcastle, 1997). This approach uses a pumped sample concentration measurement at

one height above the bed as a calibration concentration. Using this explicit approach allows the concentration

to be computed as:

c=R2/[Ro2/Mo – Ir]

with: Ir= ror (4 sR

2)dr, R=[V r/ks]e2r , V=backscattered voltage, r=range from transducer, =departure from

spherical spreading within transducer field, ks and s=acoustic backscattering and attenuation parameters,

=attenuation due to water absorption, Mo=calibration concentration at range ro, Ro=value of R at r=0.

Figure 5 shows time-averaged concentration profiles (over 17 min) for typical cases of regular and irregular

wave conditions. Over the 17 min period there was generally some ripple migration below the ABS-

transducers and therefore the measured concentration profiles were considered to be approximately quasi-

ripple-averaged profiles. The results from the three ABS-frequencies are comparable with one another.

However, there are differences, due in part to the calibration of the voltage transfer function for the system

and also to our present limited knowledge of the variability of the backscattering and attenuation

characteristics of different sediments. The concentration averaged over the three ABS-frequencies shows

generally good agreement with the pumped sample data, though there is some divergence at greater heights

above the bed. A comparison between all the contemporaneously measured pumped samples and acoustic

concentrations is shown in Figure 6.

Grasmeijer et al. (2005) have successfully used a point-measuring (in 5 points above the bed) acoustic sand

transportmeter (ASTM; 4.5 MHz) and an acoustic profiling instrument (ABS; three frequencies). Both

instruments were operated from two stand-alone tripods in the Dutch sector of the North Sea. The local water

depth was about 13 m. The seabed consisted of fine sand with d50 of about 0.22 mm. One major storm with

significant wave heights up to 4 m did occur during the deployment period. Peak tidal velocities were of the

order of 0.6 to 0.8 m/s. On average, the suspended transport rates derived from the ABS-data were somewhat

smaller (factor 2) than those of the ASTM. The ABS tends to underestimate the concentrations of particles

smaller than 0.2 mm due to the frequency used (2 MHz).

Visser (1997) has studied the backscattered signals from ADCP data (RD type DR-BB, frequency of 1200

KHz, beam angle of 20 degrees, mounted at 1.25 m beneath the water surface) collected from a moored ship

in the Western Scheldt estuary, The Netherlands. For calibration water-sediment samples were taken at 5

heights in the water column every 2 hours. The local water depth varied between 12 and 16 m. The

suspended sediment samples were used to determine the various sediment size classes and sediment

concentrations per size class. The inversion model of Thorne et al. (1995) was used to translate the acoustic

signals to sediment concentration profiles. It was found that the acoustic concentrations are very sensitive to

the –coefficient which represents the acoustic attenuation due to water phase. Figure 7 shows –values in


the range of 0.01 to 0.1 to obtain reasonable agreement between the acoustic concentrations and the water-

sample concentrations (calibration samples in the range of 10 to 300 mg/l). Increasing the –value from

about 0.042 to 0.08 results in an increase of the concentrations by a factor of 2 to 5. The –values were

found to vary from case to case (without any systematic dependency).

Hoitink and Hoekstra (2005) have successfully used a standard 1.2 MHz ADCP-instrument in combination

with an in-situ calibrated optical backscatter instrument (OBS) to measure the mud concentration profiles

(<20 mg/l) at a field site in Indonesia. The response of the ADCP showed a linear behaviour. The

interpretation of the ADCP signal was complicated by the presence of flocculated materials and by the

prevalence of anomalous scatters due to phytoplankton in the water column.

DRL software Ltd has tried to determine the suspended sediment concentrations from bottom-mounted

ADCP signals in USA coastal waters (near Cape Fear at Bald Head Island; August 2001). For comparison

data from other instruments were used (OBS, LISST, water samples). At each site a single calibration was

applied to the full data set. Many in-situ calibration surveys are required to interpret long-term monitoring

data to deal with the environmental variability of particle sizes, concentrations and other suspended

materials.

Merckelbach and Ridderinkhof (2005) have successfully used an acoustic backscatter ADCP instrument

and an optical backscatter sensor OBS to measure the suspended sediment concentration in the mouth of a

large-scale tidal inlet (Marsdiep, The Netherlands). The OBS sensor was carefully calibrated on the site

using in-situ water samples. The acoustic backscatter signal was found to consist of a Rayleigh

backscattering component proportional to SSC (suspended sediment concentration) and a backscatter

component generated by turbulence-induced variability of SSC. Comparing data from the OBS and ADCP, it

was found that the deviations between both time series were, on average, within 10 mg/l. Occasionally, the

deviations were as large as 20 mg/l.

Technical specifications of ABS (Aquascat) of Aquatecgroup (www.aquatecgroup.com)

frequencies: up to 4 frequencies, up to 5 MHz

transducers: typically 10 mm diameter ceramic cells

vertical range: 128 cm standaard

transmission rate: 1W transmitted pulse

data averaging: cell ensembles averaged over time by powers of 2 up to 64 before storage

range cells: 10 mm standaard

logging period: from 1 to 60 minutes

power requirement: 10V to 28V dc; typically 2W when logging, 4W when writing to disc

Advantages

1. non-intrusive profiler covering the lowest 1 meter of the water column where most of the suspended sand

transport takes place; bed detection included;

2. time-resolution of concentrations within wave cycle can be accurately determined;

3. particle sizes can be determined using multiple frequencies (calibration required);

4. usable in coastal environments (stand-alone tripod deployments);

5. almost no effects of biological fouling.

Disadvantages

1. in-situ calibration is required for accurate results; in-situ calibration is problematic in coastal

environments;

2. not suitable for upper concentration range (>10 gr/l);

3. measurement of velocity requires multiple sensors (cross-correlation);

4. sensitive to air bubbles and organic materials in the water column; not suitable for breaking wave

conditions (surf zone).


References

Coates, R.F.W., 1990. Underwater Acoustic Systems. Macmillan, Basingstoke. Downing, A., Thorne, P.D., and Vincent, C.E., 1995. Backscattering from a suspension in the near

field of a piston transducer. Journal of the Acoustical Society of America, 97 (3), p. 1614-1620. DRL Software Ltd, 2001. Monitoring of experiment disposal mound at Cape Fear: sediview

calibration of ADCPs and comparison with other measurement techniques. DRL Software, Godalming, Surrey, UK (www.drl.com)

Grasmeijer, B.T., Dolphin, T., Vincent, C. and Kleinhans, M.G., 2005. Suspended sand concentrations and transports in tidal flow with and without waves. Paper U in Sandpit book ISBN90-800356-7-x, edited by Van Rijn et al. Aqua Publications, The Netherlands (www.aquapublications.nl)

Green, M.O., 1996. Introducing ALICE , Water & Atmosphere (NIWA), 4 (2), p. 8-10. Green, M.O. and Vincent, C.E., 1991. Field measurements of time-averaged suspended-sediment

profiles in a combined wave and current flow. In: Soulsby, R. and Bettess, E. (Editors), Sand Transport in Rivers, Estuaries and the Sea. A.A. Balkeema, Rotterdam, p. 25-30.

Green, M.O., Vincent, C.E., McCave, I.N., Dickson, R.R., Rees, J.M., and Pearson, N.D., 1995. Storm sediment transport: observations from the British North Sea shelf. Continental Shelf Research, 15, p. 889-912.

Hay, A.E., 1983. On the remote acoustic detection of suspended sediment at long wavelengths. Journal of Geophysical Research, 88 (C12), p. 7525-7542.

Hay, A.E. and Sheng, J., 1992. Vertical profiles of suspended sand concentration and size from multifrequency acoustic backscatter. Journal of Geophysical Research, 97 (C10), p. 15661-15677.

Heyse, I., Chamley, H., De Batist, M., De Moor, G., De Schaepmeester, G., De Winne, E., Houthuys,

R., Lankneus, J., Marsset, T., Pichot, G., Pollentier, A., Porter, C., Stolk, A., Terwindt, J.,

Tessier, B., van Cauwenberghe, C., Van Wesenbeeck, V., Vincent, C., 1995. Sediment transport and bedform mobility in a sandy shelf environment. In: Marine Science and Technologies. Second MAST Days and Euromar Market. Project Reports. Weydert, M., Lipiatou, E., Goñi, R., Fragakis, C., Bohle-Carbonell, M., Barthel, K.-G. (Eds.). Commission of European Communities, Brussels, p. 1393-1408.

Hoitink, A.J.F. and Hoekstra, P., 2005. Observations of suspended sediment from ADCP and OBS measured in a mud-dominated environment. Coastal Engineering, Vol. 52, p. 103-118

Huck, M.P. et al., 1999. Vertical and horizontal coherence length scales of suspended sediments. Coastal Sediments, p. 225-240

Libicki, C., Bedford, K.W., and Lynch, J.F., 1989. The interpretation and evaluation of a 3-Mhz acoustic backscatter device for measuring benthic boundary layer sediment dynamics. Journal of the Acoustical Society of America, 85 (4), p. 1501-1511.

Merckelbach, L.M. and Ridderinkhof, H., 2005. Estimating suspended sediment concentration from ADCP backscatterance at a site with strong tidal currents. Submitted to Ocean Dynamics, ([email protected])

Osborne, P.D., Vincent, C.E. and Greenwood, B., 1994. Measurement of suspended sand concentrations in the nearshore. Continental Shelf Research, Vol. 14, No. 23, p. 159-174

Smerdon, A.M., 1996. AQ59:C - ABS System User Manual. Aquatec Electronics Limited, Hartley Wintney.

Smerdon, A.M., Rees, J.M. and Vincent, C.E., An acoustic backscatter instrument to measure near-bed sediment processes. www.aquatecgroup.com

Soulsby, R.L., Atkins, R., Waters, C.B., and Oliver, N, 1991. Field measurements of suspended sediment over sandwaves. In: Soulsby, R. and Bettess, E. (Editors), Sand Transport in Rivers, Estuaries and the Sea. A.A. Balkeema, Rotterdam, p. 155-162.

Thorne, P.D. and Hardcastle, P.J., 1991. Application of acoustic backscattering to measuring suspended sediment processes. In: Soulsby, R. and Bettess, E. (Editors), Sand Transport in Rivers, Estuaries and the Sea. A. A. Balkeema, Rotterdam, p. 111-116.

Thorne, P.D. and Campbell, S.C., 1992. Backscattering by a suspension of spheres. Journal of the Acoustical Society of America, 92 (2), Pt 1, p. 978-986.


Thorne, P.D., Vincent, C.E., Hardcastle, P.J., Rehman, S., and Pearson, N., 1991. Measuring suspended sediment concentrations using acoustic backscatter devices. Marine Geology, 98, p. 7-16.

Thorne, P.D., Hayhurst, L., and Humphrey, V.F., 1992. Scattering by non-metallic spheres. Ultrasonics, 30 (1), p. 15-20.

Thorne, P.D., Hardcastle, P.J. and Soulsby, R.L., 1993. Analysis of acoustic measurements of suspended sediments. Journal of Geophysical Research, 88 (C1), p. 899-910.

Thorne, P.D., Manley, C., and Brimelow, J., 1993. Measurements of the form function and total scattering cross section for a suspension of spheres. Journal of the Acoustical Society of America, 93 (1), p. 243-248.

Thorne, P.D., Waters, K.R. and Brudner, T.J., 1995. Acoustic measurements of scattering by objects of irregular shape. Journal of the Acoustical Society of America, 97 (1), p. 242-251.

Thorne, P.D., Holdaway, G.P. and Hardcastle, P.J., 1995. Constraining acoustic backscatter estimates of suspended sediment concentration profiles using the bed echo. Journal of the Acoustical Society of America, 98 (4), p. 2280-2288.

Thorne, P.D. and Hardcastle, P.J., 1997. Acoustic measurements of suspended sediments in turbulent currents and comparison with in-situ sampling. Journal of Acoustic Society Am., Vol. 101, p. 2603-2614

Thorne, P.D., Williams, J.J. and Davies, A.G., 2002. Suspended sediments under waves measured in a large-scale flume facility. Journal of Physical Research, Vol. 107, No. C8, p. 4.1-4.16

Thorne, P.D. and Hanes, D.M., 2002. A review of acoustic measurement of small-scale sediment processes. Continental Shelf Research, Vol. 22, p. 603-632

Van Hardenberg, B., Hay, A.E., Sheng, Y. and Bowen, A.J., 1991. Field measurements of the vertical structure of suspended sediment. Coastal Sediments '91. A.S.C.E., New York, p. 300-312.

Vincent, C.E. and Green, M.O., 1991. Patterns of suspended sand. In: Soulsby, R. and Bettess, E. (Editors), Sand Transport in Rivers, Estuaries and the Sea. A.A. Balkema, Rotterdam, p. 117-124.

Vincent, C.E., Hanes, D.M., and Bowen, A.J., 1991. Acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness. Marine Geology, 96, p. 1-18.

Vincent, C.M. et al., 1998. Sand suspension and transport on the Middelkerke Bank by storms and tidal currents. Marine Geology

Vincent, C.M. and Green, M.O., 1999. The control of resuspension over megaripples on the continental shelf. Coastal Sediments, p. 269-280

Visser, R., 1997. Relationship between the reflection of sound in water and suspended sediment concentration (in Dutch). Kamminga BV, Zoetermeer, The Netherlands


Figure 1

Acoustic head of ABS instrument

Figure 2

Typical example of ABS profiles (height above bed on vertical axis) as function of time (on horizontal axis) with high concentrations just above the bed.


Figure 3

ABS concentrations versus OBS concentrations; wave-averaged values of half wave cycles; Osborne et al. (1994)

Figure 4

Left: Time-averaged sand concentration profiles calculated for F1, F2 , F4, F1-F2 pair simultaneously and F1 -F4 pair simultaneously based on a mean sand size of 0.33 mm; Pacific East Coast New Zealand

Right: Time-averaged values of suspended sand size derived from F1 -F2 signals and from F1 -F4

signals (Vincent and Green, 1999)


Figure 5

Sand concentration profiles in regular waves (left) and irregular waves (right), Thorne et al. (2002)

Figure 6

Regression plot of mean acoustic concentration and pumped sample concentration (calibration curve); Thorne et al. (2002)


Figure 7

Sand concentration profiles from ADCP backscattered signal (lines) and water samples (solid squares) in Western Scheldt (The Netherlands); concentration (in gr/l) on vertical axis, height above bed on horizontal axis); Visser (1997)


5.6.5 Impact sensor

5.6.5.1 General aspects

Impact probes are based on the momentum-transfer principle. The high density of sediment particles gives

them excess momentum over the surrounding water so that they tend to strike a transducer placed in the

stream rather than follow the path of the water particles. This effect discriminates between sand and silt

particles. Silt particles do not possess sufficient excess momentum to impact. The sand concentration can be

determined from the impact rate and the independently measured water velocity. The height of the voltage

pulse is proportional to the momentum of the sand grains so that a crude grain size distribution can be

obtained by pulse height discrimination. The impact transfers some of the sand particle momentum and

energy to an impact sensor which converts it to an electric signal. The output is a count of frequency of

impacts.

References

Salkfield, A.P., Le Good, G.P. and Soulsby, R.L., 1981. Impact Sensor for Measuring Suspended Sand Concentration. Conf. on Electronics for Ocean Technology, Birmingham, England

5.6.5.2 IOS-impact sensor

Principle

The IOS-impact probe consists of a 1 x 10 mm stainless steel strip coupled to a slab of piezo-electric ceramic

(see Figure 1A). The ceramic slab is totally enclosed, protected and screened from electrical pick-up. The

body of the probe is formed from a stainless steel/glassepoxy laminate/stainless steel sandwich, covered in a

2-part epoxy coating.

The sand concentration can be derived from the impact rate and the (independently) measured velocity.

Thus, a separate instrument for measuring the fluid velocity is necessary.

Practical operation

1. lower (or raise) sensor to sampling position (use echo sounder)

2. select sampling period (u3e time-integrator)

3. read output signals


5. note data on measuring sheet


Probe calibration is necessary to obtain a relationship between count rate and sand concentration. A typical

field calibration (using pump sampler) is given in Figure 1B. The impacts per litre were calculated by

assuming that the geometric area of the probe was the effective active area and from simultaneously

measured velocities.


dimensions: 0.03x0.03x0.15 m

weight: 1 kg


response period: 0.2 s

cycle period: 5 min

Advantages

1. small sensor

2. small response period

Disadvantages

1. calibration required

2. not suitable for silt


Figure 1

IOS impact sensor


5.6.6 Nuclear sensor

Principle

Nuclear samplers for suspended sediment concentrations have been used in Russia, Hungary, Poland and

China. The principle is based on the absorption of radio-active energy by the sediment particles. In China the

nuclear sampler consists of a PU238 X-ray source. Other sources are Cs137, Am241 and Cd109. The radio-

activity is measured by (radiation) counters. Calibration is required. The concentration range is 0.3 to 1000

kg/m3 with an inaccuracy of 20% for low concentrations and 5% for high concentrations.

References

Basinski, T., 1989. Field Studies on Sand Movement in the Coastal Zone. Polska Akademia Nauk, Instytut Budownictwa Wodnego, Gdansku, Poland

Liu Yu-Ren, 1987. Summary on the Development of Nuclear Suspended Sediment Flux Gauge HDD-l Institute of Hydraulic Research, Yellow River Conservancy Commission, China


5.6.7 Conductivity sensor

Principle

Delft Hydraulics has developed a small-scale conductivity sensor (CCM) for measuring sand concentrations

in the high concentration regime (100 to 2000 kg/m3). The sensor (size of 0.01 m) measures the conductivity

of the fluid sediment mixture near the sensor points. The sensor has been used to measure sand

concentrations in the sheet flow layer close to the bed.

CHAPTER 6: INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY January 2006 Manual Sediment Transport Measurements Page: 6.1

6. MEASURING INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY

6.1 General aspects

Particle size information is of essential importance for the estimation of erosion, transport and deposition

rates.

As the sizes of sediment particles vary over extremely wide ranges, sediment particles are therefore

measured in very large numbers and grouped into specific, but arbitrary size classes according to various

analysis methods and definitions (see Figures 1 and 2). Sediment particles not only vary widely with respect

to size, but also with respect to specific weight and shape. Therefore, different particles of a given physical

size will behave different in the hydraulic environment as though they are larger or smaller, depending on

how their shape and specific weight vary from the defined size class.

Because of the wide range of particle characteristics, particle size usually needs to be defined in terms of the

method of analysis. Large sizes including boulders and cobbles can be measured directly by immersion and

weighing. Intermediate sizes of gravel and sand are measured semi-directly by sieving resulting in sieve

diameters. Small sizes of silts and clays are measured hydraulically by sedimentation or settling methods

resulting in the particle fall velocity and the standard fall diameter (Figure 1). The relationship between the

median sieve diameter" and the standard fall diameter is a measure of the effect of shape, roughness and

specific gravity on the settling velocity of a particle.

This leads to the fact that there are essentially two types of measurements:

size- or volume-measurements

fall velocity measurements (sedimentation method).

The size- or volume-measurements include the determination of the:

diameter by means of photographs, sieves or the diffraction of coherent light beams (Laser

granulometer);

volume by means of immersion or conductivity (Coulter Counter).

The fall velocity measurements, usually, consists of the determination of sediment accumulation as a

function of time using a:

dispersed suspension for silt particles (pipet-withdrawal tube, bottom-withdrawal tube, balance-

accumulation tube);

stratified suspension for sand particles (visual accumulation tube, manual accumulation tube, balance

accumulation tube).

In a dispersed suspension the (silt) particles begin to settle from an initially uniform suspension. The lower

size limit is approximately 5 µm because the settling of smaller particles is hindered by the Brownian

motion.

In a stratified suspension the particles start from a common source at the upper end of the tube and become

stratified according to their settling velocities. As settling medium, water is generally prefered because it is

the universal environment of (fluvial and marine) sediments. Greatest consideration must be given to the

effect of proximity of particles to each other. Concentrations larger than 5000 mg/1 result in hindered settling

and hence smaller settling velocities than for individual particles. It is also conceivable that a group of

particles creates a region of high concentration and will act as one "large" particle resulting in a relatively

large settling velocity. It may be noted that a sample containing silt, clay and coarse material will require

analysis by two or more methods because of the limitation on the range of sizes that can be analyzed by each

specific method. Because suspended sediment samples often contain a very small quantity of particles, it is

only feasible to analyze those samples obtained during periods of relatively high sediment concentrations or

those samples which consist of accumulated single samples collected under similar flow conditions. On the

other hand, some streams may at times contain rather large concentrations so that a single sample must be

splitted to obtain the optimum quantity for size analysis. A detailed description of the measuring instruments

is given in Paragraph 6.5.


6.1.1 In-situ sampling

Suspended sediment particles in estuaries and coastal seas generally consist of solid and aggregated (flocs)

materials with densities as low as 1050 kg/m3. Particle surfaces may be coated with absorbed humuc

molecules.

In-situ measurements of sediment particles and flocs in these conditions is essential as natural flocs are

disrupted easily by physical manipulation such as sampling by bottles or pumps. True particle size

distributions of natural suspended sediments can only be achieved by in-situ systems. Most optical particle

size methods are potentially non-disruptive.

Eisma et al. (1991) have used an in-situ photocamera to measure size distributions of suspended sediment in

various West-European estuaries. In addition, they have also determined the size distributions of sediment

samples collected in bottles using the traditional pipette analysis method (sedimentation method) and the

Coulter Counter method. The bottle samples were either quickly brought back to the laboratory on land or

were analysed several hours later when the survey ship was at anchor with the engines off (as the pipette

method is sensitive to mechanical vibrations and temperature-induced circulations in the sample). The

analysis results reveal no relation between the in-situ size distributions (based on photocamera method) and

the size distributions from the bottle samples (pipette or Coulter Counter method). The maximum size of the

in-situ sediments was about 800 m and the maximum size of the bottle sediments was a bout 125 m. Both

the pipette and Coulter Counter analyis methods were performed on suspended samples that were sampled

and brought to the laboratory. During sample analysis in the laboratory the original flocs were disrupted so

that actually the size of the individual solids and/or floc fragments were measured. Both methods (pipette

and Coulter Counter methods) gave similar but erroneous results. The results also depended on the way the

samples were treated and stored before analysis.

Phillips and Walling (1995) using a field-portable Laser-reflectance particle size analyser (PARTEC

200/300) have also shown that in-situ determination of particle size distributions of fluvial sediments is of

essential importance, either by making direct in-situ measurements in the water column or by taking bottle

samples and measuring the particle sizes directly after sampling. On-site measurements (immediately after

sample collection) of bottle samples were broadly similar to direct in-situ measured size distributions.

Analysis results of water-sediment samples collected in bottles and returned to the laboratory showed

significant differences in particle size distributions due to floccutation of sediments in the bottle samples,

even if the sediments in the bottle were artificially resuspended. In general the longer a sample was

allowed to settle the greater the increase in volume mean size upon resuspension. The bonding of flocs

formed during the settling period appears to become stronger with time.

6.1.2 Formulae particle fall velocity

The particle size and the fall velocity can be related to each other by the following formulae:

ws=(1/18 )((s-1)g D2) for particles in range of 1 to 100 m (1)

ws=(10 /D)[(1+(0.01(s-1)gD3/ 2))0.5 -1] for particles in range of 100 to 1000 m (2)

ws=(1.1)((s-1)g D)0.5 for particles larger than 1000 m (3)

with: ws= settling velocity (m/s), D= particle diameter (m), s= s/ =relative density ( s =2650 kg/m3),

=kinematic viscosity coefficient (m2/s).

The viscosity coefficient can be determined as:

=[1.14-0.031(T-15)+0.00068(T-15)2]10-6 (4)

with: T=water temperature in Celsius.


Figures 3 and 4 present graphs for the settling (fall) velocity and viscosity coefficient.

6.1.3 Definitions of sediment sizes

The nominal diameter of a particle is the diameter of a sphere that has the same volume as the particle.

The sieve diameter of a particle is the diameter of a sphere equal to the length of the side of a square sieve

opening through which the given particle will just pass.

The standard fall velocity of a particle is the average rate of fall that the particle would attain if falling

alone in quiescent, distilled water of infinite extent and at a temperature of 24°C.

The standard fall diameter, or simply fall diameter, of a particle is the diameter of a sphere that has a

specific gravity of 2.65 and has the same standard fall velocity as the particle.

The sedimentation diameter of a particle is the diameter of a sphere that has the same specific gravity and

terminal uniform settling velocity as the given particle in the same sedimentation fluid.

The standard sedimentation diameter of a particle is the diameter of a sphere that has the same specific

gravity and has the same standard fall velocity as the given particle.

The size distribution, or simple distribution, when applied in relation to any of the size concepts, is the

distribution of material by percentages of proportions by weight.

References

Eisma, D. et al., 1991. Suspended matter particle size in some west-european estuaries; part I: particle size-distribution. Netherlands Journal of Sea Research, Vol. 28, No. 3, p. 193-214

Phillips, J.M. and Walling, D.E., 1995. An assessment of the effects of sample collection, storage and resuspension on the representativeness of measurements of the effective particle size distribution of fluvial sediment. Water resources, Vol. 29, N0. 11, p. 2498-2508


Class Name Millimeters Micrometers Phi Value*

Phi= -2log(D)

Boulders >256 <-8

Cobbles 256 - 64 -8 to -6 Gravel 64 - 2 -6 to -1

Very coarse sand 2.0 - 1.0 2,000 - 1,000 -1 to 0

Coarse sand 1.0 - 0.50 1,000 - 500 0 to +1Medium sand 0.50 - 0.25 500 - 250 +1 to +2Fine sand 0.25 - 0.125 250 - 125 +2 to +3Very fine sand 0.125 - 0.062 125 - 62 +3 to +4

Coarse silt 0.062 - 0.031 62 - 31 +4 to +5

Medium silt 0.031 - 0.016 31 - 16 +5 to +6Fine silt 0.016 - 0.008 16 - 8 +6 to +7Very fine silt 0.008 - 0.004 8 - 4 +7 to +8

Coarse clay 0.004 - 0.0020 4 - 2 +8 to +9

Medium clay 0.0020 - 0.0010 2 - 1 +9 to +10Fine clay 0.0010 - 0.0005 1 - 0.5 +10 to +11Very fine clay 0.0005 - 0.00024 0.5 - 0.24 +11 to +12Colloids <0.0024 <0.24 >+12

Figure 1

Definitions and size scale


Sediment characteristics Strength Structure

compact

ness

Field test Term Field indentification

coarse

grained

uniform boulders

cobbles

gravel

hard can be excavated with spade; 2

inch wooded peg can easily be

driven in

homogeneous deposit consisting

essentially to one type

sands compact Require pick for excavation

non-

cohesive

graded slightly

cemented

Visual examination. Pick removes

soil in lumps

stratified alternatively layers of

varying types

low plasticity silts soft Easily moulded in fingers

Particles mostly barely or not

visible; dries moderately



firm Can be moulded by strong

pressure in fingers

alternatively layers of

varying types

fine

grained

medium

plasticity

very soft Exudes between fingers when

squeezed in first

fissured breaks into polyhedral

fragments along

fissure planes

cohesive high plasticity clays soft Easily moulded in fingers intact no fissures


pressure in fingers

(dry lumps can be broken, but not

powdered; disintegrates under

water; sticks to fingers; dries

slowly with cracks

homogeneous

stratified

deposit consisting


alternating layers of

varying types if layers

are thin, the soil may

be described as

laminated

stiff Cannot be moulded in fingers

hard Brittle or very tough weathered usually exhibit

scrumbs or columner

structure

organic peats firm Fibre compressed together; colour

brown to black

spongy Very compressible and open

structure, colour brown to black

Figure 2

Sediments and structural characteristics

CHAPTER 6: INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY February 2006

Manual Sediment Transport Measurements Page: 6.6

Figure 3

Particle fall velocity for silt particles and kinematic viscosity coefficient



Figure 4

Particle fall velocity for sand particles



6.2 Instrument characteristics

In this Paragraph 6.2 some characteristics (size range, required sample quantity and analysis period) of the

various measuring methods for particle size and fall velocity are summarized. The results are presented in the

following Table 1:

Methods size range ( m) required quantity analysis

period

Photographic 100-100000 - 1 hour

sieves dry

wet

air jet

50-50000

10-100

10-1000

1 -1000 gram

0.1-1 gram

10 -100 gram

30 min

60 min

30 min

VA-tube 50-2000 1 -10 gram 5 min

MA-tube 50-2000 1 -10 gram 30 min

large 50-2000 0.1-10 gram 15 min BA-tube

small 5-100 500-5000 mg/1 3 hours

Bottom-withdrawal tube 5-100 500-5000 mg/1 3 hours

sedimen

tation

methods

pipet-withdrawal tube 5-100 100-5000 mg/1 3 hours

conductivity coulter coulter 1-500 10 -100 mg/1 30 min

(in-situ) laser diffraction 5-1000 10 -1000 mg 15 min

(in-situ) laser reflectance 5-1000 10 -1000 mg 15 min

(in-situ) video camera 10-1000 100-1000 mg 15 min

Table 1

Instrument characteristics



6.3 Selection of instruments

In Paragraph 6.5 the instruments for determining the size and/or settling velocity of silt and sand particles are

described extensively. In this paragraph a summary of the most appropriate instruments for a specific

sediment sample is given (see following Table 1).

It should be stressed that the settling velocity of silt particles should be determined by means of an in-situ

instrument only, using the field pipet-withdrawal tube or the field bottom-withdrawal tube.

Bed material

samples

Suspended

sediment

samples

Instrument

silt sand gravel silt sand

Required sample

size

Inaccu-

racy

photocamera X -

dry X X X 1-1000 gr 10% sieves

wet X X X 0.1-10 gr 10%

laboratory X 0.5-3 liter 10% pipet-withdrawal

tubefield X 3 liter 20%

laboratory X 2 liter 10% bottom-

withdrawal tube field X 2 liter 20%

VAT X X X 1-10 gr 10%

MAT X X X 1-10 gr 5%

small X 0.5 liter 5%

accumulation

tubes

BAT

large X X 0.1-10 gr 5%

Coulter counter X X X 0.5 liter 10%

(in-situ) laser diffraction X X X X 0.01-1 gr 10%

(in-situ) laser reflectance X X 0.01-1 gr 10%

(in-situ) video camera X X X X 0.01-1 gr 10%

Table 1




6.4 Comparison of instruments

6.4.1 BAT and VAT for sand particles

The fall velocity distributions of two sand samples (D50=120 m and D50 = 230 m) were determined by

means of a sophisticated Balance Accumulation Tube (BAT) as designed by the Delft University of

Technology, The Netherlands (Geldof and Slot, 1979) and a simple Visual Accumulation Tube (VAT). The

instruments are described in Par. 6.5. The sophisticated BAT has an effective settling length of 1.66 m and

an internal diameter of 0.17 m. The tube is equipped with an under-water balance accurate to 1 mg. The

overall inaccuracy of the instrument is supposed to be smaller than 5%.

The simple VAT consisted of a perspex tube with an effective settling length of 1.95 m and an internal

diameter of 0.03 m. The sand particles are accumulated in a small capillary tube (length 150 mm, diameter 4

mm) suspended at the underside of the tube (see Par. 6.5.3.2). Sand samples with a dry weight of 1.5 grams

have been used, which result in a total deposit height in the capillary tube of about 100 mm. Figure 1A

shows the fall velocity distribution according to the BAT and VAT. The agreement is rather good. From

these results it can be concluded that the VAT is sufficiently accurate for the routine determination of the fall

velocity of suspended sand samples. Figure 1B shows the size distribution of the same two samples

according to the dry-sieving method and computed from the measured (VAT) fall velocity distribution. The

agreement between both distributions is remarkably good.

References

Geldof, H.J. and Slot, R.E., 1979. Design Aspects and Performance of Settling Tube System. Delft University of Technology, Dep.Civ.Eng., Report No. 4-79, The Netherlands



Figure 1

Comparison of BAT and VAT



6.4.2 BAT, PWT, Wet-sieving and Coulter-Counter for fine particles

Quartz and limestone powder

Figure 1A shows size-distribution curves determined by Colon (1968) using the small Balance

Accumulation Tube (BAT, Sartorius), the Pipet-Withdrawal Tube (PWT, Andreasen-Eisenwein), the Wet-

Sieving method and the Coulter-Counter. The instruments are described in Par. 6.5. The results show rather

large deviations upto 50% in the characteristic sizes D10, D50 and D90. The Pipet-Withdrawal Tube (PWT)

gives smaller size-values than the Balance Accumulation Tube (BAT) for both materials. The Wet-Sieving

method shows good agreement with the Pipet-Withdrawal Tube results.

Silt sample

Karelse and Polhuys (1981) compared the Balance-Accumulation Tube (Sartorius) and the Pipet-

Withdrawal Tube (Andreasen-Eisenwein) for a silt sample (see Figure 1B). The results show good agreement

with a maximum error of about 10%.

References

Colon, F.J. de, 1968. Particle Size Analysis (in Dutch). CTI-TNO Publication No. 68, The NetherlandsKarelse, M. and Polhuys, J.M., 1981. Investigation of Accumulation Balance for Particle Size

Measurement (in Dutch). Delft Hydraulics, Report S320, The Netherlands



Figure 1

Comparison of BAT, PWT, Wet-sieving, Coulter-Counter



6.4.3 PWT, BWT and BAT for fine particles

The laboratory designs of the Pipet-Withdrawal Tube (PWT) and the Bottom-Withdrawal Tube (BWT) were

used to determine the fall velocity distribution of a silt sample with particles in the range of 0 to 70 m. The

results of the PWT and the BWT were compared with the results of a small (commercially available)

Balance Accumulation Tube (BAT, Sartorius). The latter instrument is supposed to give the most accurate

results (Van Rijn, 1986). All instruments are described in Par. 6.5.

The PWT consisted of a perspex tube with a length of about 300 mm and an internal diameter of 120 mm, as

shown in Fig. 1B, Par. 6.5.3.4. The withdrawal volume was about 200 ml. The initial settling height above

the point of withdrawal was 290 mm. The operational procedure is described in Paragraph 6.5.3.4.

The BWT consisted of a perspex tube with a length of 1020 mm and an internal diameter of 50 mm, as

shown in Figure 1A, Par. 6.5.3.3. The operational procedure as described in Paragraph 6.5.3-3-Figure 1A

shows the fall velocity distribution based on the PWT and BAT methods for various initial silt

concentrations in the range of 20 to 2000 mg/1. The agreement between the results of both methods is rather

good, even for small (initial) concentrations of 20 mg/1.

Figure 1B shows the results based on the BWT and BAT. The agreement between the results of both

methods is reasonably good for initial concentrations larger than about 200 mg/1. For initial concentrations

smaller than 200 mg/1 the results of the BWT begin to deviate, showing rather large deviations for c = 20

mg/1. The relatively poor results of the BWT for small initial concentrations are, probably, caused by

insufficient removal of the sediment particles during the withdrawals. Sediment particles may (partly) stick

to the inside of the contracted section of the tube (Van Rijn, 1986). Therefore, the BWT is not supposed to

be an optimal instrument for the in-situ analysis of silt suspensions, because the results may not be very

accurate for initial concentrations smaller than 200 mg/1, the latter being quite common values for field

conditions.

Figure 2 shows comparative results for the field design (see Figs. 2, 3 of Par. 6.5.3.4) of the PWT based on

measurements in a flume. The results of the PWT are compared with the results of water-sediments samples

analyzed in the Sartorius balance accumulation tube (BAT).

References

Van Rijn, L.C., 1986. The determination of Settling Velocities. Delft Hydraulics Laboratory, Report S3O4, The Netherlands

Van Rijn, L.C. and Nienhuis, L.E.A., 1985. In-situ Determination of Fall Velocity of Suspended Sediment. 21st IAHR-Congres, Melbourne, Australia



Figure 1

Comparison of PWT, BWT and BAT

Figure 2

Comparison of PWT, and BAT for silt particles in a flume



6.5 Description of instruments

The following instruments are described:

photographic instrument,

sieving instruments,

sedimentation instruments,

Coulter Counter,

Laser diffraction,

Laser reflectance,

video camera.

6.5.1 Photographic instrument

Principle

The method is based on taking photographs of the (dry) stream bed. The height of the camera depends on the

size of the bed material and the lens system. A reference scale must appear in the photograph. The

photograph is printed on thin paper to be inspected on a lightbox with special optical equipment. By

adjusting the optical equipment, the diameter of a sharply defined circular lightspot appearing on the photo-

graph can be changed and its area made equal to that of the individual particles.

An automatic counting system can be used for registration of the particles. After registration each particle

must be marked on the photograph.

References

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. United States Government Printing Office, Washington, USA

Vanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, New York, USA



6.5.2 Sieving instruments


Sieve analysis is one of the simplest, most widely used methods of particle size analysis, that covers the

approximate size range from 50 m to 50000 m using standard woven wire sieves. Micromesh sieves

extend the range down to 5 m and punched plate sieves extend the upper range.

Sieve results can be highly reproducable (within 5%). Inaccuracies may be caused by:

size of total sample and size of particle fractions on each sieve

presence of aggregated lumps of particles

inaccuracies in size and shape of the sieve openings

the duration of the sieving operation.

For several countries standard (woven-wire) sieves are in use according to the national standard

specifications:

The Netherlands: N480

Germany: DIN 4188

England: BS 410 (1976)

France: AFN0R NFX 11-501

USA: ASTM E 11-70

6.5.2.2 Dry sieving

Principle

The sieving analysis is carried out by stacking the sieves in ascending order of aperture size and placing the

sediment sample of the top sieve. A closed pan (receiver) is placed at the bottom of the stack to collect the

fines and a lid is placed on top of the stack of sieves to prevent loss of particles. A stack usually consists of

five or six sieves in a root-two progression of aperture size. The stack is vibrated for a fixed time (20 min)

and the residual weight of particles on each sieve determined.

Practical operation

1. weigh the required quantity of dry sediment sample.

2. stack the required sieves in ascending order of size with a closed pan under the bottom-sieve,

3. put the dry sample on the top-sieve (with cover plate),

4. vibrate for 20 minutes,

5. weigh the particles on each sieve and the bottom-pan,

6. check that the total cumulative weight is substantially the same as the original weight of the sample,

7. note data on measuring sheet (Figure 2 and 3).

Remarks

The optimum quantity of the sediment sample depends on the mesh size of the sieve, the diameter of the

sieve and the number of sieves. As a rule, the quantity of the material on any sieve at the completion of

sieving should not exceed a layer of one particle deep over the sieving surface. Otherwise, the sample should

be recombined, splitted and sieved again.


Usually, the results are given in terms of the cumulative weight percentage passing the sieve as a function of

the sieve size, as shown in Figure 4. The geometric standard deviation is defined as (Vanoni, 1980):

s=0.5[D16/D50 + D50/D84]

For a log-normal size-distribution it follows that:

D10=( s)-1.3

D50

D16=( s)-1 D50

D84=( s)1 D50

D90=( s)1.3 D50



The mean particle diameter is defined as:

DM=( 0.5(Di+Di+1)gi)/ gi=( 0.5(Di+Di+1)pi)/100

in which:

gi = dry weight of particles on sieve i (diameter Di)

pi= weight percentage of particles on sieve i (diameter Di)

For a symmetrical distribution the mean diameter (DM) is equal to the median diameter (D50): DM = D50

The minimum sample weight required for a proper representation of the bed material and hence required for

an accurate sieving result is given by De Vries (1971), as shown in Figure 1.

References

Allen, T, 1981. Particle Size Measurement, Third Edition. Chapman and Hall, London-New York De Vries, M., 1971. On the Accuracy of Bed Material Sampling. Delft Hydraulics Laboratory, Publication

No. 90, The Netherlands Vanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice, No.54, New

York

Figure 1

Dry sieving



Sieve Analysis Delft Hydraulics

Sample No =

Location =

Date =

Observer =

D10 =

D50 =

D90 =

DM =

Sieve diameter

Di

(mm)

Average diameter

DM,i

(mm)

Weight on sieve

gi

(mg)

Weight percentage

pi

(-)

Cumulative weight

percentage

(-)

piDM,i

11.2

8.0 9.6

5.6 6.8

4.0 4.8

2.0 3.0

1.4 1.7

1.0 1.2

0.85 0.925

0.71 0.78

0.6 0.655

0.5 0.55

0.42 0.46

0.35 0.385

0.3 0.325

0.212 0.256

0.18 0.196

0.15 0.165

0.125 0.138

0.106 0.114

0.09 0.098

0.075 0.083

0.063 0.069

0.053 0.058

0.045 0.049

0.038 0.042

pan Ttotal 100%

Figure 2

Measuring sheet sieve analysis



Figure 3

Sieve analysis



Figure 4

Sieve curve form



6.5.2.3 Wet sieving

Principle

The method can be used for particles in the range of 10-100 m. The sieves consist of

nickel plates in which electrolytic holes are made with an accuracy of 2 m (micro-precision sieves).

Usually, the sieves are stacked on top of each other and the sample is placed on the top sieve and

washed with a liquid (water with a detergent, 0.1% Teepol) while the stack of sieves is being

vibrated.

The vibration can be accomplished by placing the stack of sieves in an ultrasonic bath. Before the sieving

operation the sieves are dried and weighed.

Practical operation

1. dry and weigh all sieves,

2. stack the sieves in ascending order of size,

3. wash the sample on the top sieve (use water with a detergent, 0.1% Teepol solution),

4. continue washing process while the sieves are vibrating (use ultrasonic bath) so that the

water drains through all sieves,

5. remove top sieve with subfraction of sediment,

6. wash the next sieve and so on,

7. dry and weigh all sieves including sediment fractions,

8. determine weight of sediment samples,

9. note data on measuring sheet (see Par. 6.5.2.2).

References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapman and Hall, London-New York

6.5.2.4 Air-jet sieving

Principle

The air-jet sieve is an instrument using an air-current to agitate the dry sediment particles on the sieve. A

single sieve is placed above a rotating vane in an airtight container. The air-jet is blown through the rotating

vane and the sieve above the vane. The air and the particles then passes down the sieve on both sides of the

vane. The particles are collected on a filter paper. Firstly, the finest of the sieves is used and so on untill all

sieves have been used. The method has been found useful for sieving low density materials and very fine

sediments. Materials such as coal, wood and polystyrene particles can be sieved more efficiently.

References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New York Kiff, P.R., 1977. Sedimentation Methods Manual. Hydraulic Research Station Wallingford, England



6.5.3 Sedimentation instruments


Basically, two methods are used for particle size analysis:

stratified suspensions,

dispersed suspensions.

Stratified systems

In a stratified system the particles start from a common source and become stratified at the bottom of the

tube according to the settling velocities. Generally, this method is only used for sand particles. The stratified

sediment layers at the bottom of the tube can be measured by means of a small capillary tube (Visual

Accumulation Tube, VAT). Another possibility is to weigh the settled sediment particles directly by means

of an under-water balance or to extract the settled sediment particles at pre-fixed time intervals by means of a

mechanical method. The latter two methods produce the accumulated sediment weight as a function of time.

Using the known settling height (L), the weight percentage of the particles with a fall velocity larger than

Wi(=L/Ti) can be determined (Figure 1A).

Dispersed system

In a dispersed system the particles begin to settle from an initially uniform dispersion (equal concentration).

Generally, this method is only used for silt or fine sand particles (5 to 150 m). Usually, the sediment weight

is determined as a function of time by means of an under-water balance (Figure 1B).

At time T, being the ratio of the settling length L and the fall velocity W, all particles are settling at a weight

rate per unit time: (dG/dt)T. During the period 0-T all particles with a fall velocity smaller than W have

settled (as a group) at the same constant rate (dG/dt)T. Consequently, the total weight of particles with a fall

velocity smaller than W on the balance can be represented by T(dG/dt)T. However, the total weight of all

particles on the balance at time T is GT. Hence, the difference represents all particles of the whole sample

with a fall velocity larger than W:

G=GT – T(dG/dt)T (1)

In terms of weight percentages:

P=PT – T(dP/dt)T=PT – (log e) (dP/dlogt)T (2)

in which:

P = weight percentage of particles with a fall velocity larger than W (oversize).

Equation (2) is known as the Oden-equation.

References

Allen, T., 1980. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New YorkVanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice, No. 54,

New York, USA



Figure 1

General principles Sedimentation method



6.5.3.2 Accumulation tube

Principle

An accumulation tube can be operated as a stratified system for sand particles in the range 50-2000 m or as

a dispersed system for silt and fine sand particles smaller than 150 m. Typical examples of the

accumulation tube method are:

Visual Accumulation Tube (VAT)

Manual Accumulation Tube (MAT)

Balance Accumulation Tube (BAT)

The VAT, which operates as a stratified system, consists of a settling tube with a length of about 2 m and a

diameter of about 0.03 m (Figure 1A) . The sample can be released on top of the tube by means of a simple

clamp device or by means of rotating lamellae (Figure 1A, IB). Under the settling tube a small capillary tube

is suspended in which the deposit height can be determined as a function of time. Based on this information,

the fall velocity distribution can be determined. The capillary tube has a length of about 150 mm and an

internal diameter of 4 mm for particles of 50-500 m and 10 mm for particles of 500-2000 m. The method

may not be very accurate due to hindered settling of the particles in the contracted section and the capillary

tube. To minimize this effect, the length of the settling tube should be relatively large compared with that of

the capillary. The settling is not properly defined and can be calibrated using samples of known diameter and

settling velocity (glass beads, see Kleinhans, 1998).

The overall inaccuracy of the fall velocity distribution is about 10% for particles in the range 50-500 m (see

Paragraph 6.4.1). The major advantage of the VAT-method is the rapid determination of the fall velocity

distribution of a sand sample. A routine analysis can be easily done in about 5 minutes (included elaboration

of the measuring results).

Gibbs (1972) found that the tube diameter should be larger than 0.1 m and the sample mass should be 1 to 2

grams to obtain accurate results (< 5%).

The MAT is quite similar to the VAT, but another method is applied to determine the weight increase of the

settled sediment particles as a function of time. The particles are collected in small cups placed under the

settling tube at pre-set times using a manual slide mechanism. The slide mechanism may be designed as a

waterproof system or as an under-water system, as shown in Figure IB. In the latter case the tube must be

closed directly after release of the sample by means of a rubber stop. By drying and weighting of the

particles, the cumulative weight curve can be determined. The overall accuracy is about 5%.

The BAT is based on the weighting of the settled particles by means of an under-water balance (see Figure

2). The method can be operated as a stratified system using a long tube of about 2 m or as a dispersed system

using a short tube of about 0.5 m for silt particles. In the latter case the tube should be equipped with a

temperature control system because the measuring period may be rather large (24 hours) to determine the

total weight of all particles.

Commercially available for the analysis of silt particles is the Sartorius-Balance (Sartorius, 1981). At the

Delft University of Technology (The Netherlands) a large tube is available for the analysis of sand particles

(Geldof, 1979). The overall accuracy of the BAT is about 5% (Karelse and Polhuys, 1981).



Practical operation

VAT

1. fill the tube with water (upto 0.02 m below the top),

2. measure the water temperature,

3. level the bottom side of the capillary with the measuring scale,

4. close the sample release system,

5. wash the (wet) sample in the container (1.5 gram for particles 50- 500 m and 10 grams for particles of

500-2000 m).

6. release the sample and start the clock when the particles pass the water surface level,

7. note the clock times (Ti) when the deposit height passes the values Hi=5, 10, 20, 30, …… mm,

8. measure the total deposit height (H) and determine the cumulative height curve as a function of time.

(the percentage of particles with a settling velocity larger than Wi= L/Ti is equal to Hi/H;

L= effective settling height of the tube, Ti= clock time of deposit height Hi, H= total deposit height).

9. determine the fall velocity curve.

Figure 3 shows a measuring sheet; Figure 5 and 6 show a graph for the fall velocity distribution.

MAT

1. see above,

2. see above,

3. see above,

4. see above,

5. see above,

6. see above,

7. collect the sediment particles in cups placed under the tube at pre-set times (which should be

pre-determined by a trial test),

8. dry and weigh the sediment samples,

9. determine the cumulative weight curve,

10. determine the fall velocity distribution.

Remarks

If an under-water slide mechanism is used, the tube must be closed at the bottom side before sample release

and closed at the top directly after sample release.

Figure 4 shows a measuring sheet; Figure 5 shows a graph for plotting the fall velocity distribution.

References

Geldof, H.J. and Slot, R.E., 1979. Settling Tube Analysis of Sand. Delft University of Technology, Dep. of Civ. Eng., Internal Report No. 4-79

Geldof, H.J. and Slot, R.E., 1979. Design Aspects and Performance of a Settling Tube System Delft University of Technology, Dep. of Civ. Eng. Internal Report No. 6-79

Gibbs, R.J., 1972. The accuracy of Particle-Size Analysis Utilizing Settling Tubes Journal of Sedimentary Petrology, Vol. 42, No. 1, p. 141-145

Inter Agency Committee on Water Resources, 1957. The Development and Calibration of the Visual Accumulation Tube. Report No. 11, St. Anthony Falls Hydraulic Laboratory, Minneapolis, USA

Karelse, M. and Polhuys, J.M., 1981. Investigation of Accumulation Balance for Particle Size Measurement (in Dutch). Delft Hydraulics Laboratory, Report S320, The Netherlands

Kleinhans, M.G., 1998. Calibration of visual accumulation settling tube (in Dutch). Report ICG 98/13 Department of Physical Geography, University of Utrecht, Utrecht, The Netherlands

Sartorius, 1981. Aufstellungs- und Bedienungsanweisung



Figure 1

Visual and Manual Accumulation Tube

Figure 2

Balance Accumulation Tube



Figure 3

Measuring Sheet Visual Accumulation Tube



Figure 4

Measuring Sheet Manual Accumulation Tube



Figure 5

Graphs Fall velocities



Figure 6

Graph Fall velocity



6.5.3.3 Bottom Withdrawal Tube (BWT)

Principle

The instrument is based on the sedimentation of sediment particles from an uniform suspension (dispersed

system).

The bottom withdrawal tube method can be used for the fall velocity analysis in the laboratory, but also for

the in-situ determination of the fall velocity distribution. This latter possibility offers the advantage of using

an undisturbed suspension sample and native water as settling medium, which is essential for flocculated

sediments.

The laboratory instrument consists of a tube with a length of about 1 m and an internal diameter of 0.05

m (or 0.025 m). The lower end of the tube is contracted into a nozzle (Figure 1A). A pinch-clamp on a

short piece of rubber is used to enable quick withdrawals.

The field instrument consists of a stainless steel tube with a length of about 1 m and an internal diameter

of 0.05 m.

The tube is used for the collection of the sample as well as for the determination of the fall velocity

distribution by means of a settling test. Therefore, the tube is equipped with two valves on both ends

(Figure 1B) and a double wall for temperature control. The tube is lowered to the sample location in a

horizontal position with opened valves. After closing the valves, the tube is placed in an upright position

(start of settling process) and hoisted on board of the survey vessel. Commercially available is the

BRAYSTOKE Sediment Sampler SK 80 (Dartmouth, Devon, England), as shown in Figure 2. The bottom

withdrawal tube is also known as the Owen tube.

Usually, eight equal volume fractions of about 0.2 liter are withdrawn at prefixed time intervals

chosen in such a way as to best define the accumulative weight curve.

A suitable schedule for particles in the range of 5 to 100 m is withdrawals at 3, 6, 10, 20, 40, 60 and 120

minutes. The initial concentration should not be smaller than about 200 mg/1 (see Par.6.4.3).

Dearnaley (1996) studied the settling process in the Owen tube using a miniature video camera (standard

CCD video camera with c-mount to Nikon mount adaptor, 200 mm bellows and standard 135 mm Nikon

lens). The camera and light source are set up perpendicular to one another and focussed on the transparent

settling column. Typical images of about 3 by 4 mm can be obtained with this set up. This gives an

approximate resolution of 20 microns. The depth of focus in the image is about 0.1 mm. The median settling

velocity based on the gravimetric analysis is an order of magnitude less than that derived from video image

analysis. Individual flocs can readily be observed by eye within an Owen tube. It can also be observed that

the flocs do not always appear to move vertically downwards. Often, the fluid motion is highly turbulent

during and after subsample withdrawal. At times of subsample withdrawal the fluid motion inside the tube

was found to be in the range of 20 to 30 mm/s, two orders of magnitude greater than the gravimetrically

determined median settling velocity producing turbulence that affects the settling and flocculation/break-up

process. Dearnaley concludes that by analysing video images obtained shortly after the sample is obtained,

the results should be more representative of the size and settling velocity distribution of the flocs in the field

than that inferred from the results of the gravimetric analysis over about an hour.

Practical operation

Laboratory instrument

1. prepare a 2-litre suspension of about 1000 mg/1,

2. fill tube and measure water temperature,

3. close tube (rubber stop),

4. mix suspension (turning over of the tube),

5. put tube in an upright position, remove stop (or open upper valve) and start the clock,

6. withdraw eight equal volume fractions at pre-fixed time intervals (see measuring form); the

actual withdrawal is started 5 seconds before the chosen withdrawal time (the last withdrawal

can be done directly after the last but one),

7. determine the volume of each withdrawal,



8. determine the dry weight of the sediment particles in each withdrawal (filter method, use a

balance with an accuracy of 0.1 mg).

RemarksBefore opening the pinch-clamp, it is advisable to loosen the sediment particles settled on the inside of the

contracted section by ticking with a small stick against the contracted section (about 30 seconds). After that

the pinch-clamp must be opened fully to assure that the water will flush all deposited particles.

Field instrument

1. lower tube in a horizontal position with opened valves to sampling location,

2. close valves (by messenger),

3. put tube in a vertical position (start of settling process, t =0; start clock),

4. raise tube on board of survey vessel

5. withdraw eight equal volume fractions at pre-fixed time intervals (see measuring sheet); the

actual withdrawal is started 5 seconds before the selected withdrawal time (the last withdrawal can

be taken directly after the last but one),

6. determine volume of each withdrawal,

7. determine dry weight of sediment particles in each withdrawal (filter method, use balance accurate to 0.1

mg).

Figures 6 and 7 show a measuring sheet and a graph for plotting the fall velocity distribution (see also

Figure 6 of Par. 6.5.3.2).


Analysis of the measuring results is, as follows (see Figure 3, 4, 5):

1. Compute equivalent sample heights (column 6, Fig. 3): hi=Vi/A=Vi/(0.25 D2) (mm),

2. Compute effective fall height (column 7) by summation of the equivalent sample heights: h= hi (mm),

3. Compute the cumulative sediment weight (column 9) by summation of the silt sample weights (column

4): G= Gi (mg),

4. Compute the depth factor (column 10) to correct the results to a standard fall height of 1000

mm: Depth-factor=1000/ hi,

5. Compute the corrected cumulative sediment weights (column 11) by multiplying the values of

column 9 with the depth-factor,

6. Compute the corrected settling times (column 13) by multiplying the values of column 8 with the

depth-factor,

7. Compute the cumulative weight percentages (column 12) by using the values of column 11,

8. Compute the settling times (column 15) for a standard settling height of 1000 mm by using

the settling velocities of column 14,

9. Plot the cumulative weight percentages (column 12) as a function of time (column 13), as shown in

Figure 5,

10. Indicate the settling times (column 15) on the time axis (as an example the value of 42

min for a settling velocity of 0.4 mm/s is indicated, (Figure 5),

11. Plot the tangents to the curve at each indicated time and construct a triangle with a basis equal to log e

(Figure 5),

12. Plot a horizontal line through the top of the triangle, read the (weight) percentage "smaller"

on the vertical axis (Figure 5) and note the value in column 16,

13. Plot the settling velocity distribution by using the values in column 14 and 16 (Figure 3,4).

Based on comparative experimental studies, the overall inaccuracy is found to be about 10% for

initial concentrations larger than 200 mg/1.



Disadvantages

The bottom-withdrawal tube has two major disadvantages.

1. Sediment particles sticking to the inside of the contracted section may not be removed during withdrawal

of the sample. Laboratory experiments have shown that the last withdrawal may contain more sediment

particles than the last but one sample.

2. The complete procedure for determining the particle fall velocity distribution is rather time consuming

compared with other methods (pipet-withdrawal tube, balance accumulation tube).

References

Dearnaley, M.P., 1996. Direct measurements of settling velocities in the Owen tube: a comparison with gravimetric analysis. Journal of Sea research, Vol. 36, No. 1-2, p. 41-47

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. Geological Survey, USA Government Printing Office, Washington, USA

Owen, M.W., 1976. Determination of the Settling Velocities of Cohesive Muds. Hydraulics Research Station Wallingford, Report No. IT 161, England

Van Rijn, L.C., 1986. Determination of Settling Velocities. Delft Hydraulics Laboratory, Report S304, Delft, The Netherlands

Vanoni, V.E., 1976. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, New York, USA

Figure 1

Bottom Withdrawal Tube



Figure 2

Bottom Withdrawal Tube, Braystoke Sampler



Figure 3

Measuring Sheet Bottom Withdrawal Tube



Figure 4




Figure 5

Oden curve; settling velocity distribution



Figure 6




Figure 7

Graphs



6.5.3.4 Pipet-Withdrawal Tube (PWT)

Principle

The fundamental principle of the pipet method is to determine the sediment concentrations

of an initially uniform suspension (dispersed system) at a pre-fixed depth below the water surface as a

function of settling time (Figure 1). Particles having a settling velocity greater than the ratio of the

depth and the elapsed time period will settle below the point of withdrawal after the elapsed time period.

The sediment concentration at a certain depth can be determined by withdrawing samples at that height.

Usually, eight or nine samples are withdrawn.

The pipet method can be used for the laboratory analysis of a silt sample but also for the

in-situ analysis of a silt suspension. This latter possibility offers the advantage of using

an undisturbed suspension sample and native water as settlingmedium, which is essential for flocculated

sediments.

The laboratory instrument consists of a 1 liter-cylinder with an internal diameter of 0.075 m for

suspensions with an initial concentration larger than 1000 mg/1, while a 25 ml-pipet is used for

withdrawing the samples (Figure 1A). A 3 liter-cylinder (internal diameter of 0.1 m) in

combination with a 200 ml-withdrawal volume should be used for suspensions with an initial

concentration in the range of 100 to 1000 mg/1. In the latter case a side-withdrawal method can be used,

as shown in Figure 1B. For accurate results the initial settling height should be 0.3 m. The analysis

period is about 2 hours for separation to about 5 ym. (Van Rijn, 1986).

For suspensions with an initial concentration larger than 1000 a 2000 mg/1, the Andreasen-

Eisenwein pipet (Figure 1C) can be used for routine analysis.

The field instrument consists of a stainless steel tube with a length of about 0.3 m and an internal

diameter of 0.12 m. The tube is used for sample collection as well as for the determination of the fall

velocity distribution by means of a settling test. Therefore, the tube is equipped with two

valves on both ends and a double wall for temperature control (Figures 2 and 3). The tube is lowered to the

sampling location in a horizontal position with opened valves. After closing the valves, the tube is put in a

vertical position (start of settling process, t -0) and raised. On board of the survey vessel withdrawals are

taken at pre-fixed times.

Practical operation laboratory instrument

25 ml withdrawal

1. prepare a 1 liter suspension with a concentration larger than 1000 mg/1,

2. fill the tube and measure the water temperature (frequent readings, if necessary).

3. mix the suspension thoroughly by turning the tube upside-down (close tube with rubber

stop),

4. remove the stop and start the clock,

5. withdraw 3 samples at a depth of 0.15 m below the watersurface at times t = 0, 1, 3 min (use

standard 25 ml-pipet),

6. withdraw 3 samples at a depth of 0.1 m below the watersurface at times t = 7, 15, 30 min,

7. withdraw 3 samples at a depth of 0.05 m below the watersurface at times t = 45, 60, 90 min

8. withdraw the residual suspension through the bottom of the tube directly after the last 25 ml

pipet-sample,

9. determine the dry sediment weight of all samples.

200 ml withdrawal

1. prepare a 3 liter-suspension with a concentration in the range 100-1000 mg/1,

2. fill the tube to a level of 0.3 m above the point of withdrawal (note the exact reading) and

measure water temperature,

3. mix the suspension thoroughly by turning the tube upside-down (close tube with rubber stop),


5. withdraw the first 200 ml sample directly after starting the clock (t= 0),

6. read the water surface level above the point of withdrawal,



7. withdraw samples at times t = 1, 3, 7, 15, 30, 60 and 120 min (read withdrawal depth before each

withdrawal),


withdrawal,

9. determine the volume of all samples,


Figure 5 shows a measuring sheet for laboratory conditions; Figure 7 of Par. 6.5.3.3 shows a graph for

plotting the fall velocity distribution (see also Figure 6 of Par. 6.5.3.2).

Practical operation Field Instrument

1. lower the tube in a horizontal position with opened valves to the sample location,

2. close the valves (by messenger),

3. put the tube in a vertical position (start of settling process, t = 0, start clock) and raise the tube,

4. open the top valve of the tube,

5. measure the height between the water surface level and the upper end of the tube (if tube is not

completely filled), measure water temperature,

6. withdraw 200 ml-samples at times t = 1, 3, 6, 10, 20, 40 and 60 minutes (side

withdrawals),

7. withdraw the residual suspension through the bottom of the tube directly after the last

200 ml sample (wash inside free of sediment, note volume of wash water),

8. determine the exact volume of all samples,


Remarks

1. the actual withdrawals should be started 10 seconds before the chosen withdrawal times,

2. use a balance accurate to 0.1 mg.

Figure 6 shows a measuring sheet for field conditions; Figure 7 of Par. 6.5.3.3 shows a graph for plotting the

fall velocity distribution (see also Figure 6 of Par. 6.5.3.2).



The results of the calculation method are given in Figure 4.

1. Compute the sediment concentrations (column 6) as: ci=Gi/Vi (mg/l), initial concentration (co) can be

determined from the total sediment weight and the total sample volume.

2. Compute the settling velocity at which separation is required (column 9) as: Ws,i=Hi/Ti (mm/s), in

which Hi = height of water surface above withdrawal point (mm),

3. Compute the weight percentage (%) of the particles with a settling velocity smaller than Ws,i (column

10) as: Pi =(ci/co)100% in which co = initial concentration (mg/1),

4. Plot the settling velocity distribution using the values of column 9 and 10 (see Figure 4).

Field instrument

The calculation method is shown in Figure 6.

1. Compute the sediment concentration (column 6) as: ci=Gi/Vi (mg/l), initial concentration (co) can be


2. Compute the equivalent sample height (column 7) as : hi=Vi/Ai=Vi/(0.25 D2) (mm),

3. Compute the effective settling or fall height (column 8) as: h=Ho- hi, with Ho=initial settling height=

height between initial water surface and point of withdrawal (mm),

4. Compute the settling velocity at which separation is required (column 11), as: Ws,i= (Ho- hi)/Ti,


12), as : Pi=(ci/co)100%,

6. Plot the settling velocity distribution by using the values in column 11 and 12.



References

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. Geological Survey, USA Government Printing Office, Washington, USA

Vanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, New York, USA

Van Rijn, L.C., 1986. Determination of Settling Velocities. Delft Hydraulics Laboratory, Report S304, Delft, The Netherlands

Figure 1

Laboratory Pipet-Withdrawal Tubes



Figure 2

Field Pipet-Withdrawal Tube

Figure 3




Figure 4

Measuring Sheet Laboratory Pipet Withdrawal Tube



Figure 5




Figure 6

Measuring Sheet Field Pipet Withdrawal Tube



6.5.4 Coulter Counter

Principle

The method is based on an electrical conductivity difference between particles and common diluent. Particles

act as insulators and diluents as good conductors. The particles suspended in an electrolyte are made to pass

through a small aperture through which an electrical current path has been established (Figure 1). As each

particle displaces electrolyte in the aperture, a pulse essentially proportional to the particle volume is

produced. Particles in the range of 1 to 500 m can be counted and measured volumetrically.


The results are presented in terms of a cumulative volume distribution and the percentage of particles in each

volume class. The reproducibility is claimed to be accurate to about 1%.

References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New York

Figure 1

Coulter Counter



6.5.5 Particle size and concentration by Laser Diffraction (LISST, COULTER, PARTEC)

The Laser diffraction method (Fraunhofer diffraction) offers a fundamentally superior basis for in-situ

measuring the sizes of suspended sediment particles in a point in the water column. Unlike other and simpler

optical or acoustic methods, the diffraction method does not suffer from a change in calibration with

changing sediment colour, composition or size.

When a parallel light wave strikes a particle, part of the wave enters the particle, and part is blocked by it.

The wave entering the particle senses particle composition (e.g. colour, absorption). However, this part is

scattered into a wide range at angles, very little of which appears in the original light wave direction. In

contrast, light blockage produces a diffraction pattern that dominates the light intensity in the original

direction. This pattern is bright and it is identical to the diffraction through an aperture familiar to optical

physicists (analogous to the diffraction of waves on water surface by a jetty). When a lens gathers the

scattered plus diffracted light, diffraction shows up on the lens axis. The diffraction pattern is weaker and

wider for small particles, but tall and narrow for large particles. The width helps to distinguish particle size

while the magnitude delivers concentration.

Figure 1A shows a parallel beam of light striking a spherical particle. The light that enters the particle (and that

therefore feels its composition) exits at large angles to the original beam. It makes a very small contribution to the

very small angle scattering. Only rays diffracted around the particle appear at the small angles, producing the Airy

pattern shown on right. This is why the name: Laser Diffraction. Figure 1B shows the sampling volume.

When a number of particles are present, the intensity patterns of individual particles add. The resulting

pattern is what the Laser-diffraction instruments measure and interpret. The diffraction pattern shown in

Figure 1A is intensity versus scattering angle. For mathematical reasons, the detection of diffracted light is

done with ring-shaped detectors.

The Laser diffraction method is accurate for spheres. As the multi-angle scattering is only very slightly

sensitive to particle composition, the measurement of both the size distribution and concentration for

spheres is fundamentally assured by physics to be accurate. However, shape effects do reduce accuracy as

the diffraction pattern of non-spheres exhibits differences. The principal difference is in the width of the

main diffraction peak; the peak is broader for non-spheres. Furthermore, the minima of the diffraction

from non-spheres are less deep than for spheres. These properties are not fully understood at this time.

Until such time as these shape effects are accounted for in a fundamental way, the best possible approach

appears to be to apply an empirical calibration correction.

In-situ measurements of sediment particles and flocs are essential as natural flocs are disrupted easily by

physical manipulation such as sampling by bottles or pumps. True particle size distributions of natural

suspended sediments can only be achieved by in-situ systems. Most optical methods are potentially non-

disruptive.

Early in-situ measurements using a Laser-diffraction instrument (MALVERN) were carried out by Bale et

al. (1984) and Bale and Morris (1987).

Recent instruments (LISST) can derive the particle size distributions and also the particle volumes (volume

concentration) from the measured data with an accuracy of the order of 20% (Agrawal and Pottsmith,

Sequoia Inc., 2002). Mass concentration can only be determined by assuming a constant particle density or

by measuring the settling velocity (LISST-ST, see Figure 1C). Detailed information is given by Agrawal

and Pottsmith (2000).

Laboratory instruments based on Laser diffraction method such as the Coulter LS230 are offered by

Beckman-instruments (www.beckman.com). These laboratory instruments which are used in soil studies

have a range from 0.04 to 2000 microns with 116 fractions. For the very small particles in the range of 0.04

to 0.4 microns the Polarization Intensity Differential of Scattered Light (PIDS) is used (Buurman et al.,

1997). Another method for the very fine particle size range (0.01 to 5 microns) is photon correlation

spectoscopy (nanosizers). Progress in the engineering field makes it possible to incorparate these

sophisticated instruments in submersible devices to extend the working range of in-situ size analysis.



Various Laser diffraction instruments are manufactured by Sequoia Inc, USA (www.sequoiasci.com), which

are known as the LISST–instruments (Laser In-Situ Scattering and Transmissometery), as follows:
















compensate for effects such as convection and particle interaction (Delft Hydraulics, 2003).








angle scattering data to construct the size distribution, and then summing the concentrations in the 32 size







version, and a second version that is fully recording and presents a coarse fraction concentration in

addition to the total suspended load. The LISST-25X instrument has new comet shapes built in to separate

between wash load finer than 63 micron and the sand load larger than 63 micron. The two new comet

shapes deliver the total concentration and SMD in the entire size range, and concentration and SMD in the

coarse sand range. The comet shapes assume nothing regarding the underlying size distribution of

sediments. The only requirement is spherical shape for particles. Inaccuracies of perhaps as much as 100%

may occur if the particle composition changes from mineral to biogenic.


measurements. It incorporates a Laser, optics, multi-ring detector identical to the LISST-100, and




mounted above the body itself, and a sensor is employed to count the number of its rotations in a short




points, verticals, or in the entire stream cross-section (Gray, 2004).



The limitations of the LISST instruments can be described, as follows (see also Bale, 1996):

1. The method is accurate for spherical particles; the effect of non-spherical particle shape on the

measured size distributions is not quite clear and can only be determined by calibration.

2. The method is limited in the range of turbidities that can be measured. Multiple-scattering effects (re-

scattering of scattered light) begin to appear when the optical transmission is less than 30%. The lower

the transmission, the stronger the effects of multiple-scattering. When these effects are ignored, the

recovered size distributions show a bias toward the small sizes. Maximum concentration of fine

sediments (<50 m) is about 100 to 150 mg/l (Gartner et al., 2002) and about 500 mg/l for coarser

sediments (>50 m), (Traykovski et al, 1999). If the particle concentration is too low, the diffracted

signal is too small to detect it. Upper concentration limit can be extended by using smaller optical path

lengths (5 to 10 mm); very small path lengths may however produce relatively large shear within the

measuring volume disrupting the flocs.

3. When measuring settling velocities, there is a continual concern regarding the breakup of fragile flocs

in the process of drawing of a sample from the water column. The results of LISST-ST instrument may

be affected by disrupted flocs.

4. Flocs may not consist of solid materials; if flocs have small pores, light will be diffracted through

these pores which will contribute to the diffraction pattern in a similar way as smaller particles would

be.

5. Errors are relatively large in conditions with large amounts of very fine clay sediments (<5 m) close

the wave length of the light source; very large sand particles (>500 m) are difficult to detect as

relatively large focal lengths (300 mm) are required.

6. The instruments (due to their relatively large physical size) cannot be used very close to the bed where

most of the sediment transport takes place.

7. The instruments suffer from biological fouling in long-term deployments (stand-alone tripods). The

results of the instrument LISST-ST may also be affected by vibrations of the tripods in stormy

conditions (strong orbital motions) resulting in secondary flow patterns in the settling tube.

Advantages

1. Rapid (output at a rate of 1 Hz) in-situ determination of particle size without the need to pump, store,

transport or otherwise handle water samples; accurate results in conditions with weak currents.

2. Instrument can measure in stand-alone mode without human interference.

3. Accurate size and concentration determination of non-flocculated, spherical particles in range of 5 to

250 m (volume concentration is computed from size measurements assuming spherical particles).

4. Particle composition does not affect the particle size measurements.

Disadvantages

1. The particles of a size class may consist of solid particles and flocculated particles. To determine the

mass concentrations of each size class, the effective, submerged density ( s- w) is required which can

be determined from the fall velocity curve (LISST-ST) for each size class using the Stokes fall

velocity formula (with size class diameter and fall velocity as input values) or by calibration (to

determine volume conversion factor cv) using water samples. The mass concentration of the LISST-ST

is a mass concentration of solids and flocs including the fluid in the pores of the flocs. This value is

different from the dry mass concentration obtained by drying and weighing of the sample.

2. Maximum concentration of about 150 to 500 mg/l for particles in range of 5 to 63 m in marine

applications (expected concentration levels should be known before deployment); maximum

concentration thesholds can be larger if shorter optical path lengths are used.

3. The scatter around the initial volume concentrations of the larger size classes is relatively large due to

the small number of particles present in those size classes.

4. Errors in size distributions are greatest when there is a significant amount of material outside and close

to the extremities of the measured size ranges (<5 m and >500 m).

5. The sampling volume at end of instrument may be in the lee (wake flow) of the instrument-housing

(LISST-100) disturbing proper sampling.



6. LISST-ST in-situ measurements may suffer from sediment resuspension (instrument vibrations) in the

settling column in conditions with relatively strong currents and or waves; these problems can be

overcome by taking water-sediment samples and analysis of these samples directly after collection

using the LISST-ST.

Technical specifications LISST-100 (www.sequoiasci.com)

1. Measured parameters: size distribution in range of 1.25 to 250 micron, concentration by volume,

optical transmission, pressure in range of 0 to 200 m with resolution of 0.05 m, temperature in range of

-5 to +50 degrees with resolution of 0.1 degrees.

2. Instrument size: length of about 0.8 m (housing datalogger, electronics, power supply and Laser

source, diameter of about 0.08 m, weight of about 12 kg.

3. Two external digital I/O ports; external analog input (0-5 v).

4. Optical path length: standard 5 cm; optical transmission with 12 bit resolution; 1 milli-watt Laser

beam.

5. Resolution: 64 size classes; log spaced.

6. Maximum sample speed: 5 size distributions per second.

7. Battery life: upto 1 year.

8. Maximum depth range: 200 m.

Field and laboratory experiments

Buurman et al. (1997) have summarized the practical problems related to particle size analysis by using

Laser diffraction method. It is concluded that this method has a high reproducibility, but the method

contains a large number of pitfalls for clayey samples. Problems of sample homogeneity and subsample

representation can be solved by thorough mixing and, in some cases, by dilution of the suspension in the

sample cuvette. Loss of coarse fractions by such dilution appears to be negligible if the samples are

smaller than about 1000 microns. Flocculation of sediments causes major problems. Flocculation can (if

necessary) be eliminated by using ultrasonic vibration of the samples or by adding dispersants. Another

problem is the blocking of the optical detectors when the sediment concentration is too large. Sizes of

colloid, platy particles cannot be determined absolutely by the Laser diffraction method. Calibration with

other methods remain necessary.

Traykovski et al. (1999) have tested the LISST-ST (settling tube version) to measure the particle size

distributions and concentrations of sediment suspensions of natural sediments. Suspensions were used with

sand-sized sediments based on dry sieving methods in the range of 63 to 710 m; fine sediment

suspensions (based on wet sieving methods) in the range of 25 to 63 m and other suspensions with

sediments in the range of 5 to 25 m. The maximum concentrations were about 500 mg/l for the 25-65 m

sediment range and about 150 mg/l for the 5-25 m sediment range; larger concentrations cannot be tested

because the optical transmission becomes too low and multiple scattering is introduced leading to errors.

It was found that the LISST-ST can resolve the peak size of a uniform size distribution, and can resolve

two peaks in a bimodal distribution if they are separated by approximately 1 . The LISST-ST is not able

to resolve the sand sizes larger than about 250 m. The LISST-ST was able to correctly resolve the sizes

of fine particles in the range of 5 to 63 m, but the results become confused if particles are smaller than

about 5 m.

The LISST-ST is able to determine the mass concentration of different size distributions with a single

calibration parameter (cv-value) as long as the sediment size is in the range of 5 to 250 m, and as long as

the transmission rates are not too low. This means that the LISST-ST can only be used outside the bottom

boundary layer in marine applications, where the concentrations are relatively low (<150 mg/l) for the

finer sediments. Thus, some knowledge of the expected concentration levels is required before

deployment. This problem can be partly solved by using instruments with shorter optical path lengths (1.5,

2.5 cm in stead of 5 cm).

Gartner et al. (2001) have tested the LISST-100 in laboratory and field conditions. A laboratory setup

was designed to determine the size distribution of polystyrene particles of known size with a density of

1050 kg/m3. Most of the experiments were carried out using mono-sized particles including 5, 10, 20, 50,



100, 140 and 200 m. In addition, several broad size distributions were tested, including a 1-35 m

distribution of polystyrene particles and a 1-40 m distribution of glass beads with density of 2450 kg/m3.

The design principle of the laboratory procedure was to mimic as closely as possible the instrument

operational conditions in the field. Therefore, an important consideration is that the experimental setup

must allow the full 5 cm Laser passage through the testmedia and yet keep the required volume of test

sample of well-defined polystyrene particles to a minimum. A test chamber consisting of two adjustable

cylinders was designed to fit in the region of the LISST-100 where the sample volume is located. A

peristaltic pump was used to keep the test solution circulating between a test solution reservoir (a beaker

above a magnetic stirrer) and the test chamber. Test particle suspensions consisting of known particle size,

mass and volume of water were mixed in the reservoir to produce the required mass concentrations of

known values. A Laser Coulter Counter model LS230 was used to analyze the samples independently.

Size distributions were also provided by the manufacturer of the polystyrene materials (also based on

Coulter Counter instrument). A typical result of the tests is shown in Figure 5 for a sample with a broad

particle size distribution of 1 to 40 m. The LISST-100 tends to estimate sizes that fall between those of

the other two methods. Above about 10 m, the LISST-100 tends to show cumulative volume percent

values less than the other two methods but typically within about 10% to 15%. Differences below 5 m

may be the result of the different methods because Fraunhofer-diffraction method begins to loose

applicability due to less distinct scattered light patterns. Since the detector rings in the LISST-100 are

logarithmically-spaced (the upper size of each bin is 1.18 times the lower size), instrument resolution

becomes poorer with increasing particle size range. Traykovski et al. (1999) found that the LISST-100

tends to slightly overestimate the size of natural marine sediments tested in laboratory conditions.

Once the particle size distribution is measured, the volume concentration of suspended particles can be

estimated if a volume conversion factor (cv) is provided to post-processing software. Gartner et al. (2001)

have determined the best-fitting values of cv for each size class by minimizing the percent difference

between the known volume concentration values and the LISST-determined volume concentrations. The

cv-values are shown in Figure 6 for two LISST-100 instruments used by Gartner et al. (2001) Rather

than being independent of particle size, the cv-value varies over a factor of 3 inversely proportional to

log(size) in the range between 5 and 200 m. Furthermore, the cv-value differ between the two

instruments, although the trends are consistent. It is concluded that the cv-parameter is an experimentally

calibrated value. There is no theoretical reason to explain the observed variance of the cv-parameter.

Because the volume concentration depends on the third power of the particle diameter, any error in

particle size will produce a proportionally larger error in volume concentration. Size errors are larger for

larger particles. Hence, the volume concentration errors will be largest for the largest particles. This partly

explains the trend of decreasing cv-values with increasing particle size.

Gartner et al. (2001) have also used the LISST-100 in field conditions with fine muddy, sediment beds,

(near San Mateo bridge and Dumbarton bridge, San Francisco Bay, USA; October 1998). Results of field

data are expected to deviate from the laboratory data with the polystyrene particles because the

characteristics of naturally occurring suspended materials are substantially different in shape, structure and

density.

The LISST-100 instrument was deployed together with OBS sensors for concentration values and ADCP

sensors for current speeds. The LISST-100 was mounted at 220 cm above the bed while the OBS sensors

were mounted at 41, 71, 107 and 220 cm above the bed. The OBS at 200 cm did not function properly.

The LISST-100 was programmed to record an average of 16 scans (taking about 4 s) once every 15 min.

The OBS sensors recorded an average of 99 samples (taking less than 1 s) once every 15 min (=burst

length). The OBS sensors were calibrated using suspensions of known concentration based on bed

material samples from the field sites. This calibration procedure was not useful for the LISST-100,

because it may not fully represent the (flocculated) particle size distributions present in the water column

at the field site. An apparent dry density (of 190 kg/m3; wet density of 1140 kg/m3) related to flocculated

aggregates was used to estimate the mass concentration by LISST-100.

Mass concentrations of suspended materials at the field site based on the LISST-100 (assuming dry floc

density of 190 kg/m3 resulting in cv-factor of 6000) and the OBS are shown in Figure 7. Estimates of both

instruments correlate very well, except that the mass concentration estimates from LISST-100 are

generally missing during the time of the peak concentrations above about 100 to 150 mg/l, because the

percent optical transmission was too low.



Measured particle sizes of LISST-100 are shown in Figure 8. Size distributions near some of the

maximum flood currents cannot be determined because the optical transmission was too low

(concentration levels exceed the threshold of multiple scattering). The particle sizes vary around a mean

value of about 60 m (variation in range of 40 to 70 m). No independent measurements of the size

distribution at the field site were made. As flocculation of suspended material my produce aggregates in

the hundreds of microns in estuaries, the LISST-100 values in the range of 40 to 70 m may not be fully

representative of the entire spectrum of particle sizes at the field site. Flocs sizes in the range of 100 to 500

microns have been measured at other (nearby) sites of San Francisco Bay using other instruments.

Fugate and Friedrichs (2002) have used the LISST-100 at a field site in the lower Chesapeake Bay

(USA) with relatively low concentrations in the range of 10 to 50 mg/l (fine sediments). The measured

mass concentrations are in good agreement with pumped sample concentration values. Estimation of mass

concentrations by the volume concentrations measured by the LISST are not improved by adding grain

size distribution information, which suggests for this field site that sediment density is not a strong

function of size.

Van Wijngaarden and Roberti (2002) have tested the LISST-ST (settling tube) to determine the particle

size and fall velocities in the Hollands Diep and Haringvliet, two large fresh-water basin in the south-west

part of The Netherlands. The flow velocities at these sites are of the order of 0.1 to 0.2 m/s. The tidal range

is limited to about 0.1 to 0.2 m. Salt intrusion is absent. The measurement period was between November

8 and 22 in 1999, between April 3 and 17 in 2000 and between August 7 and 20 in 2000. During the first

deployment the settling period was taken to be 12 hours and 24 hours during the second deployment. The

LISST-ST was mounted in a stand-alone frame on the bottom of the basin. Water samples were taken at

three depths at the start of each deployment and before the pick-up of the frame.

Figure 9 shows the settling behaviour of 8 size classes (concentration versus time); a decreasing trend can

be observed. To improve the accuracy of the larger particle sizes, 8 classes in stead of 32 classes were

used. The particles of a size class may consist of solid particles and flocculated particles. The scatter

around the initial volume concentrations of the larger size classes is relatively large due to the small

number of particles present in those size classes. The fall velocity curves can be determined from these

plots. To determine the mass concentrations of each size class, the effective, submerged density ( s- w) is

required which can be determined from the fall velocity curve for each size class using the Stokes fall

velocity formula (with size class diameter and fall velocity as input values). The effective dry densities

were found to be in the range of 10 to 1000 kg/m3; about 10 kg/m3 for the larger flocs and about 1000

kg/m3 for the smaller flocs. The mass concentration of the LISST-ST can obtained from the volume

concentrations and the effective density. The mass concentration of the LISST-ST is a mass concentration

of solids and flocs including the fluid in the pores of the flocs. This value is different from the dry mass

concentration obtained by drying and weighing of the sample. Figure 10 shows the dry mass concentration

of water samples and the LISST-ST values. The real dry mass concentrations are about 5 to 6 times larger

than those of the LISST-ST.

The weighted mean particle diameters were found to be in the range of 10 to 20 m during the April

deployment, in the range of 10 to 50 m during the November deployment and in the range of 10 to 20 m

in the August period. These values are somewhat smaller than those found in earlier studies using in-situ

video recordings. The weighted mean settling velocity was found to be in the range of 0.01 to 0.04 mm/s

for the November period, in the range of 0.04 to 0.12 mm/s for the April period and in the range of 0.02 to

0.05 mm/s for the August period. These settling velocities are much smaller than those measured earlier by

the in-situ video camera recordings.

It is concluded that for the calculation of mass concentration, the combination of both a settling velocity

distribution and a mass concentration distribution is essential. Although the flocs in the upper size classes

do take up only a small fraction of the mass concentration, their high settling velocity accounts for the fact

that they dominate the total mass flux. This is however not always the case for flocs in the largest size

classes: these aggregates can be of low density and hardly settle out. The use of mean values for

concentration and settling velocity would average out such effects.

Thonon et al. (2005) have successfully used the LISST-ST (Type C) to measure in-situ particle size

distributions and settling velocities in rivers in The Netherlands. In 2002 and 2004, the LISST-ST was used in



combination with sediment traps in two river floodplains in The Netherlands: one along the unembanked

IJssel River, another along the embanked Waal River. LISST-ST can measure particle sizes between 2.5

and 500 m. However, particles smaller than 2.5 m and small particles that are aggregated into flocs

also give a signal in the class with the smallest size fraction. The model by Van Wijngaarden and

Roberti (2002) for the estimation of the settling velocities was used. To have enough data in each size

class, the 32 ring data were resampled to eight size classes with class midpoints in the range of 3 to 360 m).

The model fits the data with a bimodal settling velocity distribution. Using the fitted bimodal settling

velocity distribution, the model calculates the corresponding floc density distribution using Stokes's law. For

each particle size class, two floc densities are fitted: a higher and a lower one. For the size classes with the

smallest particles often only the lower fitted densities were physically plausible. This is explained by the

following: when Laser light falls on larger flocs, it is also partially diffracted by their smaller constituent

particles, thereby creating aliases ('ghost particles') in the smaller size fractions. These ghost particles settle

at the same rate as the larger flocs. Therefore, it seems as if a part of the smaller size fractions is falling with

a higher settling velocity. This higher settling velocity resulted in anomalously high densities for the smaller

size classes. Therefore, for these size classes the lower floc densities and settling velocities were selected.

The floc densities and their corresponding settling velocities were finally used to calculate dry mass

concentrations and mass-weighted mean grain sizes.

To check the concentration measurements of LISST-ST independently, an optical backscatter sensor

(OBS) was also used (Seapoint Sensors). The OBS measures the turbidity of the water in Formazin

Turbidity Units (FTU), which vary under ideal conditions linearly with suspended concentration (SSC).

A disadvantage of the OBS technique is its size dependency: the smaller the particles, the stronger the

signal at constant concentration. This often makes field calibration necessary. As a second check of the

LISST-ST, water samples were taken (close to the measuring frame). The sediments from the water

samples were analysed in the laboratory by using a Coulter LS 230 Laser diffraction device which can

measure particle size in the range of 0.04 to 2000 m (Beckman-Coulter, Fullerton, CA, USA). Finally,

sediment traps were mounted on the bottom close to the measuring frame. The particle size distribution

of the trapped sediments was analysed in the laboratory using the Coulter LS 230.

The results of suspended sediment concentrations (SSC) show that the SSC as measured by the

LISST-ST is much lower than the SSC of the water samples in some cases, which is concluded to be

caused by improper (non-isokinetic) sampling of the LISST-ST instrument.

Figure 11 shows particle size distributions based on LISST-ST method and on the Coulter LS230 for

the water samples and sediment trap samples. The data only represents particle size fractions larger than

2.5 m, since the LISST cannot measure the particle sizes smaller than 2.5 m accurately. It can be

concluded that the LISST yields significant smaller fractions in the size classes < 13 m and larger

fractions for particles > 25 m. It should be noted however that the LISST-data are real in-situ

measurements whereas the other two methods refer to samples analysed in the laboratory (sample

transfer).

Rijkwaterstaat (2005) has used the LISST-100 and LISST-ST instruments during a field deployment

in the Dutch coastal zone of the North Sea during summer and winter periods in 2003 and 2004. The

LISST-100 instrument was found to be a robust and well-suited instrument for prolonged deployment.

The fouling problem is similar to that of other instruments; in summer the biofouling effects start to

occur after approx. 2 weeks. Even in this period, obvious changes in grain size distribution caused by

wave events could still be detected.

During the winter deployment, the LISST-ST instrument stopped functioning after 5 cycles. The

December storm was missed, and the successful cycles were all taken at quiet conditions. The LISST-ST

signal of the data at quiet conditions shows a small but clear tidal signal implying that the settling within

the tube is affected by the tidal current. As the instrument was only deployed for 3 days, it is not l ikely

that the problem is caused by fouling and improper closure of the lids. It is thus more likely that current-

induced vibrations cause resuspension of particles from the bottom or wall of the tube. Analysis of the data

shows rather small (unrealistic) sediment densities, which may be caused by resuspension of sediments in

the settling column due to instrument vibrations.

In the summer deployment (20 June - 24 July) heavy barnacle growth had covered all instruments by the

end of the deployment. Nevertheless, the LISST-ST instrument continued to measure for about 30



cycles and some results are useful, although it cannot be decided whether leakages occurred due to a bad

closure of the lids.

It is concluded that the LISST-ST is mechanically not sufficiently well developed and definitely not

suited for prolonged deployment in coastal conditions. The lids are fragile and the least growth of

barnacles impedes its proper functioning. The necessary cleaning procedure may have made the

instrument even more vulnerable to corrosion. Glued parts in the Perspex tube detached in the long run.

References

Agrawal, Y.C. and Pottsmith, H.C. 2002. Laser Diffraction Method: two new sediment sensors. Sequoia Inc., USA (www.sequoiasci.com)

Agrawal, Y.C. and Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observations in sediment transport. Marine Geology, Vol. 168, p. 89-114

Bale, A.J., Morris, A.W. and Howland, R.J.M, 1984. Size distribution of suspended material in the surface waters of an estuary as measured by Laser Faunhofer diffraction. In: Transfer processes in cohesive sediment systems edited by Parker and Kinsman. Plenum Publising, New york

Bale, A.J. and Morris, A.W., 1987. In-situ measurements of particle size in estuarine waters. Coastal and Shelf Science, Vol. 24, p. 253-263

Bale, A.J., 1996. In-situ Laser opticle particle sizing. Journal of Sea Research, Vol. 36, p. 31-36 Buurman, P., Pape, T. and Muggler, C.C., 1997. Laser grain size determination in soil genetic studies. 1.

practical problems. Soil Science, Vol. 162, No. 3, p. 211-228 Delft Hydraulics, 2003. Analysis of LISST-ST. Report Z3671. Delft, The Netherlands Fugate, D.C. and Friedrichs, C.T., 2002. Determining concentration and fall velocity of estuarine particle

populations using ADV, OBS and LISST. Continental Shelf Research, Vol. 22, p. 1867-1886 Gartner, J.W., Cheng, R.T., Wang, P.F. and Richter, K., 2001. Laboratory and field evaluations of the

LISST-100 instrument for suspended particle size determinations. Marine geology, Vol. 175, p. 199-219

Gray, J.R., 2004. The LISST-SL streamlined isokinetic suspended-sediment profiler. Proc. 19th Int. Symp. on River Sedimentation, Yichang, China.

Rijkswaterstaat/RIKZ, 2004. LISST-100 and LISST-ST in the Sandpit Project. Preliminary Note RIKZ/OS/2004/106W, RIKZ, The Hague, The Netherlands

Thonon, I., Roberti, J.R., Middelkoop, H., Van der Perk, M. and Burrough, P.A., 2005. In-situ measurements of sediment settling characteristics in flood plains using LISST-ST. Earth Surface Processes and Landforms, Vol. 30, p. 1327-1343

Traykovski, P., Latter, R.J. and Irish, J.D., 1999. A laboratory evaluation of the Laser in situ scattering and transmissometry instruments using natural sediments. Marine Geology, Vol. 159, p. 353-367

Van Wijngaarden, M. and Roberti, J.R., 2002. In situ measurements of settling velocity and particle size with the LISST-ST. In: Fine Sediment Dynamics in the Marine Environment. Edited by Winterwerp and Kranenburg, Elseviers Science



Figure 1A

Laser Diffraction patterns

Figure 1B

Schematic representation of the optical path and sampling volume; light scattered at the same angle from particles in different locations in the beam is focussed on to the same ring detector; unscattered and forward scattered light passes directly through a hole in the center of the ring detector to measure transmission (Traykovski et al., 1999)

Figure 1C

LISST-Settling Tube instrument with the impeller and sliding doors. Top and bottom lids are not shown. The column is 5 cm in diameter, and 30 cm tall from the inlet to the Laser beam. Right: LISST-ST mounted on a tripod (Agrawal and Pottsmith, 2000).



Figure 2

LISST 100 instrument; particles in the flow scatter light; a receiving lens collects the scattered light, which is detected by the ring collector; a hole in the center of the ring detector permits the focused Laser beam to pass through, where its power is sensed; this constitutes a transmission measurement, which corrects for attenuation of the scattered light that is sensed by the rings.

Figure 3

The use of shaped focal plane detector in LISST-25.

Figure 4

The LISST-SL instrument



Figure 5

Comparison of particle size distributions of polystyrene material in range of 1 to 40 micron based on three methods (LISST-100; Laser Coulter Counter LS230 and Coulter Counter analysis of supplier Duke Scientific Inc.), (Gartner et al., 2001)

Figure 6

Calibration constants determined in the laboratory using polystyrene spheres plotted against particle size; results from two LISST- instruments from USGS (US Geological Survey and SPAWAR Space and Naval Warfare Systems Center), (Gartner et al., 2001)



Figure 7

Time-series plots of suspended sediment concentration estimated by the LISST-100 and the OBS-instruments; LISST values are truncated above about 100 to 150 mg/l when concentration levels resulted in percent transmission less than 20% (Gartner et al., 2001)



Figure 8

Time series plots (10/19/1998 to 10/23/1998) of: A: water levels B: current speeds C: suspended concentrations by OBS D: mean particle size of suspended sediments; overall mean is 62 micron E: percent suspended material of 20 m size class (open circles) and percent of 100 m size class (filled circles) F: ratio percent of 20 m size class to percent of 100 m size class



Figure 9

Volume concentration as a function of time for 8 size classes, August 2000 * data points ____ model fit (+ deviation between model fit and data points)

Figure 10

Calibration curve of dry mass concentration from water samples and LISST-ST values



0

5

10

15

20

25

30

35

40

3.5 6.8 13.1 25.4 49.2 95.5 185 359

Grain size class [µm]

Fra

ction o

f to

tal G

SD

[%

LISST-ST

Sediment traps

Water samples

Figure 11

Particle size distribution for floodplain of IJssel River in The Netherlands (SW dataset)



6.5.6 In-situ photo and video camera

Principle

Eisma et al. (1990) has described the development of an in-situ photocamera (and image-analysis software)

for in-situ measurement of solid particles and aggregates (flocs) larger 4 m. It can be used in depths up to

4000 m with concentrations up to 200 mg/l. In very clear ocean waters the system is not effcient because of

the large number of photographs that have to be taken to obtain a reliable size distribution.

The camera system consists of a steel frame (1.8 x 2 m) in which 3 cameras are mounted in such a way that

there is a minimum disturbance of the water flow through the frame. A vane keeps the camera windows

approximately parallel to the main flow sothat the particles are photographed approximately perpendicular to

their direction of movement. Two cameras are each directed horizontally towards a flash light mounted

opposite the camera at a distance of 32 mm. A third camera is mounted above the other two cameras with a

flashlight oblique directed at an angle of 60 degrees towards the view-area of the camera. This camera is

used to obtain photographs of the larger particles and other materials that may be present in the water. The

cameras are professional NIKON F3 cameras with a 250-exposure magazine. The lenses are 35 and 50 mm

(f/1.4). Teflon beads of known size have been used to calibrate the photographic results. The camera system

produces a large number of photographs, which are analyzed by an automated particle image-analysis

system.

Figure 2 shows a photograph of flocculated sediments in the Scheldt estuary, The Netherlands

Van Leussen and Cornelisse (1991, 1993) and Fennessy et al. (1994) have developed in-situ video camera

instruments which can determine both the size and the settling velocity of the solid particles and the

flocculated sediments.

The in-situ video camera (VIS, see Figure 3) of Van Leussen and Cornelisse consists of a small vertical tube

with a closed end at the bottom in which particles are settling down in still water. Two small windows are

present in the tube for enlighting (light beam) and for video-recordings. The instrument is connected by a

signal cable to the survey ship which floats with the current during sampling. Floc sizes and settling

velocities are obtained from the recordings by computer analysis. Figure 4 shows measured settling

velocities through the tidal cycle in the Ems-Dollard estuary (Van Leussen, 1994). It can be observed that

the settling velocities measured by the VIS are significantly larger than those obtained from a mechanical in-

situ settling tube (Owen tube). This is caused by two effects: 1) the in-situ video camera VIS has a lower

limit of about 50 m, the finer particles are not detected and 2) the flocs trapped in the mechanical settling

tube may have been disrupted during the sampling process. Van der Lee (2000) has succesfully used the in-

situ camera in the Dollard estuary, The Netherlands. He has also assessed the erros involved in the

determination of the floc sizes and settling velocities (from consecutive images at intervals of 0.08 s). The

lower detection limit of the VIS was determined to be about 80 m. Analysis of some enlarged images from

a second camera shows that about 10% to 20% of the (smaller) flocs cannot be detected. Hence, there is an

overestimation of the size of the flocs by VIS. Determination of the settling velocity of the flocs is

complicated by the presence of vertical water motions in the settling tube of the VIS. Van der Lee estimated

the water velocity to be equal to the observed settling velocity of the smallest 5% of the flocs. This value was

subracted from all measured results. On average, the relative errors in floc size and settling velocity were

estimated to be in the range of 10% to 20%. The results are biased to the largest flocs with the largest settling

velocity. This may not be too serious, as the larger flocs are most relevant for sedimentation processes.

Fennessy et al. (1994) and Manning and Dyer (2002) have developed an in-situ settling velocity instrument

(INSSEV) based on video camera recordings. The instrument comprises a computer controlled chamber

(decelerator) with closing doors to slowly collect a sample of water and sediments, from which some of the

suspended materials is allowed to enter the top of a settling tube (settling length of 110 mm). The settling

flocs are viewed using a miniature video system, see Figure 5. Subsequent analysis of video tapes can

provide direct observations of the size and settling velocity of individual flocs in the size ranges of 20 to

1000 m. The video recordings show large numbers of low density flocs with multiple structures linked by

fine organic fibres. To ensure stability and remove vertical motions during the settling process, the

instrument is to be attached to a bed-mounted tripod. The camera system uses high resolution videocamera

and six high-intensity red LED’s for illumination of the flocs arranged in an annulus around the camera lens.



Figure 6 shows time series of floc sizes and settling velocities based on INSSEV observations (Dyer et al.,

2002) at 0.5 m above the bed during spring tide in the Tamar estuary (UK). Micro (<150 m) and macro

(>150 m) flocs are separately shown. The concentrations are as large as 3000 mg/l. The mean floc diameter

increase from about 100 to 200 m and the settling velocities increase with size to about 2 mm/s. The

effective density ( s- w) decreases from 330 to about 150 kg/m3.

Dyer et al. (1996) have tested two in-situ video systems (VIS and INSSEV) and various mechanical in-situ

settling tubes in an intercomparison experiment carried out in the turbidity maximum in the mouth of the

Elbe Estuary in Germany (water depth of about 18 m; peak velocities of about 1 to 1.5 m/s). The mechanical

settling tubes generally produce settling velocities which are an order of magnitude smaller than the direct

video measurements which is an indication that the mechanical tubes disrupt the flocs on sampling. The

video systems VIS and INSSEV appear to give comparable results. The settling processes in one of the

mechanical tubes (OWEN tube) were studied by using a high magnification video camera (Dearnaley,

1996). These results reveal floc breakup, reflocculation and generation of circulation within the tube during

and after sample withdrawal. The settling velocites inside the tube based on video recordings are larger than

those derived from the standard gravimetric method from the same tube. The settling velocities of the video

are similar to those of the VIS and INSSEV.

Advantages

1. simultaneous measurement of particle size and fall velocity

2. reliable information of floc sizes

3. statistical information of particle size variations and fall velocity variations

4. no destruction of samples during measuring period

Disadvantages

1. very labour-intensive analysis of data;

2. automatic analysis software may not be sufficiently accurate (verification required)

3. can not be used in stand-alone mode

4. relatively large equipment; cannot be used close to bed

References

Dearnaley, M.P., 1996. Direct measurements of the settling velocities in the Owen tube: a comparison with gravimetric analysis. Journal of Sea Research, Vol. 36, p. 41-47

Dyer, K.R. et al., 1996. A comparison of in situ techniques for estuarine floc settling velocity measurements. Journal of Sea Research, Vol.36 (1/2), p. 15-29

Dyer, K.R. et al., 2002. The turbidity maximum in a mesotidal estuary, the Tamar Estuary, UK: Part II, The floc properties. In: Fine Sediment Dynamics in the Marine Environment edited by Winterwerp and Kranenburg, Elseviers Science

Eisma, D. et al., 1990. A camera and image-analysis system for in-situ observation of flocs in natural waters. Netherlands Journal of sea resaerch, Vol. 27, No. 1, p. 43-56

Fennessy, M.J., Dyer, K.R. and Huntley, D.A., 1994. INSSEV: an instrument to measure the size and settling velocity of flocs in situ. Marine Geology Vol. 117, p. 107-117

Manning, A.J. and Dyer, K.R., 2002. The use of optics for the in-situ determination of flocculated mud characteristics. Journal of Optics A: Pure and Applied Optics, Vol. 4, p. S71-S81

Van der Lee, W.T.B., 2000. The settling of mud flocs in the Dollard estuary, The Netherlands. Doctoral Thesis. Dep. of Physical Geography, University of Utrecht, Utrecht, The Netherlands

Van Leussen, W., 1994. Estuarine macroflocs and their role in fine-grained sediment transport. Doctoral Thesis. Dep. of Physical Geography, University of Utrecht, Utrecht, The Netherlands

Van Leussen, W. and Cornelisse, J.M., 1991. Direct measurements of sizes and settling velocities of mud flocs in the Ems estuary. Paper WGSL 18/91, Delft Hydraulics, Delft, The Netherlands

Van Leussen, W. and Cornelisse, J.M., 1993. The determination of sizes and settling velocities of estuarine flocs by an underwater video system. Netherlands Journal of sea research, Vol. 31, p. 231-241



Figure 1

In-situ photocamera (Eisma, et al., 1990)

Figure 2

Photograph image of flocculated material in Scheldt estuary, (Eisma, 1990)



Figure 3

In-situ video camera VIS (Van der Lee, 2000)

Figure 4

Settling velocities through the tidal cycle by VIS in Ems-Dollard estuary (Van Leussen and Cornelisse, 1993)



Figure 5

INSSEV video camera system for flocs and settling velocity of flocs (Fennessy et al., 1994; Manning and Dyer, 2002)



Figure 6

Time series of floc sizes and settling velocities based on INSSEV observations (Dyer et al., 2002) Top: Settling velocity (ws), sediment concentration (SPM) and floc size Middle: Effective density and bed-shear stress parameter (G) Bottom: sediment concentration (SPM) and bed-shear stress parameter (G)



6.5.7 Particle size and velocity by Phase Doppler Anemometry (PDA)

PDA is an extension of Laser Doppler anemometry (LDA) and can determine not only the Doppler

shift frequency of light refracted by a particle within the flow (hence its velocity) but also the phase shift

as observed at three different receiving locations which can be utilized to derive the diameter of the

scattering particle. Assuming constant density and spherical particles, the volume concentration can be

determined. Hence, the simultaneous measurement of particle size, velocity and concentration can be

obtained using phase Doppler anemometry as an extension of the principles of LDA (Buchhave, 1987;

Bachalo, 1994). As a transparent particle passes through the measurement volume, the interference fringes

are refracted by an amount proportional to the diameter of the grain. Three detectors, located at different

positions within the receiving optics, will observe the same frequency but with a relative phase shift

proportional to the detector spacing divided by the projected fringe spacing. The separation of the

projected fringes at a large distance from the measuring volume is proportional to the effective focal

length of the particle, which, for a spherical particle, is proportional to the particle diameter. Hence the

measured phase shift is proportional to the particle diameter.

A commercial instrument is provided by DANTEC (www.dantecdynamics.com).

Laboratory experiments

Bennet and Best (1995) have used the PDA method (two-component DANTEC 100 mW argon-ion laser

with fibre-optics operated in side-forward scatter mode) to determine its potential for measuring

suspended sediment transport. The flow in a small perspex container with an oscillating grid was

analyzed using a DANTEC Flow Velocity Analyzer processor (correlation-type), 40 MHz frequency

shift, and a 600 mm focal length lens with a beam separation distance of 0.025 m at the lens surface. In

the processing software, the maximum phase error was set at 15 degrees and the maximum spherical

deviation was set at 35 percent. With this configuration, typical uncertainty estimates were 1 percent for

the velocity measurements (with a resolution of 0.006 m/s) and 4 percent for the particle size

measurements (with a resolution of 2 m in the range of 5 to 500 m). The Laser intersection volume

was located within 10-15 mm from a grid bar intersection. At this fixed coordinate, vertical velocity

profiles (one component only) were taken through the container to determine the vertical fluid and

sediment velocities and suspended sediment fluxes as a function of both height away from the grid and

oscillation frequency. A 300 second sample was taken at heights above the grid of 5, 10, 15, 20, 25, 30,

35, 40, 45, 50, 60, 70, 80, 100, 120, and 150 mm. Typical sampling rates varied from 30 to 1000 Hz

(normally 400 Hz) depending upon the chosen frequency and height above the grid.

The instrument was operated from outside the experimental container, which consisted of a perspex

container (0.254 x 0.254 x 0.45 m high) with a grid of square bars, 0.01 m wide, 0.22 m long and spaced

0.05 m apart, located near and parallel to the base of the container. This grid was fixed by a central arm to

an eccentric drive which provided an oscillation amplitude (perpendicular to the base of the container) of

approximately 0.019 m. The center of the grid was maintained at a constant mean height above the base

in all experiments. Grid oscillation frequency was controlled by a voltage regulator and four frequencies

(in range of 1 to 3.4 Hz) were chosen so that each sediment population within a particular experiment

remained immobile at the lowest frequency. The entire apparatus was mounted to a baseboard, which had

a scissor jack attached at each corner. This configuration allowed the container and its controls to be

raised and lowered with an accuracy of ±0.5 mm using millimeter-graduated scales fixed to the sides of

the base, thus permitting acquisition of vertical velocity profiles without requiring movement of the Laser

optics. The mixing container was filled with unfiltered tap water to a depth of 0.25 m.

The sediment grains used were glass beads (density of 2600 kg/m3 with a refractive index of 1.5),

chosen because: (1) their shape is close to spherical and they are of quartz density; (2) their

transparency allows the PDA to be used in refraction mode; and (3) large quantities sufficient for mobile

bed flume experiments can be purchased at moderate cost. Three size distributions of glass beads were

used with nominal sieved diameters of 75-95 m (fine), 180-210 m (medium), and 300-355 m

(coarse). The total dry mass of sediment (concentration) added to the container for the fine, medium,

and coarse populations was 0.31 g/1, 2.48 g/1 and 6.2 g/l, respectively. Each sediment population was

initially spread evenly across the base of the box. In most experiments, the sediment supply available for

suspension was not exhausted and therefore the flow could reach an equilibrium capacity, although no

particles remained on the base of the box at the highest oscillation frequencies using the fine sediment.



Figure 1 shows the frequency distributions for all PDA measured grain diameters for each sediment

population at two different oscillation frequencies at a fixed height of 5 mm within the container.

The majority of grain sizes observed within the pure fluid experiments (>90 percent) were less than

50 m, with 85 percent less than 30 m. These measured grain sizes represent colloidal and natural

particulate matter in the water. The frequency distributions for the fine sediment population show peaks

at 50-70 m and 60-90 m for the low and high oscillation frequencies, respectively. At the lower

frequency (1.9 Hz), the signal illustrates the equal detection of fluid particles and sediment grains,

while at the higher frequency (3.4 Hz) the signal is completely dominated by the sediment particles in

suspension. For the two other sediment populations used, few sediment grain sizes are observed at the

lower frequency whereas appropriate peaks are observed at 200-230 m for the medium sediment and

at 310-340 m for the coarse sediment at the higher frequency. It is apparent that, for a given range of

grain sizes introduced into the container, discrimination can easily be made between the particles

representing the fluid (<30 m) and the sediment particles suspended in the flow. However, a larger

range of grain sizes is observed by the PDA when compared to the sieved grain size range. The range

of grain sizes measured by the PDA varies from about 25-125 m for the fine sediment, 100-300 m

for the medium sediment, and 200-400 m for the coarse sediment.

Best et al. (1997) and Bennett et al. (1998) discuss the errors involved using the PDA instrument in flume

experiments (see Figure 2), which arise from:

variable sample size and concentration between different measurement locations;

cross talk between the two phases, in which fluid and particles may be incorrectly classified, may be

caused by errors in grain sizing caused by asphericity of sediment and contamination of the flow;

positioning and relocation of the measurement volume.

Particle sizing errors were also assessed through comparison of the PDA-derived grain size-distribution

with that obtained through Coulter Counter analysis, yielding relative errors smaller than 3% for sand-

sized particles. Errors in concentration are relatively large (DANTEC, 1994), especially at higher

concentrations (>1 gr/l) where signal attenuation becomes problematic. Figure 3 shows good agreement of

PDA concentrations and siphon sampler concentrations in the upper half of the flow where concentrations

are relatively small (<1 g/l). The PDA concentrations become progressively less than the siphon

concentrations as the bed is approached. The PDA method is not capable of recording every sediment

grain that passes through the measurement volume when sediment concentrations in the flow are high.

Advantages

1. rapid and non-intrusive determination of simultaneous particle velocity, size (5 to 500 m) and

concentration;

2. fast response enabling turbulence measurements.

Disadvantages

1. calibration required for non-spherical particles;

2. not usable in high-concentration flows (>1 g/l);

3. laboratory instrument; problematic in field conditions.

References

Bachalo, W.D., 1994. Experimental methods in multi-phase flows. International Journal of Multi-Phase Flows. Vol. 20, Suppl., p. 261-295

Bennett, S.J. and Best, J.L., 1995. Particle size and velocity discrimination in a sediment laden turbulent flow using Phase Doppler Anemometry. Journal of Fluids engineering, Vol. 117, p. 505-511

Bennett, S.J., Bridge, J.S. and Best, J.L., 1998. Fluid and sediment dynamics of upper stage plane beds. Journal of Geophysical Research, Vol. 103, No. C1, p. 1239-1274

Best, J., Bennett, S., Bridge, J. and Leeder, M., 1997. Turbulence modulation and particle velocities over flat sand beds at low transport rates. Journal of Hydraulic Engineering, Vol. 123, No. 12, p. 1118-1125

Buchhave, P., 1987. A new instrument for the simultaneous measurement of size and velocity of spherical particles based on the Laser Doppler method. Dantec Electronics, New Yersey, USA



Figure 1

Grain size frequency distributions for all sediment populations at a height of 5 mm above the grid for oscillating frequencies of 1.9 and 3.4 HZ (bin size of 10 m), (Bennett and Best, 1995)

Figure 2

Experimental set-up in laboratory flume (Bennett et al., 1998)

Figure 3

Measured concentration profiles uding PDA method and Siphon samples (Bennett et al., 1998)



6.5.8 Particle size by Laser Reflectance (PARTEC Laser)

In-situ Laser diffraction techniques are severely limited in their use by the presence of high sediment

concentrations larger than about 0.5 to 1 g/l. This limitation can be overcome by sing in-situ Laser

reflectance techniques (Law et al. 1997), see Figures 1 and 2. The PARTEC 100 is a commercially

available, Laser reflectance particle-sizing instrument which was initially designed for process control in the

grinding and milling industries with concentrations in the range of 10 to 100 g/l. The sensor is computer-

operated and the output of the PARTEC 100 consists of a histogram of 38 logarithmic size intervals over the

size range 2 to 1000 m.

The measuring principle employs an optical beam which is directed through a lens located eccentrically on a

rotating disc within the reflectance probe such that the focal point describes circles of 8.4 mm in diameter.

The light source is a semi-conducting Laser diode. As the focal point is typically smaller than the suspended

particles and moving with a greater velocity, reflected light signals are assumed to be related to individual

particles. When the sensor probe is immersed in a sample, measurements of reflected pulses are accumulated

for a set period, typically 3 to 25 s depending upon particle numbers, and a particle chord size distribution is

calculated. A correction algorithm, which assumes the particles are spheres, allows a distribution of spherical

equivalent diameters to be calculated. Using this data, the system software generates size distributions which

may be presented as a percentage of either the total number of particles or the total volume of particles

encountered. The measuring zone is confined to a small volume in the vicinity of the focal point by the use

of an electro-optical discrimination procedure based on the rise and decay times of the reflected signals.

Individual particles are only registered if the leading and trailing edges of their reflected signal have a

sufficiently sharp or rapid rise from background noise. Multiple reflections are ignored. If both signal edges

possess a sufficiently fast rate of change, the measured duration between the two points is added to the

sample data. These are direct measurements, which avoid signal smoothing or curve fitting techniques. The

high power of the focused PARTEC 100 Laser beam allows the instrument to operate in conditions with

high particle concentrations.

To facilitate in-situ measurements in water depths up to 100 m, the probe unit is mounted in a watertight

cylinder made from PVC plastic.


Law et al. (1997) have evaluated the PARTEC 100 under laboratory conditions using commercially

available calibration particles which include Pollen (with mean diameters of 7.4, 29.7 and 78 m), latex (19

m) and glass beads (8, 31.5 and 85.7 m). A number of secondary standards (mean diamaters of 6.5, 23.6,

39.5, 89.4 and 171.7 m) were produced by fractionating ashed sediment using settling time criteria.

Samples of large particles were obtained by sieving sand to produce various factions (up to 766 m). The

size distributions of the secondary standards were determined using electro-resistive (Coulter Counter) and

Laser diffraction (Malvern) instruments for the finer materials and optical microscopy combined with an

image-digitising package for the large grains.

Focused beam reflectance measurements of these standards were carried out with the sensor probe immersed

in a suspension of the particles contained within a glass beaker. During each series of measurements the

particles were kept in suspension using a variable speed, electrically driven impeller. The influence of

particle concentration was studied by using suspensions in the range of 10 to 50,000 mg/l.

For all types of sediments the size distributions compared well with those obtained from alternative systems.

Analysis of materials such as latex and glass gave erroneous results due to insufficient reflectance.

Furthermore, the PARTEC 100 progressively oversized particles below 150 m with increasing errors for

decreasing sizes (up to 30%), whilst undersizing particles above 500 m (up to 10%). These systematic

deviations can be eliminated by calibration. Measurements of particle size distributions at a number of

concentrations indicate the presence of a slight trend towards larger mean sizes with increasing

concentrations (about 30% increase in mean size for concentrations between 100 and 50,000 mg/l), which is

caused by the inability of the instrument to resolve separate particles at extremely high concentrations. At all

concentrations a minimum of about 1000 reflected counts is needed to be registered to obtain statistically

reliable results. This means that the size distribution of 10 mg/l with particles of 5 to 10 m can be analysed

in about 3 minutes, whereas 10 mg/l with sand sized particles would require about 30 minutes. The particle

velocity was not found to have any effect on the size distribution. It is concluded that estuarine particles in



the size range of 20 m and larger can be measured with some confidence using a fixed focal distance of 0.8

mm. If the sample consists primarily of particles smaller than 20 m, a focal distance setting nearer to 0.2

mm is required.

Law et al. (1997) have also performed field trials using the PARTEC100 Laser reflectance technique in the

Tamar and Humber estuaries (UK). Particle size measurements were taken at 1 m intervals from the surface

throughout the water column, taking typically 2 to 5 minutes at each depth, dependent upon particle

concentration (based on OBS method). The results confirmed the presence of large flocs in the range of 50 to

500 m. The method cannot be used in conditions with relatively large organic materials (insufficient

reflectance).


200/300) have shown that in-situ determination of particle size distributions of fluvial sediments is of


samples and measuring the particle sizes directly after sampling. On site measurements of bottle samples

were broadly similar to direct in-situ measured size distributions. Analysis results of water-sediment

samples collected in bottles and returned to the laboratory show significant differences in particle size

distributions due to floccutation of sediments in the bottle samples, even if the sediments in the bottle are

artificially resuspended. In general the longer a sample was allowed to settle the greater the increase in

volume mean size upon resuspension. The bonding of flocs formed during the settling period appears to

become stronger with time.

Advantages

1. rapid in-situ analysis of particle size distributions

2. usable in conditions with relatively large concentrations (>0.5 gr/l)

3. very accurate for sand particles larger than 30 m

Disadvantages

1. results based on assumption of spherical particles; finer particles are oversized, larger particles are

undersized; calibration is required to reduce errors

2. particles should have sufficiently high reflectance; flocs with organic materials cannot be detected

3. different focal distances are required for finer and coarser particle ranges

4. long counting times in case of low concentrations

References

Law, D.J., Bale, A.J. and Jones, S.E., 1997. Adaptation of focused beam reflectance measurements to in-situ particle sizing in estuaries and coastal waters. Marine Geology Vol. 140, p. 47-59

Phillips, J.M. and Walling, D.E., 1995. An assessment of the effects of sample collection, storage and resuspension on the representativeness of measurements of the effective particle size distribution of fluvial sediment. Water resources, Vol. 29, No. 11, p. 2498-2508



Figure 1

Details of the Laser probe head and measuring geometry (Phillips and Walling, 1995)

Figure 2

a) comparison of the volumetric size distribution for 447.6 m sand measured by PARTEC 100 (open circles; upper curve) and optical microscopy (open diamonds; lower curve); b) comparison of mean particle sizes by PARTEC100 (on vertical axis) and other methods (on horizontal axis) for a range of standard materials (Law et al., 1997) diamonds=Pollen circles= Ashed sediments squares=Sands triangles= Algal cells Open symbols represent pre-calibrated results; closed symbols represent calibrated values.

CHAPTER 6: INSTRUMENTS FOR PARTICLE SIZE AND FALL VELOCITY January 2006


6.1 General aspects

Particle size information is of essential importance for the estimation of erosion, transport and deposition

rates.

As the sizes of sediment particles vary over extremely wide ranges, sediment particles are therefore

measured in very large numbers and grouped into specific, but arbitrary size classes according to various

analysis methods and definitions (see Figures 1 and 2). Sediment particles not only vary widely with respect

to size, but also with respect to specific weight and shape. Therefore, different particles of a given physical

size will behave different in the hydraulic environment as though they are larger or smaller, depending on

how their shape and specific weight vary from the defined size class.

Because of the wide range of particle characteristics, particle size usually needs to be defined in terms of the

method of analysis. Large sizes including boulders and cobbles can be measured directly by immersion and

weighing. Intermediate sizes of gravel and sand are measured semi-directly by sieving resulting in sieve

diameters. Small sizes of silts and clays are measured hydraulically by sedimentation or settling methods

resulting in the particle fall velocity and the standard fall diameter (Figure 1). The relationship between the

median sieve diameter" and the standard fall diameter is a measure of the effect of shape, roughness and

specific gravity on the settling velocity of a particle.

This leads to the fact that there are essentially two types of measurements:

· size- or volume-measurements

· fall velocity measurements (sedimentation method).

The size- or volume-measurements include the determination of the:

· diameter by means of photographs, sieves or the diffraction of coherent light beams (Laser

granulometer);

· volume by means of immersion or conductivity (Coulter Counter).

The fall velocity measurements, usually, consists of the determination of sediment accumulation as a

function of time using a:

· dispersed suspension for silt particles (pipet-withdrawal tube, bottom-withdrawal tube, balance-

accumulation tube);

· stratified suspension for sand particles (visual accumulation tube, manual accumulation tube, balance

accumulation tube).

In a dispersed suspension the (silt) particles begin to settle from an initially uniform suspension. The lower

size limit is approximately 5 µm because the settling of smaller particles is hindered by the Brownian

motion.

In a stratified suspension the particles start from a common source at the upper end of the tube and become

stratified according to their settling velocities. As settling medium, water is generally prefered because it is

the universal environment of (fluvial and marine) sediments. Greatest consideration must be given to the

effect of proximity of particles to each other. Concentrations larger than 5000 mg/1 result in hindered settling

and hence smaller settling velocities than for individual particles. It is also conceivable that a group of

particles creates a region of high concentration and will act as one "large" particle resulting in a relatively

large settling velocity. It may be noted that a sample containing silt, clay and coarse material will require

analysis by two or more methods because of the limitation on the range of sizes that can be analyzed by each

specific method. Because suspended sediment samples often contain a very small quantity of particles, it is

only feasible to analyze those samples obtained during periods of relatively high sediment concentrations or

those samples which consist of accumulated single samples collected under similar flow conditions. On the

other hand, some streams may at times contain rather large concentrations so that a single sample must be

splitted to obtain the optimum quantity for size analysis. A detailed description of the measuring instruments

is given in Paragraph 6.5.

6.1.1 In-situ sampling



Suspended sediment particles in estuaries and coastal seas generally consist of solid and aggregated (flocs)

materials with densities as low as 1050 kg/m3. Particle surfaces may be coated with absorbed humuc

molecules.

In-situ measurements of sediment particles and flocs in these conditions is essential as natural flocs are

disrupted easily by physical manipulation such as sampling by bottles or pumps. True particle size

distributions of natural suspended sediments can only be achieved by in-situ systems. Most optical particle

size methods are potentially non-disruptive.

Eisma et al. (1991) have used an in-situ photocamera to measure size distributions of suspended sediment in

various West-European estuaries. In addition, they have also determined the size distributions of sediment

samples collected in bottles using the traditional pipette analysis method (sedimentation method) and the

Coulter Counter method. The bottle samples were either quickly brought back to the laboratory on land or

were analysed several hours later when the survey ship was at anchor with the engines off (as the pipette

method is sensitive to mechanical vibrations and temperature-induced circulations in the sample). The

analysis results reveal no relation between the in-situ size distributions (based on photocamera method) and

the size distributions from the bottle samples (pipette or Coulter Counter method). The maximum size of the

in-situ sediments was about 800 mm and the maximum size of the bottle sediments was a bout 125 mm. Both

the pipette and Coulter Counter analyis methods were performed on suspended samples that were sampled

and brought to the laboratory. During sample analysis in the laboratory the original flocs were disrupted so

that actually the size of the individual solids and/or floc fragments were measured. Both methods (pipette

and Coulter Counter methods) gave similar but erroneous results. The results also depended on the way the

samples were treated and stored before analysis.


200/300) have also shown that in-situ determination of particle size distributions of fluvial sediments is of


samples and measuring the particle sizes directly after sampling. On-site measurements (immediately after

sample collection) of bottle samples were broadly similar to direct in-situ measured size distributions.

Analysis results of water-sediment samples collected in bottles and returned to the laboratory showed

significant differences in particle size distributions due to floccutation of sediments in the bottle samples,

even if the sediments in the bottle were artificially resuspended. In general the longer a sample was

allowed to settle the greater the increase in volume mean size upon resuspension. The bonding of flocs

formed during the settling period appears to become stronger with time.

6.1.2 Formulae particle fall velocity

The particle size and the fall velocity can be related to each other by the following formulae:

ws=(1/18n)((s-1)g D2) for particles in range of 1 to 100 mm (1)

ws=(10n/D)[(1+(0.01(s-1)gD3/n2))0.5 -1] for particles in range of 100 to 1000 mm (2)

ws=(1.1)((s-1)g D)0.5 for particles larger than 1000 mm (3)

with: ws= settling velocity (m/s), D= particle diameter (m), s=rs/r=relative density (rs =2650 kg/m3),

n=kinematic viscosity coefficient (m2/s).

The viscosity coefficient can be determined as:

n=[1.14-0.031(T-15)+0.00068(T-15)2]10-6 (4)

with: T=water temperature in Celsius.

Figures 3 and 4 present graphs for the settling (fall) velocity and viscosity coefficient.



6.1.3 Definitions of sediment sizes

The nominal diameter of a particle is the diameter of a sphere that has the same volume as the particle.

The sieve diameter of a particle is the diameter of a sphere equal to the length of the side of a square sieve

opening through which the given particle will just pass.

The standard fall velocity of a particle is the average rate of fall that the particle would attain if falling

alone in quiescent, distilled water of infinite extent and at a temperature of 24°C.

The standard fall diameter, or simply fall diameter, of a particle is the diameter of a sphere that has a

specific gravity of 2.65 and has the same standard fall velocity as the particle.

The sedimentation diameter of a particle is the diameter of a sphere that has the same specific gravity and

terminal uniform settling velocity as the given particle in the same sedimentation fluid.

The standard sedimentation diameter of a particle is the diameter of a sphere that has the same specific

gravity and has the same standard fall velocity as the given particle.

The size distribution, or simple distribution, when applied in relation to any of the size concepts, is the

distribution of material by percentages of proportions by weight.

References

Eisma, D. et al., 1991. Suspended matter particle size in some west-european estuaries; part I: particle size-distribution. Netherlands Journal of Sea Research, Vol. 28, No. 3, p. 193-214

Phillips, J.M. and Walling, D.E., 1995. An assessment of the effects of sample collection, storage andresuspension on the representativeness of measurements of the effective particle size distribution offluvial sediment. Water resources, Vol. 29, N0. 11, p. 2498-2508



Class Name Millimeters Micrometers Phi Value*

Phi= -2log(D)

Boulders >256 <-8

Cobbles 256 - 64 -8 to -6Gravel 64 - 2 -6 to -1

Very coarse sand 2.0 - 1.0 2,000 - 1,000 -1 to 0

Coarse sand 1.0 - 0.50 1,000 - 500 0 to +1Medium sand 0.50 - 0.25 500 - 250 +1 to +2Fine sand 0.25 - 0.125 250 - 125 +2 to +3Very fine sand 0.125 - 0.062 125 - 62 +3 to +4

Coarse silt 0.062 - 0.031 62 - 31 +4 to +5

Medium silt 0.031 - 0.016 31 - 16 +5 to +6Fine silt 0.016 - 0.008 16 - 8 +6 to +7Very fine silt 0.008 - 0.004 8 - 4 +7 to +8

Coarse clay 0.004 - 0.0020 4 - 2 +8 to +9

Medium clay 0.0020 - 0.0010 2 - 1 +9 to +10Fine clay 0.0010 - 0.0005 1 - 0.5 +10 to +11Very fine clay 0.0005 - 0.00024 0.5 - 0.24 +11 to +12Colloids <0.0024 <0.24 >+12

Figure 1

Definitions and size scale



Sediment characteristics Strength Structure

compact

ness

Field test Term Field indentification

coarse

grained

uniform boulders

cobbles

gravel

hard can be excavated with spade; 2

inch wooded peg can easily be

driven in



sands compact Require pick for excavation

non-

cohesive

graded slightly

cemented

Visual examination. Pick removes

soil in lumps

stratified alternatively layers of

varying types

low plasticity silts soft Easily moulded in fingers

Particles mostly barely or not

visible; dries moderately




pressure in fingers

alternatively layers of

varying types

fine

grained

medium

plasticity

very soft Exudes between fingers when

squeezed in first

fissured breaks into polyhedral

fragments along

fissure planes

cohesive high plasticity clays soft Easily moulded in fingers intact no fissures


pressure in fingers

(dry lumps can be broken, but not

powdered; disintegrates under

water; sticks to fingers; dries

slowly with cracks

homogeneous

stratified

deposit consisting


alternating layers of

varying types if layers

are thin, the soil may

be described as

laminated

stiff Cannot be moulded in fingers

hard Brittle or very tough weathered usually exhibit

scrumbs or columner

structure

organic peats firm Fibre compressed together; colour

brown to black

spongy Very compressible and open

structure, colour brown to black

Figure 2

Sediments and structural characteristics



Figure 3



Particle fall velocity for silt particles and kinematic viscosity coefficient

Figure 4

Particle fall velocity for sand particles



6.2 Instrument characteristics

In this Paragraph 6.2 some characteristics (size range, required sample quantity and analysis period) of the

various measuring methods for particle size and fall velocity are summarized. The results are presented in the

following Table 1:

Methods size range (mm) required quantity analysis

period

Photographic 100-100000 - 1 hour

sieves dry

wet

air jet

50-50000

10-100

10-1000

1 -1000 gram

0.1-1 gram

10 -100 gram

30 min

60 min

30 min

VA-tube 50-2000 1 -10 gram 5 min

MA-tube 50-2000 1 -10 gram 30 min

large 50-2000 0.1-10 gram 15 minBA-tube

small 5-100 500-5000 mg/1 3 hours

Bottom-withdrawal tube 5-100 500-5000 mg/1 3 hours

sedimen

tation

methods

pipet-withdrawal tube 5-100 100-5000 mg/1 3 hours

conductivity coulter coulter 1-500 10 -100 mg/1 30 min

(in-situ) laser diffraction 5-1000 10 -1000 mg 15 min

(in-situ) laser reflectance 5-1000 10 -1000 mg 15 min

(in-situ) video camera 10-1000 100-1000 mg 15 min

Table 1


6.3 Selection of instruments

In Paragraph 6.5 the instruments for determining the size and/or settling velocity of silt and sand particles are

described extensively. In this paragraph a summary of the most appropriate instruments for a specific

sediment sample is given (see following Table 1).

It should be stressed that the settling velocity of silt particles should be determined by means of an in-situ

instrument only, using the field pipet-withdrawal tube or the field bottom-withdrawal tube.

Bed material

samples

Suspended

sediment

samples

Instrument

silt sand gravel silt sand

Required sample

size

Inaccu-

racy

photocamera X -

dry X X X 1-1000 gr 10%sieves

wet X X X 0.1-10 gr 10%

laboratory X 0.5-3 liter 10%pipet-withdrawal

tubefield X 3 liter 20%

laboratory X 2 liter 10%bottom-

withdrawal tubefield X 2 liter 20%

VAT X X X 1-10 gr 10%

MAT X X X 1-10 gr 5%

small X 0.5 liter 5%

accumulation

tubes

BAT

large X X 0.1-10 gr 5%

Coulter counter X X X 0.5 liter 10%

(in-situ) laser diffraction X X X X 0.01-1 gr 10%

(in-situ) laser reflectance X X 0.01-1 gr 10%

(in-situ) video camera X X X X 0.01-1 gr 10%

Table 1


6.4 Comparison of instruments

6.4.1 BAT and VAT for sand particles

The fall velocity distributions of two sand samples (D50=120 mm and D50 = 230 mm) were determined by

means of a sophisticated Balance Accumulation Tube (BAT) as designed by the Delft University of

Technology, The Netherlands (Geldof and Slot, 1979) and a simple Visual Accumulation Tube (VAT). The

instruments are described in Par. 6.5. The sophisticated BAT has an effective settling length of 1.66 m and an

internal diameter of 0.17 m. The tube is equipped with an under-water balance accurate to 1 mg. The overall

inaccuracy of the instrument is supposed to be smaller than 5%.

The simple VAT consisted of a perspex tube with an effective settling length of 1.95 m and an internal

diameter of 0.03 m. The sand particles are accumulated in a small capillary tube (length 150 mm, diameter 4

mm) suspended at the underside of the tube (see Par. 6.5.3.2). Sand samples with a dry weight of 1.5 grams

have been used, which result in a total deposit height in the capillary tube of about 100 mm. Figure 1A

shows the fall velocity distribution according to the BAT and VAT. The agreement is rather good. From

these results it can be concluded that the VAT is sufficiently accurate for the routine determination of the fall

velocity of suspended sand samples. Figure 1B shows the size distribution of the same two samples

according to the dry-sieving method and computed from the measured (VAT) fall velocity distribution. The

agreement between both distributions is remarkably good.

References

Geldof, H.J. and Slot, R.E., 1979. Design Aspects and Performance of Settling Tube System. DelftUniversity of Technology, Dep.Civ.Eng., Report No. 4-79, The Netherlands

Figure 1

Comparison of BAT and VAT

6.4.2 BAT, PWT, Wet-sieving and Coulter-Counter for fine particles

Quartz and limestone powder

Figure 1A shows size-distribution curves determined by Colon (1968) using the small Balance

Accumulation Tube (BAT, Sartorius), the Pipet-Withdrawal Tube (PWT, Andreasen-Eisenwein), the Wet-

Sieving method and the Coulter-Counter. The instruments are described in Par. 6.5. The results show rather

large deviations upto 50% in the characteristic sizes D10, D50 and D90. The Pipet-Withdrawal Tube (PWT)

gives smaller size-values than the Balance Accumulation Tube (BAT) for both materials. The Wet-Sieving

method shows good agreement with the Pipet-Withdrawal Tube results.

Silt sample

Karelse and Polhuys (1981) compared the Balance-Accumulation Tube (Sartorius) and the Pipet-

Withdrawal Tube (Andreasen-Eisenwein) for a silt sample (see Figure 1B). The results show good agreement

with a maximum error of about 10%.

References

Colon, F.J. de, 1968. Particle Size Analysis (in Dutch). CTI-TNO Publication No. 68, The NetherlandsKarelse, M. and Polhuys, J.M., 1981. Investigation of Accumulation Balance for Particle Size

Measurement (in Dutch). Delft Hydraulics, Report S320, The Netherlands

Figure 1

Comparison of BAT, PWT, Wet-sieving, Coulter-Counter

6.4.3 PWT, BWT and BAT for fine particles

The laboratory designs of the Pipet-Withdrawal Tube (PWT) and the Bottom-Withdrawal Tube (BWT) were

used to determine the fall velocity distribution of a silt sample with particles in the range of 0 to 70 mm. The

results of the PWT and the BWT were compared with the results of a small (commercially available)

Balance Accumulation Tube (BAT, Sartorius). The latter instrument is supposed to give the most accurate

results (Van Rijn, 1986). All instruments are described in Par. 6.5.

The PWT consisted of a perspex tube with a length of about 300 mm and an internal diameter of 120 mm, as

shown in Fig. 1B, Par. 6.5.3.4. The withdrawal volume was about 200 ml. The initial settling height above

the point of withdrawal was 290 mm. The operational procedure is described in Paragraph 6.5.3.4.

The BWT consisted of a perspex tube with a length of 1020 mm and an internal diameter of 50 mm, as

shown in Figure 1A, Par. 6.5.3.3. The operational procedure as described in Paragraph 6.5.3-3-Figure 1A

shows the fall velocity distribution based on the PWT and BAT methods for various initial silt

concentrations in the range of 20 to 2000 mg/1. The agreement between the results of both methods is rather

good, even for small (initial) concentrations of 20 mg/1.

Figure 1B shows the results based on the BWT and BAT. The agreement between the results of both

methods is reasonably good for initial concentrations larger than about 200 mg/1. For initial concentrations

smaller than 200 mg/1 the results of the BWT begin to deviate, showing rather large deviations for c = 20

mg/1. The relatively poor results of the BWT for small initial concentrations are, probably, caused by

insufficient removal of the sediment particles during the withdrawals. Sediment particles may (partly) stick

to the inside of the contracted section of the tube (Van Rijn, 1986). Therefore, the BWT is not supposed to

be an optimal instrument for the in-situ analysis of silt suspensions, because the results may not be very

accurate for initial concentrations smaller than 200 mg/1, the latter being quite common values for field

conditions.

Figure 2 shows comparative results for the field design (see Figs. 2, 3 of Par. 6.5.3.4) of the PWT based on

measurements in a flume. The results of the PWT are compared with the results of water-sediments samples

analyzed in the Sartorius balance accumulation tube (BAT).

References

Van Rijn, L.C., 1986. The determination of Settling Velocities. Delft Hydraulics Laboratory, Report S3O4,The Netherlands

Van Rijn, L.C. and Nienhuis, L.E.A., 1985. In-situ Determination of Fall Velocity of Suspended Sediment.21st IAHR-Congres, Melbourne, Australia

Figure 1

Comparison of PWT, BWT and BAT

Figure 2

Comparison of PWT, and BAT for silt particles in a flume

6.5.1 Photographic instrument

Principle

The method is based on taking photographs of the (dry) stream bed. The height of the camera depends on the

size of the bed material and the lens system. A reference scale must appear in the photograph. The

photograph is printed on thin paper to be inspected on a lightbox with special optical equipment. By

adjusting the optical equipment, the diameter of a sharply defined circular lightspot appearing on the photo-

graph can be changed and its area made equal to that of the individual particles.

An automatic counting system can be used for registration of the particles. After registration each particle

must be marked on the photograph.

References

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. United States GovernmentPrinting Office, Washington, USA

Vanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, NewYork, USA

6.5.2 Sieving instruments


Sieve analysis is one of the simplest, most widely used methods of particle size analysis, that covers the

approximate size range from 50 mm to 50000 mm using standard woven wire sieves. Micromesh sieves

extend the range down to 5 mm and punched plate sieves extend the upper range.

Sieve results can be highly reproducable (within 5%). Inaccuracies may be caused by:

· size of total sample and size of particle fractions on each sieve

· presence of aggregated lumps of particles

· inaccuracies in size and shape of the sieve openings

· the duration of the sieving operation.

For several countries standard (woven-wire) sieves are in use according to the national standard

specifications:

The Netherlands: N480

Germany: DIN 4188

England: BS 410 (1976)

France: AFN0R NFX 11-501

USA: ASTM E 11-70

6.5.2.2 Dry sieving

Principle






Practical operation








Remarks








ss=0.5[D16/D50 + D50/D84]


D10=(ss)-1.3 D50

D16=(ss)-1 D50

D84=(ss)1 D50

D90=(ss)1.3 D50


DM=(S0.5(Di+Di+1)gi)/Sgi=(S0.5(Di+Di+1)pi)/100

in which:






References

Allen, T, 1981. Particle Size Measurement, Third Edition. Chapman and Hall, London-New YorkDe Vries, M., 1971. On the Accuracy of Bed Material Sampling. Delft Hydraulics Laboratory, Publication

No. 90, The NetherlandsVanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice, No.54, New

York

Figure 1

Dry sievingSieve Analysis Delft Hydraulics

Sample No =

Location =

Date =

Observer =

D10 =

D50 =

D90 =

DM =

Sieve diameter

Di

(mm)

Average diameter

DM,i

(mm)

Weight on sieve

gi

(mg)

Weight percentage

pi

(-)

Cumulative weight

percentage

(-)

SpiDM,i

11.2

8.0 9.6

5.6 6.8

4.0 4.8

2.0 3.0

1.4 1.7

1.0 1.2

0.85 0.925

0.71 0.78

0.6 0.655

0.5 0.55

0.42 0.46

0.35 0.385

0.3 0.325

0.212 0.256

0.18 0.196

0.15 0.165

0.125 0.138

0.106 0.114

0.09 0.098

0.075 0.083

0.063 0.069

0.053 0.058

0.045 0.049

0.038 0.042

panTtotal 100%

Figure 2


Figure 3

Sieve analysis

Figure 4

Sieve curve form

6.5.2.3 Wet sieving

Principle

The method can be used for particles in the range of 10-100 mm. The sieves consist of

nickel plates in which electrolytic holes are made with an accuracy of 2 mm (micro-precision sieves).



vibrated.



Practical operation











References



Principle







References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New YorkKiff, P.R., 1977. Sedimentation Methods Manual. Hydraulic Research Station Wallingford, England

6.5.2.2 Dry sieving

Principle






Practical operation








Remarks








ss=0.5[D16/D50 + D50/D84]


D10=(ss)-1.3 D50

D16=(ss)-1 D50

D84=(ss)1 D50

D90=(ss)1.3 D50


DM=(S0.5(Di+Di+1)gi)/Sgi=(S0.5(Di+Di+1)pi)/100

in which:






References

Allen, T, 1981. Particle Size Measurement, Third Edition. Chapman and Hall, London-New YorkDe Vries, M., 1971. On the Accuracy of Bed Material Sampling. Delft Hydraulics Laboratory, Publication

No. 90, The NetherlandsVanoni, V.A., 1980. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice, No.54, New

York

Figure 1

Dry sieving

Sieve Analysis Delft Hydraulics

Sample No =

Location =

Date =

Observer =

D10 =

D50 =

D90 =

DM =

Sieve diameter

Di

(mm)

Average diameter

DM,i

(mm)

Weight on sieve

gi

(mg)

Weight percentage

pi

(-)

Cumulative weight

percentage

(-)

SpiDM,i

11.2

8.0 9.6

5.6 6.8

4.0 4.8

2.0 3.0

1.4 1.7

1.0 1.2

0.85 0.925

0.71 0.78

0.6 0.655

0.5 0.55

0.42 0.46

0.35 0.385

0.3 0.325

0.212 0.256

0.18 0.196

0.15 0.165

0.125 0.138

0.106 0.114

0.09 0.098

0.075 0.083

0.063 0.069

0.053 0.058

0.045 0.049

0.038 0.042

panTtotal 100%

Figure 2


Figure 3

Sieve analysis

Figure 4

Sieve curve form

6.5.2.3 Wet sieving

Principle





vibrated.



Practical operation











References


6.5.2.3 Wet sieving

Principle





vibrated.



Practical operation











References



Principle







References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New YorkKiff, P.R., 1977. Sedimentation Methods Manual. Hydraulic Research Station Wallingford, England

6.5.3 Sedimentation instruments



· stratified suspensions,

· dispersed suspensions.

Stratified systems









Dispersed system


Generally, this method is only used for silt or fine sand particles (5 to 150 mm). Usually, the sediment weight











in which:



References


New York, USA

Figure 1



Principle

An accumulation tube can be operated as a stratified system for sand particles in the range 50-2000 mm or as

a dispersed system for silt and fine sand particles smaller than 150 mm. Typical examples of the


· Visual Accumulation Tube (VAT)

· Manual Accumulation Tube (MAT)

· Balance Accumulation Tube (BAT)






internal diameter of 4 mm for particles of 50-500 mm and 10 mm for particles of 500-2000mm. The method





The overall inaccuracy of the fall velocity distribution is about 10% for particles in the range 50-500 mm (see




















Practical operation

VAT





5. wash the (wet) sample in the container (1.5 gram for particles 50- 500 mm and 10 grams for particles of

500-2000 mm).








MAT

1. see above,

2. see above,

3. see above,

4. see above,

5. see above,

6. see above,






Remarks




References

Geldof, H.J. and Slot, R.E., 1979. Settling Tube Analysis of Sand. Delft University of Technology, Dep. ofCiv. Eng., Internal Report No. 4-79

Geldof, H.J. and Slot, R.E., 1979. Design Aspects and Performance of a Settling Tube System DelftUniversity of Technology, Dep. of Civ. Eng. Internal Report No. 6-79

Gibbs, R.J., 1972. The accuracy of Particle-Size Analysis Utilizing Settling TubesJournal of Sedimentary Petrology, Vol. 42, No. 1, p. 141-145

Inter Agency Committee on Water Resources, 1957. The Development and Calibration of the VisualAccumulation Tube. Report No. 11, St. Anthony Falls Hydraulic Laboratory, Minneapolis, USA

Karelse, M. and Polhuys, J.M., 1981. Investigation of Accumulation Balance for Particle SizeMeasurement (in Dutch). Delft Hydraulics Laboratory, Report S320, The Netherlands

Kleinhans, M.G., 1998. Calibration of visual accumulation settling tube (in Dutch). Report ICG 98/13Department of Physical Geography, University of Utrecht, Utrecht, The Netherlands


Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6

Graph Fall velocity


Principle


system).




sediments.





of 0.05 m.










A suitable schedule for particles in the range of 5 to 100 mm is withdrawals at 3, 6, 10, 20, 40, 60 and 120
















Practical operation
















Field instrument










mg).





1. Compute equivalent sample heights (column 6, Fig. 3): hi=Vi/A=Vi/(0.25 pD2) (mm),

2. Compute effective fall height (column 7) by summation of the equivalent sample heights: h=Shi (mm),


4): G=SGi (mg),


mm: Depth-factor=1000/Shi,




depth-factor,





Figure 5,




(Figure 5),






Disadvantages







References

Dearnaley, M.P., 1996. Direct measurements of settling velocities in the Owen tube: a comparison withgravimetric analysis. Journal of Sea research, Vol. 36, No. 1-2, p. 41-47

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. Geological Survey, USAGovernment Printing Office, Washington, USA

Owen, M.W., 1976. Determination of the Settling Velocities of Cohesive Muds. Hydraulics ResearchStation Wallingford, Report No. IT 161, England

Van Rijn, L.C., 1986. Determination of Settling Velocities. Delft Hydraulics Laboratory, Report S304,Delft, The Netherlands

Vanoni, V.E., 1976. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, NewYork, USA

Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6


Figure 7

Graphs


Principle










sediments.


















25 ml withdrawal




stop),







pipet-sample,


200 ml withdrawal









withdrawal),


withdrawal,













withdrawals),





Remarks















Field instrument




2. Compute the equivalent sample height (column 7) as : hi=Vi/Ai=Vi/(0.25pD2) (mm),

3. Compute the effective settling or fall height (column 8) as: h=Ho-Shi, with Ho=initial settling height=


4. Compute the settling velocity at which separation is required (column 11), as: Ws,i= (Ho-Shi)/Ti,


12), as : Pi=(ci/co)100%,


References




Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6




· stratified suspensions,

· dispersed suspensions.

Stratified systems









Dispersed system


Generally, this method is only used for silt or fine sand particles (5 to 150 mm). Usually, the sediment weight











in which:



References


New York, USA

Figure 1



Principle





































Practical operation

VAT






500-2000 mm).








MAT

1. see above,

2. see above,

3. see above,

4. see above,

5. see above,

6. see above,






Remarks




References








Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6

Graph Fall velocity


Principle





































Practical operation

VAT






500-2000 mm).








MAT

1. see above,

2. see above,

3. see above,

4. see above,

5. see above,

6. see above,






Remarks




References








Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6

Graph Fall velocity


Principle


system).




sediments.





of 0.05 m.










A suitable schedule for particles in the range of 5 to 100 mm is withdrawals at 3, 6, 10, 20, 40, 60 and 120
















Practical operation
















Field instrument










mg).





1. Compute equivalent sample heights (column 6, Fig. 3): hi=Vi/A=Vi/(0.25 pD2) (mm),

2. Compute effective fall height (column 7) by summation of the equivalent sample heights: h=Shi (mm),


4): G=SGi (mg),


mm: Depth-factor=1000/Shi,




depth-factor,





Figure 5,




(Figure 5),






Disadvantages







References

Dearnaley, M.P., 1996. Direct measurements of settling velocities in the Owen tube: a comparison withgravimetric analysis. Journal of Sea research, Vol. 36, No. 1-2, p. 41-47


Owen, M.W., 1976. Determination of the Settling Velocities of Cohesive Muds. Hydraulics ResearchStation Wallingford, Report No. IT 161, England


Vanoni, V.E., 1976. Sedimentation Engineering. ASCE-Manuals and Reports on Eng.Practice No. 54, NewYork, USA

Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6


Figure 7

Graphs


Principle










sediments.


















25 ml withdrawal




stop),







pipet-sample,


200 ml withdrawal









withdrawal),


withdrawal,













withdrawals),





Remarks















Field instrument




2. Compute the equivalent sample height (column 7) as : hi=Vi/Ai=Vi/(0.25pD2) (mm),

3. Compute the effective settling or fall height (column 8) as: h=Ho-Shi, with Ho=initial settling height=


4. Compute the settling velocity at which separation is required (column 11), as: Ws,i= (Ho-Shi)/Ti,


12), as : Pi=(ci/co)100%,


References




Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6


6.5.4 Coulter Counter

Principle

The method is based on an electrical conductivity difference between particles and common diluent. Particles

act as insulators and diluents as good conductors. The particles suspended in an electrolyte are made to pass

through a small aperture through which an electrical current path has been established (Figure 1). As each

particle displaces electrolyte in the aperture, a pulse essentially proportional to the particle volume is

produced. Particles in the range of 1 to 500 mm can be counted and measured volumetrically.


The results are presented in terms of a cumulative volume distribution and the percentage of particles in each

volume class. The reproducibility is claimed to be accurate to about 1%.

References

Allen, T., 1981. Particle Size Measurement, Third Edition. Chapmann and Hall, London-New York

Figure 1

Coulter Counter

6.5.5 Particle size and concentration by Laser Diffraction (LISST, COULTER, PARTEC)

The Laser diffraction method (Fraunhofer diffraction) offers a fundamentally superior basis for in-situ

measuring the sizes of suspended sediment particles in a point in the water column. Unlike other and simpler

optical or acoustic methods, the diffraction method does not suffer from a change in calibration with

changing sediment colour, composition or size.

When a parallel light wave strikes a particle, part of the wave enters the particle, and part is blocked by it.

The wave entering the particle senses particle composition (e.g. colour, absorption). However, this part is

scattered into a wide range at angles, very little of which appears in the original light wave direction. In

contrast, light blockage produces a diffraction pattern that dominates the light intensity in the original

direction. This pattern is bright and it is identical to the diffraction through an aperture familiar to optical

physicists (analogous to the diffraction of waves on water surface by a jetty). When a lens gathers the

scattered plus diffracted light, diffraction shows up on the lens axis. The diffraction pattern is weaker and

wider for small particles, but tall and narrow for large particles. The width helps to distinguish particle size

while the magnitude delivers concentration.

Figure 1A shows a parallel beam of light striking a spherical particle. The light that enters the particle (and that

therefore feels its composition) exits at large angles to the original beam. It makes a very small contribution to the

very small angle scattering. Only rays diffracted around the particle appear at the small angles, producing the Airy

pattern shown on right. This is why the name: Laser Diffraction. Figure 1B shows the sampling volume.

When a number of particles are present, the intensity patterns of individual particles add. The resulting

pattern is what the Laser-diffraction instruments measure and interpret. The diffraction pattern shown in

Figure 1A is intensity versus scattering angle. For mathematical reasons, the detection of diffracted light is

done with ring-shaped detectors.

The Laser diffraction method is accurate for spheres. As the multi-angle scattering is only very slightly

sensitive to particle composition, the measurement of both the size distribution and concentration for

spheres is fundamentally assured by physics to be accurate. However, shape effects do reduce accuracy as

the diffraction pattern of non-spheres exhibits differences. The principal difference is in the width of the

main diffraction peak; the peak is broader for non-spheres. Furthermore, the minima of the diffraction

from non-spheres are less deep than for spheres. These properties are not fully understood at this time.

Until such time as these shape effects are accounted for in a fundamental way, the best possible approach

appears to be to apply an empirical calibration correction.

In-situ measurements of sediment particles and flocs are essential as natural flocs are disrupted easily by

physical manipulation such as sampling by bottles or pumps. True particle size distributions of natural

suspended sediments can only be achieved by in-situ systems. Most optical methods are potentially non-

disruptive.

Early in-situ measurements using a Laser-diffraction instrument (MALVERN) were carried out by Bale et

al. (1984) and Bale and Morris (1987).

Recent instruments (LISST) can derive the particle size distributions and also the particle volumes (volume

concentration) from the measured data with an accuracy of the order of 20% (Agrawal and Pottsmith,

Sequoia Inc., 2002). Mass concentration can only be determined by assuming a constant particle density or

by measuring the settling velocity (LISST-ST, see Figure 1C). Detailed information is given by Agrawal

and Pottsmith (2000).

Laboratory instruments based on Laser diffraction method such as the Coulter LS230 are offered by

Beckman-instruments (www.beckman.com). These laboratory instruments which are used in soil studies

have a range from 0.04 to 2000 microns with 116 fractions. For the very small particles in the range of 0.04

to 0.4 microns the Polarization Intensity Differential of Scattered Light (PIDS) is used (Buurman et al.,

1997). Another method for the very fine particle size range (0.01 to 5 microns) is photon correlation

spectoscopy (nanosizers). Progress in the engineering field makes it possible to incorparate these

sophisticated instruments in submersible devices to extend the working range of in-situ size analysis.

Various Laser diffraction instruments are manufactured by Sequoia Inc, USA (www.sequoiasci.com), which

are known as the LISST–instruments (Laser In-Situ Scattering and Transmissometery), as follows:
















compensate for effects such as convection and particle interaction (Delft Hydraulics, 2003).








angle scattering data to construct the size distribution, and then summing the concentrations in the 32 size







version, and a second version that is fully recording and presents a coarse fraction concentration in

addition to the total suspended load. The LISST-25X instrument has new comet shapes built in to separate

between wash load finer than 63 micron and the sand load larger than 63 micron. The two new comet

shapes deliver the total concentration and SMD in the entire size range, and concentration and SMD in the

coarse sand range. The comet shapes assume nothing regarding the underlying size distribution of

sediments. The only requirement is spherical shape for particles. Inaccuracies of perhaps as much as 100%

may occur if the particle composition changes from mineral to biogenic.


measurements. It incorporates a Laser, optics, multi-ring detector identical to the LISST-100, and




mounted above the body itself, and a sensor is employed to count the number of its rotations in a short




points, verticals, or in the entire stream cross-section (Gray, 2004).

The limitations of the LISST instruments can be described, as follows (see also Bale, 1996):

1. The method is accurate for spherical particles; the effect of non-spherical particle shape on the

measured size distributions is not quite clear and can only be determined by calibration.

2. The method is limited in the range of turbidities that can be measured. Multiple-scattering effects (re-

scattering of scattered light) begin to appear when the optical transmission is less than 30%. The lower

the transmission, the stronger the effects of multiple-scattering. When these effects are ignored, the

recovered size distributions show a bias toward the small sizes. Maximum concentration of fine

sediments (<50 mm) is about 100 to 150 mg/l (Gartner et al., 2002) and about 500 mg/l for coarser

sediments (>50 mm), (Traykovski et al, 1999). If the particle concentration is too low, the diffracted

signal is too small to detect it. Upper concentration limit can be extended by using smaller optical path

lengths (5 to 10 mm); very small path lengths may however produce relatively large shear within the

measuring volume disrupting the flocs.

3. When measuring settling velocities, there is a continual concern regarding the breakup of fragile flocs

in the process of drawing of a sample from the water column. The results of LISST-ST instrument may

be affected by disrupted flocs.

4. Flocs may not consist of solid materials; if flocs have small pores, light will be diffracted through

these pores which will contribute to the diffraction pattern in a similar way as smaller particles would

be.

5. Errors are relatively large in conditions with large amounts of very fine clay sediments (<5 mm) close

the wave length of the light source; very large sand particles (>500 mm) are difficult to detect as

relatively large focal lengths (300 mm) are required.

6. The instruments (due to their relatively large physical size) cannot be used very close to the bed where

most of the sediment transport takes place.

7. The instruments suffer from biological fouling in long-term deployments (stand-alone tripods). The

results of the instrument LISST-ST may also be affected by vibrations of the tripods in stormy

conditions (strong orbital motions) resulting in secondary flow patterns in the settling tube.

Advantages

1. Rapid (output at a rate of 1 Hz) in-situ determination of particle size without the need to pump, store,

transport or otherwise handle water samples; accurate results in conditions with weak currents.

2. Instrument can measure in stand-alone mode without human interference.

3. Accurate size and concentration determination of non-flocculated, spherical particles in range of 5 to

250 mm (volume concentration is computed from size measurements assuming spherical particles).

4. Particle composition does not affect the particle size measurements.

Disadvantages

1. The particles of a size class may consist of solid particles and flocculated particles. To determine the

mass concentrations of each size class, the effective, submerged density (rs-rw) is required which can

be determined from the fall velocity curve (LISST-ST) for each size class using the Stokes fall

velocity formula (with size class diameter and fall velocity as input values) or by calibration (to

determine volume conversion factor cv) using water samples. The mass concentration of the LISST-ST

is a mass concentration of solids and flocs including the fluid in the pores of the flocs. This value is

different from the dry mass concentration obtained by drying and weighing of the sample.

2. Maximum concentration of about 150 to 500 mg/l for particles in range of 5 to 63 mm in marine

applications (expected concentration levels should be known before deployment); maximum

concentration thesholds can be larger if shorter optical path lengths are used.

3. The scatter around the initial volume concentrations of the larger size classes is relatively large due to

the small number of particles present in those size classes.

4. Errors in size distributions are greatest when there is a significant amount of material outside and close

to the extremities of the measured size ranges (<5 mm and >500 mm).

5. The sampling volume at end of instrument may be in the lee (wake flow) of the instrument-housing

(LISST-100) disturbing proper sampling.

6. LISST-ST in-situ measurements may suffer from sediment resuspension (instrument vibrations) in the

settling column in conditions with relatively strong currents and or waves; these problems can be

overcome by taking water-sediment samples and analysis of these samples directly after collection

using the LISST-ST.

Technical specifications LISST-100 (www.sequoiasci.com)

1. Measured parameters: size distribution in range of 1.25 to 250 micron, concentration by volume,

optical transmission, pressure in range of 0 to 200 m with resolution of 0.05 m, temperature in range of

-5 to +50 degrees with resolution of 0.1 degrees.

2. Instrument size: length of about 0.8 m (housing datalogger, electronics, power supply and Laser

source, diameter of about 0.08 m, weight of about 12 kg.

3. Two external digital I/O ports; external analog input (0-5 v).

4. Optical path length: standard 5 cm; optical transmission with 12 bit resolution; 1 milli-watt Laser

beam.

5. Resolution: 64 size classes; log spaced.

6. Maximum sample speed: 5 size distributions per second.

7. Battery life: upto 1 year.

8. Maximum depth range: 200 m.

Field and laboratory experiments

Buurman et al. (1997) have summarized the practical problems related to particle size analysis by using

Laser diffraction method. It is concluded that this method has a high reproducibility, but the method

contains a large number of pitfalls for clayey samples. Problems of sample homogeneity and subsample

representation can be solved by thorough mixing and, in some cases, by dilution of the suspension in the

sample cuvette. Loss of coarse fractions by such dilution appears to be negligible if the samples are

smaller than about 1000 microns. Flocculation of sediments causes major problems. Flocculation can (if

necessary) be eliminated by using ultrasonic vibration of the samples or by adding dispersants. Another

problem is the blocking of the optical detectors when the sediment concentration is too large. Sizes of

colloid, platy particles cannot be determined absolutely by the Laser diffraction method. Calibration with

other methods remain necessary.

Traykovski et al. (1999) have tested the LISST-ST (settling tube version) to measure the particle size

distributions and concentrations of sediment suspensions of natural sediments. Suspensions were used with

sand-sized sediments based on dry sieving methods in the range of 63 to 710 mm; fine sediment

suspensions (based on wet sieving methods) in the range of 25 to 63 mm and other suspensions with

sediments in the range of 5 to 25 mm. The maximum concentrations were about 500 mg/l for the 25-65 mm

sediment range and about 150 mg/l for the 5-25 mm sediment range; larger concentrations cannot be tested

because the optical transmission becomes too low and multiple scattering is introduced leading to errors.

It was found that the LISST-ST can resolve the peak size of a uniform size distribution, and can resolve

two peaks in a bimodal distribution if they are separated by approximately 1f. The LISST-ST is not able

to resolve the sand sizes larger than about 250 mm. The LISST-ST was able to correctly resolve the sizes

of fine particles in the range of 5 to 63 mm, but the results become confused if particles are smaller than

about 5 mm.

The LISST-ST is able to determine the mass concentration of different size distributions with a single

calibration parameter (cv-value) as long as the sediment size is in the range of 5 to 250 mm, and as long as

the transmission rates are not too low. This means that the LISST-ST can only be used outside the bottom

boundary layer in marine applications, where the concentrations are relatively low (<150 mg/l) for the

finer sediments. Thus, some knowledge of the expected concentration levels is required before

deployment. This problem can be partly solved by using instruments with shorter optical path lengths (1.5,

2.5 cm in stead of 5 cm).

Gartner et al. (2001) have tested the LISST-100 in laboratory and field conditions. A laboratory setup

was designed to determine the size distribution of polystyrene particles of known size with a density of

1050 kg/m3. Most of the experiments were carried out using mono-sized particles including 5, 10, 20, 50,

100, 140 and 200 mm. In addition, several broad size distributions were tested, including a 1-35 mm

distribution of polystyrene particles and a 1-40 mm distribution of glass beads with density of 2450 kg/m3.

The design principle of the laboratory procedure was to mimic as closely as possible the instrument

operational conditions in the field. Therefore, an important consideration is that the experimental setup

must allow the full 5 cm Laser passage through the testmedia and yet keep the required volume of test

sample of well-defined polystyrene particles to a minimum. A test chamber consisting of two adjustable

cylinders was designed to fit in the region of the LISST-100 where the sample volume is located. A

peristaltic pump was used to keep the test solution circulating between a test solution reservoir (a beaker

above a magnetic stirrer) and the test chamber. Test particle suspensions consisting of known particle size,

mass and volume of water were mixed in the reservoir to produce the required mass concentrations of

known values. A Laser Coulter Counter model LS230 was used to analyze the samples independently.

Size distributions were also provided by the manufacturer of the polystyrene materials (also based on

Coulter Counter instrument). A typical result of the tests is shown in Figure 5 for a sample with a broad

particle size distribution of 1 to 40 mm. The LISST-100 tends to estimate sizes that fall between those of

the other two methods. Above about 10 mm, the LISST-100 tends to show cumulative volume percent

values less than the other two methods but typically within about 10% to 15%. Differences below 5 mm

may be the result of the different methods because Fraunhofer-diffraction method begins to loose

applicability due to less distinct scattered light patterns. Since the detector rings in the LISST-100 are

logarithmically-spaced (the upper size of each bin is 1.18 times the lower size), instrument resolution

becomes poorer with increasing particle size range. Traykovski et al. (1999) found that the LISST-100

tends to slightly overestimate the size of natural marine sediments tested in laboratory conditions.

Once the particle size distribution is measured, the volume concentration of suspended particles can be

estimated if a volume conversion factor (cv) is provided to post-processing software. Gartner et al. (2001)

have determined the best-fitting values of cv for each size class by minimizing the percent difference

between the known volume concentration values and the LISST-determined volume concentrations. The

cv-values are shown in Figure 6 for two LISST-100 instruments used by Gartner et al. (2001) Rather

than being independent of particle size, the cv-value varies over a factor of 3 inversely proportional to

log(size) in the range between 5 and 200 mm. Furthermore, the cv-value differ between the two

instruments, although the trends are consistent. It is concluded that the cv-parameter is an experimentally

calibrated value. There is no theoretical reason to explain the observed variance of the cv-parameter.

Because the volume concentration depends on the third power of the particle diameter, any error in particle

size will produce a proportionally larger error in volume concentration. Size errors are larger for larger

particles. Hence, the volume concentration errors will be largest for the largest particles. This partly

explains the trend of decreasing cv-values with increasing particle size.

Gartner et al. (2001) have also used the LISST-100 in field conditions with fine muddy, sediment beds,

(near San Mateo bridge and Dumbarton bridge, San Francisco Bay, USA; October 1998). Results of field

data are expected to deviate from the laboratory data with the polystyrene particles because the

characteristics of naturally occurring suspended materials are substantially different in shape, structure and

density.

The LISST-100 instrument was deployed together with OBS sensors for concentration values and ADCP

sensors for current speeds. The LISST-100 was mounted at 220 cm above the bed while the OBS sensors

were mounted at 41, 71, 107 and 220 cm above the bed. The OBS at 200 cm did not function properly.

The LISST-100 was programmed to record an average of 16 scans (taking about 4 s) once every 15 min.

The OBS sensors recorded an average of 99 samples (taking less than 1 s) once every 15 min (=burst

length). The OBS sensors were calibrated using suspensions of known concentration based on bed

material samples from the field sites. This calibration procedure was not useful for the LISST-100,

because it may not fully represent the (flocculated) particle size distributions present in the water column

at the field site. An apparent dry density (of 190 kg/m3; wet density of 1140 kg/m3) related to flocculated

aggregates was used to estimate the mass concentration by LISST-100.

Mass concentrations of suspended materials at the field site based on the LISST-100 (assuming dry floc

density of 190 kg/m3 resulting in cv-factor of 6000) and the OBS are shown in Figure 7. Estimates of both

instruments correlate very well, except that the mass concentration estimates from LISST-100 are

generally missing during the time of the peak concentrations above about 100 to 150 mg/l, because the

percent optical transmission was too low.

Measured particle sizes of LISST-100 are shown in Figure 8. Size distributions near some of the

maximum flood currents cannot be determined because the optical transmission was too low

(concentration levels exceed the threshold of multiple scattering). The particle sizes vary around a mean

value of about 60 mm (variation in range of 40 to 70 mm). No independent measurements of the size

distribution at the field site were made. As flocculation of suspended material my produce aggregates in

the hundreds of microns in estuaries, the LISST-100 values in the range of 40 to 70 mm may not be fully

representative of the entire spectrum of particle sizes at the field site. Flocs sizes in the range of 100 to 500

microns have been measured at other (nearby) sites of San Francisco Bay using other instruments.

Fugate and Friedrichs (2002) have used the LISST-100 at a field site in the lower Chesapeake Bay

(USA) with relatively low concentrations in the range of 10 to 50 mg/l (fine sediments). The measured

mass concentrations are in good agreement with pumped sample concentration values. Estimation of mass

concentrations by the volume concentrations measured by the LISST are not improved by adding grain

size distribution information, which suggests for this field site that sediment density is not a strong

function of size.

Van Wijngaarden and Roberti (2002) have tested the LISST-ST (settling tube) to determine the particle

size and fall velocities in the Hollands Diep and Haringvliet, two large fresh-water basin in the south-west

part of The Netherlands. The flow velocities at these sites are of the order of 0.1 to 0.2 m/s. The tidal range

is limited to about 0.1 to 0.2 m. Salt intrusion is absent. The measurement period was between November

8 and 22 in 1999, between April 3 and 17 in 2000 and between August 7 and 20 in 2000. During the first

deployment the settling period was taken to be 12 hours and 24 hours during the second deployment. The

LISST-ST was mounted in a stand-alone frame on the bottom of the basin. Water samples were taken at

three depths at the start of each deployment and before the pick-up of the frame.

Figure 9 shows the settling behaviour of 8 size classes (concentration versus time); a decreasing trend can

be observed. To improve the accuracy of the larger particle sizes, 8 classes in stead of 32 classes were

used. The particles of a size class may consist of solid particles and flocculated particles. The scatter

around the initial volume concentrations of the larger size classes is relatively large due to the small

number of particles present in those size classes. The fall velocity curves can be determined from these

plots. To determine the mass concentrations of each size class, the effective, submerged density (rs-rw) is

required which can be determined from the fall velocity curve for each size class using the Stokes fall

velocity formula (with size class diameter and fall velocity as input values). The effective dry densities

were found to be in the range of 10 to 1000 kg/m3; about 10 kg/m3 for the larger flocs and about 1000

kg/m3 for the smaller flocs. The mass concentration of the LISST-ST can obtained from the volume

concentrations and the effective density. The mass concentration of the LISST-ST is a mass concentration

of solids and flocs including the fluid in the pores of the flocs. This value is different from the dry mass

concentration obtained by drying and weighing of the sample. Figure 10 shows the dry mass concentration

of water samples and the LISST-ST values. The real dry mass concentrations are about 5 to 6 times larger

than those of the LISST-ST.

The weighted mean particle diameters were found to be in the range of 10 to 20 mm during the April

deployment, in the range of 10 to 50 mm during the November deployment and in the range of 10 to 20 mm

in the August period. These values are somewhat smaller than those found in earlier studies using in-situ

video recordings. The weighted mean settling velocity was found to be in the range of 0.01 to 0.04 mm/s

for the November period, in the range of 0.04 to 0.12 mm/s for the April period and in the range of 0.02 to

0.05 mm/s for the August period. These settling velocities are much smaller than those measured earlier by

the in-situ video camera recordings.

It is concluded that for the calculation of mass concentration, the combination of both a settling velocity

distribution and a mass concentration distribution is essential. Although the flocs in the upper size classes

do take up only a small fraction of the mass concentration, their high settling velocity accounts for the fact

that they dominate the total mass flux. This is however not always the case for flocs in the largest size

classes: these aggregates can be of low density and hardly settle out. The use of mean values for

concentration and settling velocity would average out such effects.

Thonon et al. (2005) have successfully used the LISST-ST (Type C) to measure in-situ particle size

distributions and settling velocities in rivers in The Netherlands. In 2002 and 2004, the LISST-ST was used in

combination with sediment traps in two river floodplains in The Netherlands: one along the unembanked

IJssel River, another along the embanked Waal River. LISST-ST can measure particle sizes between 2.5

and 500 mm. However, particles smaller than 2.5 mm and small particles that are aggregated into flocs

also give a signal in the class with the smallest size fraction. The model by Van Wijngaarden and

Roberti (2002) for the estimation of the settling velocities was used. To have enough data in each size

class, the 32 ring data were resampled to eight size classes with class midpoints in the range of 3 to 360 mm).

The model fits the data with a bimodal settling velocity distribution. Using the fitted bimodal settling

velocity distribution, the model calculates the corresponding floc density distribution using Stokes's law. For

each particle size class, two floc densities are fitted: a higher and a lower one. For the size classes with the

smallest particles often only the lower fitted densities were physically plausible. This is explained by the

following: when Laser light falls on larger flocs, it is also partially diffracted by their smaller constituent

particles, thereby creating aliases ('ghost particles') in the smaller size fractions. These ghost particles settle

at the same rate as the larger flocs. Therefore, it seems as if a part of the smaller size fractions is falling with

a higher settling velocity. This higher settling velocity resulted in anomalously high densities for the smaller

size classes. Therefore, for these size classes the lower floc densities and settling velocities were selected.

The floc densities and their corresponding settling velocities were finally used to calculate dry mass

concentrations and mass-weighted mean grain sizes.

To check the concentration measurements of LISST-ST independently, an optical backscatter sensor

(OBS) was also used (Seapoint Sensors). The OBS measures the turbidity of the water in Formazin

Turbidity Units (FTU), which vary under ideal conditions linearly with suspended concentration (SSC).

A disadvantage of the OBS technique is its size dependency: the smaller the particles, the stronger the

signal at constant concentration. This often makes field calibration necessary. As a second check of the

LISST-ST, water samples were taken (close to the measuring frame). The sediments from the water

samples were analysed in the laboratory by using a Coulter LS 230 Laser diffraction device which can

measure particle size in the range of 0.04 to 2000 mm (Beckman-Coulter, Fullerton, CA, USA). Finally,

sediment traps were mounted on the bottom close to the measuring frame. The particle size distribution

of the trapped sediments was analysed in the laboratory using the Coulter LS 230.

The results of suspended sediment concentrations (SSC) show that the SSC as measured by the

LISST-ST is much lower than the SSC of the water samples in some cases, which is concluded to be

caused by improper (non-isokinetic) sampling of the LISST-ST instrument.

Figure 11 shows particle size distributions based on LISST-ST method and on the Coulter LS230 for

the water samples and sediment trap samples. The data only represents particle size fractions larger than

2.5 mm, since the LISST cannot measure the particle sizes smaller than 2.5 mm accurately. It can be

concluded that the LISST yields significant smaller fractions in the size classes < 13 mm and larger

fractions for particles > 25 mm. It should be noted however that the LISST-data are real in-situ

measurements whereas the other two methods refer to samples analysed in the laboratory (sample

transfer).

Rijkwaterstaat (2005) has used the LISST-100 and LISST-ST instruments during a field deployment

in the Dutch coastal zone of the North Sea during summer and winter periods in 2003 and 2004. The

LISST-100 instrument was found to be a robust and well-suited instrument for prolonged deployment.

The fouling problem is similar to that of other instruments; in summer the biofouling effects start to

occur after approx. 2 weeks. Even in this period, obvious changes in grain size distribution caused by

wave events could still be detected.

During the winter deployment, the LISST-ST instrument stopped functioning after 5 cycles. The

December storm was missed, and the successful cycles were all taken at quiet conditions. The LISST-ST

signal of the data at quiet conditions shows a small but clear tidal signal implying that the settling within

the tube is affected by the tidal current. As the instrument was only deployed for 3 days, it is not l ikely

that the problem is caused by fouling and improper closure of the lids. It is thus more likely that current-

induced vibrations cause resuspension of particles from the bottom or wall of the tube. Analysis of the data

shows rather small (unrealistic) sediment densities, which may be caused by resuspension of sediments in

the settling column due to instrument vibrations.

In the summer deployment (20 June - 24 July) heavy barnacle growth had covered all instruments by the

end of the deployment. Nevertheless, the LISST-ST instrument continued to measure for about 30

cycles and some results are useful, although it cannot be decided whether leakages occurred due to a bad

closure of the lids.

It is concluded that the LISST-ST is mechanically not sufficiently well developed and definitely not

suited for prolonged deployment in coastal conditions. The lids are fragile and the least growth of

barnacles impedes its proper functioning. The necessary cleaning procedure may have made the

instrument even more vulnerable to corrosion. Glued parts in the Perspex tube detached in the long run.

References

Agrawal, Y.C. and Pottsmith, H.C. 2002. Laser Diffraction Method: two new sediment sensors. SequoiaInc., USA (www.sequoiasci.com)

Agrawal, Y.C. and Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observationsin sediment transport. Marine Geology, Vol. 168, p. 89-114

Bale, A.J., Morris, A.W. and Howland, R.J.M, 1984. Size distribution of suspended material in the surfacewaters of an estuary as measured by Laser Faunhofer diffraction. In: Transfer processes in cohesivesediment systems edited by Parker and Kinsman. Plenum Publising, New york

Bale, A.J. and Morris, A.W., 1987. In-situ measurements of particle size in estuarine waters. Coastal andShelf Science, Vol. 24, p. 253-263

Bale, A.J., 1996. In-situ Laser opticle particle sizing. Journal of Sea Research, Vol. 36, p. 31-36Buurman, P., Pape, T. and Muggler, C.C., 1997. Laser grain size determination in soil genetic studies. 1.

practical problems. Soil Science, Vol. 162, No. 3, p. 211-228Delft Hydraulics, 2003. Analysis of LISST-ST. Report Z3671. Delft, The NetherlandsFugate, D.C. and Friedrichs, C.T., 2002. Determining concentration and fall velocity of estuarine particle

populations using ADV, OBS and LISST. Continental Shelf Research, Vol. 22, p. 1867-1886Gartner, J.W., Cheng, R.T., Wang, P.F. and Richter, K., 2001. Laboratory and field evaluations of the

LISST-100 instrument for suspended particle size determinations. Marine geology, Vol. 175, p. 199-219

Gray, J.R., 2004. The LISST-SL streamlined isokinetic suspended-sediment profiler. Proc. 19th Int. Symp. onRiver Sedimentation, Yichang, China.

Rijkswaterstaat/RIKZ, 2004. LISST-100 and LISST-ST in the Sandpit Project. Preliminary NoteRIKZ/OS/2004/106W, RIKZ, The Hague, The Netherlands

Thonon, I., Roberti, J.R., Middelkoop, H., Van der Perk, M. and Burrough, P.A., 2005. In-situmeasurements of sediment settling characteristics in flood plains using LISST-ST. Earth SurfaceProcesses and Landforms, Vol. 30, p. 1327-1343

Traykovski, P., Latter, R.J. and Irish, J.D., 1999. A laboratory evaluation of the Laser in situ scatteringand transmissometry instruments using natural sediments. Marine Geology, Vol. 159, p. 353-367

Van Wijngaarden, M. and Roberti, J.R., 2002. In situ measurements of settling velocity and particle sizewith the LISST-ST. In: Fine Sediment Dynamics in the Marine Environment. Edited by Winterwerpand Kranenburg, Elseviers Science

Figure 1A

Laser Diffraction patterns

Figure 1B

Schematic representation of the optical path and sampling volume; light scattered at the same angle fromparticles in different locations in the beam is focussed on to the same ring detector; unscattered and forwardscattered light passes directly through a hole in the center of the ring detector to measure transmission(Traykovski et al., 1999)

Figure 1C

LISST-Settling Tube instrument with the impeller and sliding doors. Top and bottom lids are not shown. Thecolumn is 5 cm in diameter, and 30 cm tall from the inlet to the Laser beam. Right: LISST-ST mounted on atripod (Agrawal and Pottsmith, 2000).

Figure 2

LISST 100 instrument; particles in the flow scatter light; a receiving lens collects the scattered light, whichis detected by the ring collector; a hole in the center of the ring detector permits the focused Laser beam topass through, where its power is sensed; this constitutes a transmission measurement, which corrects forattenuation of the scattered light that is sensed by the rings.

Figure 3

The use of shaped focal plane detector in LISST-25.

Figure 4

The LISST-SL instrument

Figure 5

Comparison of particle size distributions of polystyrene material in range of 1 to 40 micron based on threemethods (LISST-100; Laser Coulter Counter LS230 and Coulter Counter analysis of supplier Duke ScientificInc.), (Gartner et al., 2001)

Figure 6

Calibration constants determined in the laboratory using polystyrene spheres plotted against particle size;results from two LISST- instruments from USGS (US Geological Survey and SPAWAR Space and NavalWarfare Systems Center), (Gartner et al., 2001)

Figure 7

Time-series plots of suspended sediment concentration estimated by the LISST-100 and the OBS-instruments; LISST values are truncated above about 100 to 150 mg/l when concentration levels resulted inpercent transmission less than 20% (Gartner et al., 2001)

Figure 8

Time series plots (10/19/1998 to 10/23/1998) of:A: water levelsB: current speedsC: suspended concentrations by OBSD: mean particle size of suspended sediments; overall mean is 62 micronE: percent suspended material of 20 mm size class (open circles) and percent of 100 mm size class (filled circles)F: ratio percent of 20 mm size class to percent of 100 mm size class

Figure 9

Volume concentration as a function of time for 8 size classes, August 2000* data points____ model fit (+ deviation between model fit and data points)

Figure 10

Calibration curve of dry mass concentration from water samples and LISST-ST values

0

5

10

15

20

25

30

35

40

3.5 6.8 13.1 25.4 49.2 95.5 185 359

Grain size class [ m]

Fra

ction o

f to

tal G

SD

[%

LISST-ST

Sediment traps

Water samples

Figure 11

Particle size distribution for floodplain of IJssel River in The Netherlands (SW dataset)

6.5.6 In-situ photo and video camera

Principle

Eisma et al. (1990) has described the development of an in-situ photocamera (and image-analysis software)

for in-situ measurement of solid particles and aggregates (flocs) larger 4 mm. It can be used in depths up to

4000 m with concentrations up to 200 mg/l. In very clear ocean waters the system is not effcient because of

the large number of photographs that have to be taken to obtain a reliable size distribution.

The camera system consists of a steel frame (1.8 x 2 m) in which 3 cameras are mounted in such a way that

there is a minimum disturbance of the water flow through the frame. A vane keeps the camera windows

approximately parallel to the main flow sothat the particles are photographed approximately perpendicular to

their direction of movement. Two cameras are each directed horizontally towards a flash light mounted

opposite the camera at a distance of 32 mm. A third camera is mounted above the other two cameras with a

flashlight oblique directed at an angle of 60 degrees towards the view-area of the camera. This camera is

used to obtain photographs of the larger particles and other materials that may be present in the water. The

cameras are professional NIKON F3 cameras with a 250-exposure magazine. The lenses are 35 and 50 mm

(f/1.4). Teflon beads of known size have been used to calibrate the photographic results. The camera system

produces a large number of photographs, which are analyzed by an automated particle image-analysis

system.

Figure 2 shows a photograph of flocculated sediments in the Scheldt estuary, The Netherlands

Van Leussen and Cornelisse (1991, 1993) and Fennessy et al. (1994) have developed in-situ video camera

instruments which can determine both the size and the settling velocity of the solid particles and the

flocculated sediments.

The in-situ video camera (VIS, see Figure 3) of Van Leussen and Cornelisse consists of a small vertical tube

with a closed end at the bottom in which particles are settling down in still water. Two small windows are

present in the tube for enlighting (light beam) and for video-recordings. The instrument is connected by a

signal cable to the survey ship which floats with the current during sampling. Floc sizes and settling

velocities are obtained from the recordings by computer analysis. Figure 4 shows measured settling

velocities through the tidal cycle in the Ems-Dollard estuary (Van Leussen, 1994). It can be observed that

the settling velocities measured by the VIS are significantly larger than those obtained from a mechanical in-

situ settling tube (Owen tube). This is caused by two effects: 1) the in-situ video camera VIS has a lower

limit of about 50 mm, the finer particles are not detected and 2) the flocs trapped in the mechanical settling

tube may have been disrupted during the sampling process. Van der Lee (2000) has succesfully used the in-

situ camera in the Dollard estuary, The Netherlands. He has also assessed the erros involved in the

determination of the floc sizes and settling velocities (from consecutive images at intervals of 0.08 s). The

lower detection limit of the VIS was determined to be about 80 mm. Analysis of some enlarged images from

a second camera shows that about 10% to 20% of the (smaller) flocs cannot be detected. Hence, there is an

overestimation of the size of the flocs by VIS. Determination of the settling velocity of the flocs is

complicated by the presence of vertical water motions in the settling tube of the VIS. Van der Lee estimated

the water velocity to be equal to the observed settling velocity of the smallest 5% of the flocs. This value was

subracted from all measured results. On average, the relative errors in floc size and settling velocity were

estimated to be in the range of 10% to 20%. The results are biased to the largest flocs with the largest settling

velocity. This may not be too serious, as the larger flocs are most relevant for sedimentation processes.

Fennessy et al. (1994) and Manning and Dyer (2002) have developed an in-situ settling velocity instrument

(INSSEV) based on video camera recordings. The instrument comprises a computer controlled chamber

(decelerator) with closing doors to slowly collect a sample of water and sediments, from which some of the

suspended materials is allowed to enter the top of a settling tube (settling length of 110 mm). The settling

flocs are viewed using a miniature video system, see Figure 5. Subsequent analysis of video tapes can

provide direct observations of the size and settling velocity of individual flocs in the size ranges of 20 to

1000 mm. The video recordings show large numbers of low density flocs with multiple structures linked by

fine organic fibres. To ensure stability and remove vertical motions during the settling process, the

instrument is to be attached to a bed-mounted tripod. The camera system uses high resolution videocamera

and six high-intensity red LED’s for illumination of the flocs arranged in an annulus around the camera lens.

Figure 6 shows time series of floc sizes and settling velocities based on INSSEV observations (Dyer et al.,

2002) at 0.5 m above the bed during spring tide in the Tamar estuary (UK). Micro (<150 mm) and macro

(>150 mm) flocs are separately shown. The concentrations are as large as 3000 mg/l. The mean floc diameter

increase from about 100 to 200 mm and the settling velocities increase with size to about 2 mm/s. The

effective density (rs-rw) decreases from 330 to about 150 kg/m3.

Dyer et al. (1996) have tested two in-situ video systems (VIS and INSSEV) and various mechanical in-situ

settling tubes in an intercomparison experiment carried out in the turbidity maximum in the mouth of the

Elbe Estuary in Germany (water depth of about 18 m; peak velocities of about 1 to 1.5 m/s). The mechanical

settling tubes generally produce settling velocities which are an order of magnitude smaller than the direct

video measurements which is an indication that the mechanical tubes disrupt the flocs on sampling. The

video systems VIS and INSSEV appear to give comparable results. The settling processes in one of the

mechanical tubes (OWEN tube) were studied by using a high magnification video camera (Dearnaley,

1996). These results reveal floc breakup, reflocculation and generation of circulation within the tube during

and after sample withdrawal. The settling velocites inside the tube based on video recordings are larger than

those derived from the standard gravimetric method from the same tube. The settling velocities of the video

are similar to those of the VIS and INSSEV.

Advantages

1. simultaneous measurement of particle size and fall velocity

2. reliable information of floc sizes

3. statistical information of particle size variations and fall velocity variations

4. no destruction of samples during measuring period

Disadvantages

1. very labour-intensive analysis of data;

2. automatic analysis software may not be sufficiently accurate (verification required)

3. can not be used in stand-alone mode

4. relatively large equipment; cannot be used close to bed

References

Dearnaley, M.P., 1996. Direct measurements of the settling velocities in the Owen tube: a comparison withgravimetric analysis. Journal of Sea Research, Vol. 36, p. 41-47

Dyer, K.R. et al., 1996. A comparison of in situ techniques for estuarine floc settling velocity measurements.Journal of Sea Research, Vol.36 (1/2), p. 15-29

Dyer, K.R. et al., 2002. The turbidity maximum in a mesotidal estuary, the Tamar Estuary, UK: Part II, Thefloc properties. In: Fine Sediment Dynamics in the Marine Environment edited by Winterwerp andKranenburg, Elseviers Science

Eisma, D. et al., 1990. A camera and image-analysis system for in-situ observation of flocs in naturalwaters. Netherlands Journal of sea resaerch, Vol. 27, No. 1, p. 43-56

Fennessy, M.J., Dyer, K.R. and Huntley, D.A., 1994. INSSEV: an instrument to measure the size andsettling velocity of flocs in situ. Marine Geology Vol. 117, p. 107-117

Manning, A.J. and Dyer, K.R., 2002. The use of optics for the in-situ determination of flocculated mudcharacteristics. Journal of Optics A: Pure and Applied Optics, Vol. 4, p. S71-S81

Van der Lee, W.T.B., 2000. The settling of mud flocs in the Dollard estuary, The Netherlands. DoctoralThesis. Dep. of Physical Geography, University of Utrecht, Utrecht, The Netherlands

Van Leussen, W., 1994. Estuarine macroflocs and their role in fine-grained sediment transport. DoctoralThesis. Dep. of Physical Geography, University of Utrecht, Utrecht, The Netherlands

Van Leussen, W. and Cornelisse, J.M., 1991. Direct measurements of sizes and settling velocities of mudflocs in the Ems estuary. Paper WGSL 18/91, Delft Hydraulics, Delft, The Netherlands

Van Leussen, W. and Cornelisse, J.M., 1993. The determination of sizes and settling velocities of estuarineflocs by an underwater video system. Netherlands Journal of sea research, Vol. 31, p. 231-241

Figure 1

In-situ photocamera (Eisma, et al., 1990)

Figure 2

Photograph image of flocculated material in Scheldt estuary, (Eisma, 1990)

Figure 3

In-situ video camera VIS (Van der Lee, 2000)

Figure 4

Settling velocities through the tidal cycle by VIS in Ems-Dollard estuary (Van Leussen and Cornelisse, 1993)

Figure 5

INSSEV video camera system for flocs and settling velocity of flocs (Fennessy et al., 1994; Manning andDyer, 2002)

Figure 6

Time series of floc sizes and settling velocities based on INSSEV observations (Dyer et al., 2002)Top: Settling velocity (ws), sediment concentration (SPM) and floc sizeMiddle: Effective density and bed-shear stress parameter (G)Bottom: sediment concentration (SPM) and bed-shear stress parameter (G)

6.5.7 Particle size and velocity by Phase Doppler Anemometry (PDA)

PDA is an extension of Laser Doppler anemometry (LDA) and can determine not only the Doppler

shift frequency of light refracted by a particle within the flow (hence its velocity) but also the phase shift

as observed at three different receiving locations which can be utilized to derive the diameter of the

scattering particle. Assuming constant density and spherical particles, the volume concentration can be

determined. Hence, the simultaneous measurement of particle size, velocity and concentration can be

obtained using phase Doppler anemometry as an extension of the principles of LDA (Buchhave, 1987;

Bachalo, 1994). As a transparent particle passes through the measurement volume, the interference fringes

are refracted by an amount proportional to the diameter of the grain. Three detectors, located at different

positions within the receiving optics, will observe the same frequency but with a relative phase shift

proportional to the detector spacing divided by the projected fringe spacing. The separation of the

projected fringes at a large distance from the measuring volume is proportional to the effective focal

length of the particle, which, for a spherical particle, is proportional to the particle diameter. Hence the

measured phase shift is proportional to the particle diameter.

A commercial instrument is provided by DANTEC (www.dantecdynamics.com).

Laboratory experiments

Bennet and Best (1995) have used the PDA method (two-component DANTEC 100 mW argon-ion laser

with fibre-optics operated in side-forward scatter mode) to determine its potential for measuring

suspended sediment transport. The flow in a small perspex container with an oscillating grid was

analyzed using a DANTEC Flow Velocity Analyzer processor (correlation-type), 40 MHz frequency

shift, and a 600 mm focal length lens with a beam separation distance of 0.025 m at the lens surface. In

the processing software, the maximum phase error was set at 15 degrees and the maximum spherical

deviation was set at 35 percent. With this configuration, typical uncertainty estimates were 1 percent for

the velocity measurements (with a resolution of 0.006 m/s) and 4 percent for the particle size

measurements (with a resolution of 2 mm in the range of 5 to 500 mm). The Laser intersection volume

was located within 10-15 mm from a grid bar intersection. At this fixed coordinate, vertical velocity

profiles (one component only) were taken through the container to determine the vertical fluid and

sediment velocities and suspended sediment fluxes as a function of both height away from the grid and

oscillation frequency. A 300 second sample was taken at heights above the grid of 5, 10, 15, 20, 25, 30,

35, 40, 45, 50, 60, 70, 80, 100, 120, and 150 mm. Typical sampling rates varied from 30 to 1000 Hz

(normally 400 Hz) depending upon the chosen frequency and height above the grid.

The instrument was operated from outside the experimental container, which consisted of a perspex

container (0.254 x 0.254 x 0.45 m high) with a grid of square bars, 0.01 m wide, 0.22 m long and spaced

0.05 m apart, located near and parallel to the base of the container. This grid was fixed by a central arm to

an eccentric drive which provided an oscillation amplitude (perpendicular to the base of the container) of

approximately 0.019 m. The center of the grid was maintained at a constant mean height above the base

in all experiments. Grid oscillation frequency was controlled by a voltage regulator and four frequencies

(in range of 1 to 3.4 Hz) were chosen so that each sediment population within a particular experiment

remained immobile at the lowest frequency. The entire apparatus was mounted to a baseboard, which had

a scissor jack attached at each corner. This configuration allowed the container and its controls to be

raised and lowered with an accuracy of ±0.5 mm using millimeter-graduated scales fixed to the sides of

the base, thus permitting acquisition of vertical velocity profiles without requiring movement of the Laser

optics. The mixing container was filled with unfiltered tap water to a depth of 0.25 m.

The sediment grains used were glass beads (density of 2600 kg/m3 with a refractive index of 1.5),

chosen because: (1) their shape is close to spherical and they are of quartz density; (2) their

transparency allows the PDA to be used in refraction mode; and (3) large quantities sufficient for mobile

bed flume experiments can be purchased at moderate cost. Three size distributions of glass beads were

used with nominal sieved diameters of 75-95 mm (fine), 180-210 mm (medium), and 300-355 mm

(coarse). The total dry mass of sediment (concentration) added to the container for the fine, medium,

and coarse populations was 0.31 g/1, 2.48 g/1 and 6.2 g/l, respectively. Each sediment population was

initially spread evenly across the base of the box. In most experiments, the sediment supply available for

suspension was not exhausted and therefore the flow could reach an equilibrium capacity, although no

particles remained on the base of the box at the highest oscillation frequencies using the fine sediment.

Figure 1 shows the frequency distributions for all PDA measured grain diameters for each sediment

population at two different oscillation frequencies at a fixed height of 5 mm within the container.

The majority of grain sizes observed within the pure fluid experiments (>90 percent) were less than

50 mm, with 85 percent less than 30 mm. These measured grain sizes represent colloidal and natural

particulate matter in the water. The frequency distributions for the fine sediment population show peaks

at 50-70 mm and 60-90 mm for the low and high oscillation frequencies, respectively. At the lower

frequency (1.9 Hz), the signal illustrates the equal detection of fluid particles and sediment grains,

while at the higher frequency (3.4 Hz) the signal is completely dominated by the sediment particles in

suspension. For the two other sediment populations used, few sediment grain sizes are observed at the

lower frequency whereas appropriate peaks are observed at 200-230 mm for the medium sediment and

at 310-340 mm for the coarse sediment at the higher frequency. It is apparent that, for a given range of

grain sizes introduced into the container, discrimination can easily be made between the particles

representing the fluid (<30 mm) and the sediment particles suspended in the flow. However, a larger

range of grain sizes is observed by the PDA when compared to the sieved grain size range. The range

of grain sizes measured by the PDA varies from about 25-125 mm for the fine sediment, 100-300 mm

for the medium sediment, and 200-400 mm for the coarse sediment.

Best et al. (1997) and Bennett et al. (1998) discuss the errors involved using the PDA instrument in flume

experiments (see Figure 2), which arise from:

· variable sample size and concentration between different measurement locations;

· cross talk between the two phases, in which fluid and particles may be incorrectly classified, may be

caused by errors in grain sizing caused by asphericity of sediment and contamination of the flow;

· positioning and relocation of the measurement volume.

Particle sizing errors were also assessed through comparison of the PDA-derived grain size-distribution

with that obtained through Coulter Counter analysis, yielding relative errors smaller than 3% for sand-

sized particles. Errors in concentration are relatively large (DANTEC, 1994), especially at higher

concentrations (>1 gr/l) where signal attenuation becomes problematic. Figure 3 shows good agreement of

PDA concentrations and siphon sampler concentrations in the upper half of the flow where concentrations

are relatively small (<1 g/l). The PDA concentrations become progressively less than the siphon

concentrations as the bed is approached. The PDA method is not capable of recording every sediment

grain that passes through the measurement volume when sediment concentrations in the flow are high.

Advantages

1. rapid and non-intrusive determination of simultaneous particle velocity, size (5 to 500 mm) and

concentration;

2. fast response enabling turbulence measurements.

Disadvantages

1. calibration required for non-spherical particles;

2. not usable in high-concentration flows (>1 g/l);

3. laboratory instrument; problematic in field conditions.

References

Bachalo, W.D., 1994. Experimental methods in multi-phase flows. International Journal of Multi-PhaseFlows. Vol. 20, Suppl., p. 261-295

Bennett, S.J. and Best, J.L., 1995. Particle size and velocity discrimination in a sediment laden turbulentflow using Phase Doppler Anemometry. Journal of Fluids engineering, Vol. 117, p. 505-511

Bennett, S.J., Bridge, J.S. and Best, J.L., 1998. Fluid and sediment dynamics of upper stage plane beds.Journal of Geophysical Research, Vol. 103, No. C1, p. 1239-1274

Best, J., Bennett, S., Bridge, J. and Leeder, M., 1997. Turbulence modulation and particle velocities overflat sand beds at low transport rates. Journal of Hydraulic Engineering, Vol. 123, No. 12, p. 1118-1125

Buchhave, P., 1987. A new instrument for the simultaneous measurement of size and velocity of sphericalparticles based on the Laser Doppler method. Dantec Electronics, New Yersey, USA

Figure 1

Grain size frequency distributions for all sediment populations at a height of 5 mm above the grid foroscillating frequencies of 1.9 and 3.4 HZ (bin size of 10 mm), (Bennett and Best, 1995)

Figure 2

Experimental set-up in laboratory flume (Bennett et al., 1998)

Figure 3

Measured concentration profiles uding PDA method and Siphon samples (Bennett et al., 1998)



6.5.8 Particle size by Laser Reflectance (PARTEC Laser)

In-situ Laser diffraction techniques are severely limited in their use by the presence of high sediment

concentrations larger than about 0.5 to 1 g/l. This limitation can be overcome by sing in-situ Laser

reflectance techniques (Law et al. 1997), see Figures 1 and 2. The PARTEC 100 is a commercially

available, Laser reflectance particle-sizing instrument which was initially designed for process control in the

grinding and milling industries with concentrations in the range of 10 to 100 g/l. The sensor is computer-

operated and the output of the PARTEC 100 consists of a histogram of 38 logarithmic size intervals over the

size range 2 to 1000 mm.

The measuring principle employs an optical beam which is directed through a lens located eccentrically on a

rotating disc within the reflectance probe such that the focal point describes circles of 8.4 mm in diameter.

The light source is a semi-conducting Laser diode. As the focal point is typically smaller than the suspended

particles and moving with a greater velocity, reflected light signals are assumed to be related to individual

particles. When the sensor probe is immersed in a sample, measurements of reflected pulses are accumulated

for a set period, typically 3 to 25 s depending upon particle numbers, and a particle chord size distribution is

calculated. A correction algorithm, which assumes the particles are spheres, allows a distribution of spherical

equivalent diameters to be calculated. Using this data, the system software generates size distributions which

may be presented as a percentage of either the total number of particles or the total volume of particles

encountered. The measuring zone is confined to a small volume in the vicinity of the focal point by the use

of an electro-optical discrimination procedure based on the rise and decay times of the reflected signals.

Individual particles are only registered if the leading and trailing edges of their reflected signal have a

sufficiently sharp or rapid rise from background noise. Multiple reflections are ignored. If both signal edges

possess a sufficiently fast rate of change, the measured duration between the two points is added to the

sample data. These are direct measurements, which avoid signal smoothing or curve fitting techniques. The

high power of the focused PARTEC 100 Laser beam allows the instrument to operate in conditions with

high particle concentrations.

To facilitate in-situ measurements in water depths up to 100 m, the probe unit is mounted in a watertight

cylinder made from PVC plastic.


Law et al. (1997) have evaluated the PARTEC 100 under laboratory conditions using commercially

available calibration particles which include Pollen (with mean diameters of 7.4, 29.7 and 78 mm), latex (19

mm) and glass beads (8, 31.5 and 85.7 mm). A number of secondary standards (mean diamaters of 6.5, 23.6,

39.5, 89.4 and 171.7 mm) were produced by fractionating ashed sediment using settling time criteria.

Samples of large particles were obtained by sieving sand to produce various factions (up to 766 mm). The

size distributions of the secondary standards were determined using electro-resistive (Coulter Counter) and

Laser diffraction (Malvern) instruments for the finer materials and optical microscopy combined with an

image-digitising package for the large grains.

Focused beam reflectance measurements of these standards were carried out with the sensor probe immersed

in a suspension of the particles contained within a glass beaker. During each series of measurements the

particles were kept in suspension using a variable speed, electrically driven impeller. The influence of

particle concentration was studied by using suspensions in the range of 10 to 50,000 mg/l.

For all types of sediments the size distributions compared well with those obtained from alternative systems.

Analysis of materials such as latex and glass gave erroneous results due to insufficient reflectance.

Furthermore, the PARTEC 100 progressively oversized particles below 150 mm with increasing errors for

decreasing sizes (up to 30%), whilst undersizing particles above 500 mm (up to 10%). These systematic

deviations can be eliminated by calibration. Measurements of particle size distributions at a number of

concentrations indicate the presence of a slight trend towards larger mean sizes with increasing

concentrations (about 30% increase in mean size for concentrations between 100 and 50,000 mg/l), which is

caused by the inability of the instrument to resolve separate particles at extremely high concentrations. At all

concentrations a minimum of about 1000 reflected counts is needed to be registered to obtain statistically

reliable results. This means that the size distribution of 10 mg/l with particles of 5 to 10 mm can be analysed

in about 3 minutes, whereas 10 mg/l with sand sized particles would require about 30 minutes. The particle

velocity was not found to have any effect on the size distribution. It is concluded that estuarine particles in



the size range of 20 mm and larger can be measured with some confidence using a fixed focal distance of 0.8

mm. If the sample consists primarily of particles smaller than 20 mm, a focal distance setting nearer to 0.2

mm is required.

Law et al. (1997) have also performed field trials using the PARTEC100 Laser reflectance technique in the

Tamar and Humber estuaries (UK). Particle size measurements were taken at 1 m intervals from the surface

throughout the water column, taking typically 2 to 5 minutes at each depth, dependent upon particle

concentration (based on OBS method). The results confirmed the presence of large flocs in the range of 50 to

500 mm. The method cannot be used in conditions with relatively large organic materials (insufficient

reflectance).


200/300) have shown that in-situ determination of particle size distributions of fluvial sediments is of


samples and measuring the particle sizes directly after sampling. On site measurements of bottle samples

were broadly similar to direct in-situ measured size distributions. Analysis results of water-sediment

samples collected in bottles and returned to the laboratory show significant differences in particle size

distributions due to floccutation of sediments in the bottle samples, even if the sediments in the bottle are

artificially resuspended. In general the longer a sample was allowed to settle the greater the increase in

volume mean size upon resuspension. The bonding of flocs formed during the settling period appears to

become stronger with time.

Advantages

1. rapid in-situ analysis of particle size distributions

2. usable in conditions with relatively large concentrations (>0.5 gr/l)

3. very accurate for sand particles larger than 30 mm

Disadvantages

1. results based on assumption of spherical particles; finer particles are oversized, larger particles are

undersized; calibration is required to reduce errors

2. particles should have sufficiently high reflectance; flocs with organic materials cannot be detected

3. different focal distances are required for finer and coarser particle ranges

4. long counting times in case of low concentrations

References

Law, D.J., Bale, A.J. and Jones, S.E., 1997. Adaptation of focused beam reflectance measurements to in-situ particle sizing in estuaries and coastal waters. Marine Geology Vol. 140, p. 47-59

Phillips, J.M. and Walling, D.E., 1995. An assessment of the effects of sample collection, storage andresuspension on the representativeness of measurements of the effective particle size distribution offluvial sediment. Water resources, Vol. 29, No. 11, p. 2498-2508



Figure 1

Details of the Laser probe head and measuring geometry (Phillips and Walling, 1995)

Figure 2

a) comparison of the volumetric size distribution for 447.6 mm sand measured by PARTEC 100 (opencircles; upper curve) and optical microscopy (open diamonds; lower curve);b) comparison of mean particle sizes by PARTEC100 (on vertical axis) and other methods (on horizontalaxis) for a range of standard materials (Law et al., 1997)diamonds=Pollencircles= Ashed sedimentssquares=Sandstriangles= Algal cellsOpen symbols represent pre-calibrated results; closed symbols represent calibrated values.

CHAPTER 7: INSTRUMENTS FOR BED MATERIAL SAMPLING February 2006


7.2 Bed material samplers: grab, dredge and scoop samplers

Principle

Grab, dredge and scoop-type samplers are used to collect a bed-surface sample (Figures 1 and 2).

A grab sampler consists of two buckets or jaws which are in an open position during lowering of the

sampler. After contact with the bed the buckets are closed by using a messenger system or by pulling the

hoisting cable. For coarse and/or firm bed material a dredge-type sampler should be used.

Simple and good samplers are the SHIPEK grab sampler and the VAN VEEN grab and dredge samplers.

A scoop-type sampler (Fig. 2) consists of a single scoop-type bucket which swings out of the bottom of the

sampler body. The bucket surrounds and encloses the bed material sample. An advantage of the US BM-54

scoop sampler is its streamlined body enabling sample collection in high-velocity conditions.


SHIPEK grab

sample size: 2 liter

weight: 70 kg

VAN VEEN grab and dredge

sample size: 0.5 liter (small grab)

2 liter (medium grab)

6 liter (large dredge)

weight: 2.5 kg (small grab)

5 kg (medium grab)

20 kg (large dredge)

US BM-54 scoop

sample size: 0.5 liter

weight: 50 kg

Disadvantages

1. collection of a disturbed surface sample,

2. loss of sediment during raising when sampler is not fully closed,

3. generation of shock waves which may wash away fine sediment fraction.



Figure 1

Grab and dredge sampler



Figure 2

US BM54 scoop sampler

7.3 Bed material samplers: core samplers

Principle

Core sampling consists of driving a tube into the bed material through the use of manpower, gravity,

hydrostatic pressure or vibration.

A simple hand corer can be used in shallow streams, which can be waded or on tidal flats. The lower end of

the sampler contains a cylinder which is pressed into the bed (Figure 1A). A piston with a handle on its upper

end passes through the sampler frame. The piston is retracted when the cylinder is pressed into the bed

material. The suction created by the piston holds the sample in the cylinder.

Box core samples can be taken by using a box corer of about 300 kg lowered to the bed by use of a cable-

winch system. A bed surface core sampler (sand to clay) is taken by mechanical penetration (box is pressed

into the bed mechanically). This device takes a core length of about 0.5 m.

The gravity (or free-fall) corer is allowed to fall freely through the water and is driven into the bed by its

weight (Fig. 1B,C). A one-way valve at the top end of the tube permits the passage of water during the

descent and prevents flushing of the sample during retrieval and raising of the sampler. A core-catcher

generally is present at the inside of the tube just above the cutting edge. Plastic liners generally are used to

minimize the problem of sample extrusion and storage. The core length is limited to 10 core diameters in

sand and 20 core diameters in firm clay. A major disadvantage of gravity corers is the compaction of the

vertical structure of the bed material during sampling.

A Piston corer can be used to reduce the compaction during sampling. This sampler is essentially a gravity

corer, but it has an internal piston which remains at the level of the water-sediment interface when the corer

penetrates into the bed (Figure 2A). The corer is attached to a trip mechanism which is released when a

counter weight hits the bed. The piston creates a slight vacuum above the sample and is supposed to reduce

friction and to prevent compaction.

Buckley et al. (1994) reviewed the problems with piston core sampling. A series of sampling experiments

using a piston corer equipped with data logging instruments has demonstrated that representative

stratigraphic intervals of soft marine sediment may never be obtained with piston corers that use a non-

reference triggering system. Incidents of sediment by-passing and flow-in can occur throughout a single core,

resulting in both foreshortening and stretching of sedimentary units. These problems are difficult to

recognize by standard visual core logging techniques. Most of the problems are initiated by accelerations in

the lowering cable that occur after tripping, with resultant accelerations in the piston, and fluctuations in the

cavity pressure inside the core liner. The non-representative sampling can result in more than 1 m of missing

stratigraphic units below the sediment interface, and as much as 1.3 m of sediment flow-in. These problems

can lead to miscalculations of sediment deposition rates by as much as a factor of 2, and can result in

obtaining samples that are of no value for geotechnical analysis.

Vibration corers are used when core samples with a length upto 10 metres are required in all types of bed

material with exception of rock and stiff clay. The corer is driven into the bed (Fig. 2B) by vibration

equipment mounted on top of the corer. Some vibrocorers use pistons, valves and compressed air to

assist sample retrieval. Sample disturbance is inevitable but this can be restricted to the outer part of the core.

Vibrocorers require a relatively large survey vessel and skilled personnel.


Hand corer

weight: 10 kg

core length: 0.2 m

core diameter: 0.05 m

Box corer

weight: 300 kg

core length: 0.5 m


Gravity corer PHLEGER

weight: 35 kg

core length: 0.60 m


BENTHOS

weight: 250 - 450 kg

core length: 3 - 6 m


VIBRO-CORER SENKOWITCH

Practical operation

1. check electronic and air supply system,

2. install core barrel (with core liner and core catcher) and attach core barrel to the vibrator by means

of closure nut (seal closure nut),

3. open air supply system and check for leakage (use an overpressure of 0.5 Ato),

4. lower frame to the bed,

5. check vertical orientation of corer (maximum angle of deviation =7°),

6. switch on vibro motor and close air supply,

7. release air-pressure in the core liner (gently),

8. regulate pressure-difference between in- and outside of core liner (pressure-difference should be

minimum),

9. check core depth (maximum depth when air pressure remains constant after closing air escape valve),

10. close air escape valve and switch off vibro motor,

11. raise corer (use vibrator if necessary),

12. replace core liner.


dimensions: Corer installation

(frame, vibrator, guiding colums)

3 x 3 x 1 m

vibro motor 0.5 x 0.2 x 0.2 m

air compressor 0.5 x 0.5 x 0.5 m

cable haspel 0.7 x 0.7 x 0.5 m

control panel 0.7 x 0.6 x 0.2 m

weight: corer installation 500 kg

accessories and spare parts 50 kg

vibro motor 20 kg

air compressor 100 kg

cable haspel 100 kg

control panel 60 kg

core length: 2 m


cable length: 70 m

maximum water depth: 100 m

energy: 220 volt (AC), 50 Hz, 660 watt

cycle period: 60 min (between two

measurements)

References

Buckley, D.E., MacKinnon, W.G., Cranston, R.E and Christian, H.A., 1994. Problems with piston coresampling: mechanical and geotechnical diagnosis. Marine geology, Vol. 117, p. 95-106.

Figure 1

Core samplers

Figure 2

Core samplers

7.4 Particle size of bed materials

7.4.1 Based on metallic trace elements (MEDUSA)

The MEDUSA system (www.medusa-online.com) can be viewed as a small soil/sediment-sensor that

determines soil composition in-situ (under water as well as in air). The system is capable of continuously

measuring very low concentrations of a number of metallic trace elements (cesium, cobalt, potassium,

uranium and thorium) to a depth of about 30 cm inside soil. Moreover, the system measures water depth

and includes sensors to determine the intensity of friction sound, generated when the detector is dragged

over the sediment bed. The gamma-radiation detector system (based on Berillium Germanium Oxyde

crystals) is towed (Figure 1) over the seabed behind a ship in lines with a spacing of about 500 m.

Software performs on-line data logging and on-line creation of data maps. After completion of the

survey, the measured data are converted to composition (percentage of clay, silt and sand) of the

sediment at each measured position. Depending on the requested level of detail, some 100 to 500 square

km can be mapped per week. Based on a sampling tow velocity of 2 m/s and a sample analysis period of

10 s (for spectral analysis), the spatial resolution is about 20 m (one measurement per 20 m).

To get an accurate description of the quantitative composition of the upper 30 cm of the sediment bed,

gamma radiation is measured. Virtually any sediment type contains tiny, but characteristic amounts of trace

elements that emit small amounts of gamma-radiation (Koomans, 2000 and Roberti, 2001). All minerals

contain different concentrations of these elements. The concentration of Uranium and Thorium in clays is

about 10 times larger than that in sand whilst the activity concentrations within grain size fractions can vary

up to a factor 2. The concentration of these trace elements is determined by measuring the natural gamma-

radiation emitted by the sediment. The MEDUSA technique uses the characteristic amounts or "fingerprint" of

sediment fractions to translate the measured trace element concentrations into a quantitative value of

sediment composition. Calibration using in-situ samples is required for accurate determination of the bed

composition.

The MEDUSA system deploys a sensor (microphone) to monitor the friction sound, generated when the

detector is towed over the sediment bed. The level of this friction sound can be related to the grain size of the

sediment: the larger the grain size, the louder the friction sound. In contrast to the gamma-radiation, friction

sound intensity provides information on the upper layer (~1 cm) of the sediment bed.

The combination of both sensors gives therefore unique information on the vertical structure of the upper

strata of the underwater bottom. In-situ calibration can be obtained by taking bottom samples (boxcore

samples) at regular intervals. Analysis of these samples should be focussed on: Rontgen-Fluorescence

measurements (XRF), particle size analysis using Laser diffraction methods and photographs of borecores.

The MEDUSA technique has been successfully used in various projects of Rijkswaterstaat, The

Netherlands (Roberti, 2001): monitoring of dredged spoil at dumping sites in the North Sea and in the

Westerschelde Estuary; sand-mud maps of Hollands Diep and Haringvliet (fresh water basins), The

Netherlands; sand-gravel maps in large-scale rivers.

References

Kooman, R.L., 2000. Sand in motion; effects of density and grain size. Doctoral Thesis, University ofGroningen, Groningen, The Netherlands

Roberti, J.R., 2001. Manual for using MEDUSA (in Dutch). Report RIKZ/2001.035.Rijkswaterstaat/RIKZ, The Hague, The Netherlands

Figure 1

Towing of MEDUSA sensor along sediment bed

7.4.2 Based on acoustic reflection (ROXANN)

RoxAnn (www.sonavision.co.uk) is a remote sensing hydro-acoustic sensor providing seabed classification

data to produce seabed bottom type maps. RoxAnn uses a patented technique to extract data on bottom

roughness and hardness from the first and second echosounder returns from the seabed. It interfaces with a

Global Positioning System (GPS) and PC enabling real-time seabed classification and mapping of geological

and biological features using RoxMap Software.

The RoxAnn Hydrographic system interfaces with the existing echosounder and GPS/DGPS on board of

the survey vessel and data is displayed via the RoxMap software program on the survey PC, creating colour

coded seabed material maps in 2 and 3 dimensions and in real-time. It is available for operation with

echosounder frequencies from 15 KHz to 210 KHz, either as a single or dual or four frequency system.

The RoxAnn Swath is the latest development of the RoxAnn System providing multi-beam mapping of

seabed materials. This allows the surveyor to increase coverage of the seabed on each pass, thus decreasing

survey time and costs. The system uses seabed reflectivity to classify material type; accurately calibrated

(200 KHz or 50 KHz classification; centimetric bathymetry array and output).

This system (www.sonavision.co.uk) has been in use for over a decade in a wide variety of applications and

has a number of benefits to the user including being easy to use.

The RoxAnn system is an entirely automatic signal processing unit designed to supply seabed sediment

hardness (similar to acoustic impedance) and sediment texture, or topographical roughness, information

derived from echo soundings. The system processing unit is connected in parallel with the onboard echo

sounder (33 KHz; 210 KHz). The system derives its information from the first and second echoes of a single

transmission from a single beam echosounder. The index E1 is derived from the first echo and is the direct

reflection from the seabed. Index E2 is produced from the second echo, or first multiple, and is hence related

to the hardness of the seabed. Since every sediment material has a unique signature, correlation of E1 and

E2 data is accomplished through appropriate sediment sampling, or ground truthing. In order to provide

meaningful E1 and E2 data for a given survey area, the RoxXann system requires an initial calibration to

adjust to the specifics of the echosounder and its transducer. This is carried out over known seabed conditions

in a specific range of water depths. The type of seabed required for calibration depends on the frequency of

operation. The manufacturer's recommendation for the 33-KHz low frequency system is to perform the

calibration over a sandy bottom in a water depth between 100 to 175 feet.
























composition.













References



Figure 1































7.5 Movement of bed material particles

7.5.1 Critical bed-shear stress for initiation of motion

The beginning of movement of bed material particles (especially mixtures of clay, silt and sand) can be

determined by using in-situ erosion flumes and erosion containers.

Houwing and Van Rijn (1998) have used an in-situ erosion flume (ISEF) to determine the critical bed-

shear stress for initiation of motion of sand beds and mixtured beds of clay, silt and sand. ISEF is a

circulating flow system in the vertical plane. It consists of a lower horizontal test section, two bend sections

and an upper section where the flow is generated by a propeller, see Figure 1. The horizontal test section and

the two bend sections have a rectangular cross-section with a height of 0.1 m and a width of 0.2 m. The

bottom part of the test section is open over a length of 0.9 m. The propeller can be rotated at various speeds

by means of an adjustable oil pump system. The flow velocity in the horizontal test section can be measured

using electromagnetic sensors. Sediment concentrations can be measured using optical sensors and pump

samples. The bed-shear stresses derived from measured velocity profiles were calibrated using various beds

of sand particles (of known sizes). The critical bed-shear stress of these sand particles can be determined

from the Shields curve.

ISEF was used to determine the bed strength of mud beds. Figure 2 shows a typical result with increasing

mud concentrations as a function of imposed flow velocity. Since the water volume in the flume and the

sediment concentrations are known, the eroded sediment layer thickness can be derived when the bulk

density of the bed material is known (from separate measurements). The bed-shear stress required to erode

each layer of sediment can be determined from the measured velocities (Houwing, 2000).

Andersen et al. (2002) have used a small-scale perspex tube with a propeller (EROMES) to determine the

critical bed-shear stress of mud bed samples collected in the Hollands Diep, The Netherlands. The field

experiments were carried out on samples taken from the bed by means of a box corer. The instrument

consists of a 100 mm diameter perspex tube which is pushed into the sediment bed sample. The tube is

gently filled with local water and the propeller is placed on top of the tube. An optical sensor is used to

measure the mud concentrations. The propeller revolutions are transferred to bed-shear stress values by use

of a calibration curve based on the onset of erosion of sand particles with known critical bed-shear stresses.

Similar instruments have been used by others (Schünemann and Kühl, 1991; Cornelisse et al. 1997; Gust

and Müller, 1997).

References

Andersen, T.J., Houwing, E.J. and Pejrup, M., 2002. On the erodibility of fine-grained sediments in aninfilling freshwater system, p. 315-328. Fine Sediment Dynamics in the Marine Environment, edited byWinterwerp and Kranenburg, Elseviers Science

Cornelisse, J.M. et al. , 1997. On the development of instruments for in situ erosion measurements, p. 175-186. In: Cohesive Sediments edited by Burt et al, John Wiley, London

Gust, G. and Müller, V., 1997. Interfacial hydrodynamics and entrainment functions of currently usederosion devices, p. 149-174. In: Cohesive Sediment, edited by Burt et al, John Wiley, London

Houwing, E.J., 2000. Sediment dynamics in the pioneer zone in the land reclamation area of the WaddenSea, Groningen, The Netherlands. Dep. of Physical Geography, University of Utrecht, Utrecht, TheNetherlands

Houwing, E.J. and Van Rijn, L.C., 1998. In Situ Erosion Flume (ISEF); determination of bed-shear stressand erosion of a kaolinite bed. Journal of Sea Research, Vol. 39, p. 243-253

Schünemann, M. and Kühl, H., 1991. A device for erosion-measurements on naturally formed muddysediments: the Eromes system Report 91/E/18,28. GKSS, Germany

Figure 1

Sketch of in-situ erosion flume (Houwing and Van Rijn, 1998).

Figure 2

Mud concentrations as function of velocity measured by ISEF (Houwing, 2000)

Figure 3

Sketch of in-situ erosion cylinder (Andersen et al., 2002)

7.5.2 Tracer studies

Increasingly, and necessarily, there is a need to describe sediment (and contaminant) transport pathways on

dynamically variable and spatially distributed scales rather than at single point localities. 'Particle tracking',

or as it is also known 'particle' or 'sediment tracing', providing certain assumptions are satisfied, offers a

practical methodology for the assessment of transport pathways of a variety of sediments across wider

temporal and spatial scales, and is available for silts, sands, granules, pebbles and cobbles. Although not a

new technique, particle tracking has experienced a resurgence of interest and application by geologists,

hydrologists and oceanographers principally as a result of the arrival of new, innovative manufacturing and

measurement technologies. These have overcome previous limitations presented by the method, and have

also provided a foundation for silt tracking that previously did not exist.

Black et al. (2006) have given an overview of tracer studies. Ingle (1966) has produced a classic paper on

tracers for beach sand movement

Historical studies

Historical attempts at tracking sediment have included the use of materials such as pulverised coal, broken

bricks, magnetic concrete and other ferromagnetically marked sediments, painted shingle, and dyed non-

fluorescent hues. By the 1960's research had begun to investigate the use of sand coated with fluorescent

paint, dye or ink and this technique gradually became the predominant and most successful of particle

tagging methods. By the time of Ingle's (1966) now classic paper, there were over 100 reported studies in the

literature employing this approach, albeit many in obscure Russian sources (refer to Ingle's bibliography for

details) and almost exclusively concerning beachface sand transport (notably along the shores of the Black

Sea). Historically, silt tracking has received little attention.

Finally, a variety of miscellaneous approaches to sediment tracking have appeared in literature sources.

These include use of the naturally fluorescent mineral fluorite, labelling of grains with Rare Earth Elements

and use of fluorescent glass beads.

It is essential that the assumptions which underpin tracer application and tracking methodology are fully

identified and tested. The fundamental assumptions are that the tracer must mimic the behaviour of target

sediments adequately (for sand), else must integrate within and be transported via floc aggregates (silts), and

this must remain so for the experimental duration. Finally, that the tracer can be monitored effectively.

Types of tracers

Two principal types of tracer have been utilised in sediment tracking studies. These are:

· Labelled (coated) natural particles,

· Labelled synthetic particles.

In both instances, the label given to particles is commonly referred to as a 'signature'. The majority of

historical studies have used coated natural particles, principally as this was the technology of the day, but

also because it retained use of the natural sediment particles. Using the natural mineral grains was - and still

is - a preferred methodology since it is relatively easy to demonstrate equivalence of hydraulic behaviour

between uncoated (native) and coated grains (Ingle, 1966), and mineral density is not an issue. More

recently, however, the emphasis has shifted to the use of manufactured, labelled synthetic particles.

Most common are the point tracer experiments, which consist of injecting the tracer material onto the bed at

one single point. Sometimes, simultaneous point injections are performed by simultaneously injecting tracer

material at many points along a line.

Coated natural tracers

A number of different signatures have been used historically to label natural sand particles, and these are

radioactivity and fluorescent colour. Although now banned for health and environmental reasons, the

radioactive technology proved eventually to have a limited application due to the cost implications for large

scale studies, and the necessity to process sediment samples immediately to avoid loss of the radioactive

signal. Fluorescent tagging of particles is perhaps the most pervasive tracer tagging methodology employed.

In the past this used to involve application of a fluorescent substance in a colloidal state to sand particles

along with a binding material (e.g. agar or a resin). This process produced particles that were entirely benign,

and therefore in contrast to radiated sand there was no need for special precautions during subsequent

sampling and processing. Fluorescent dyes such as rhodamine (red) or anthracene (yellow-green) have also

been coated to natural sand particles (Ingle, 1966).

Ingle (1966) and references therein describe in detail a number of coating application methods. Various dye

formulations and application methodologies may also be found elsewhere. Coating/dyeing sand particles for

beachface dynamics studies is not problematic, with many studies undertaking the coating process using a

cement mixer actually at the field site. Coatings applied in this manner typically have a short life-time,

particularly in the high-energy surf-zone. Ironically, this may prove an advantage in that coloured particles

then do not persist in the nearshore environment for long periods of time. Others provide accounts of the

modern usage of labelled natural sand particles. Finally, exotic substances such as gold and silver and Rare

Earth Elements have been used as coatings in particle tracking studies.

Labelled synthetic tracers

The arrival of new, innovative manufacturing technologies has given rise to the use of labelled, entirely

artificial particles in tracking studies. These particles comprise a carrier substance mixed together with a

commercially available fluorescent dye, pigment or other signature.

Some researchers explored the utility of polystyrene plastic beads embedded with a magnetic powder. The

carrier substance is frequently polymer-based but concerns related to the use of polymer-based particles in

the natural environment, particularly in ecologically sensitive regions have prompted the use of natural

materials. Black et al. have taken synthetic tracers a step further and produced a 'dual signature' tracer,

comprising both fluorescent colour and para-magnetic character. 'Para-magnetic' means that the particles are

attracted by strong permanent or electro-magnets, thereby facilitating a simple and efficient separation from

native sediment, but they are not themselves magnetic. The authors state that 4 spectrally distinctive tracer

colours are available.

Density can be controlled during the manufacturing process to within 5-10% of most common natural

mineral densities (e.g. quartz, feldspar, kaolinite}, but there is limited control on particle shape. Synthetic

particles may be manufactured with confidence across the size range 1 to 5000 microns, appropriate for the

tracking of silt and sand-gravel. Specific size fractions are obtained through sieving. Far stricter quality

assurance testing is usually required to demonstrate hydraulic equivalence of manufactured particles in

comparison to naturally dyed/painted particles.

Miscellaneous methods

Some adopted a rather different approach whilst still making use of natural sediment (sand). Ordinary beach

sand can be made magnetic by heating at 500-900 °C which converts small quantities of iron compounds on

grain surfaces to magnetic oxides. This process, termed 'thermal magnetic enhancement', increases the

magnetic signal of the material by over 300 times, and has been used previously in the terrestrial soil

transport context. The quantity of tracer sand is then measured using a field-portable magnetic susceptibility

sensor supported by laboratory methods. This technique has considerable potential as a tracer method as the

analytical equipment is reasonably cheap, magnetising the material is comparatively simple and -although

not especially straightforward - the analysis is non destructive, allowing scope for re-analysis.

Tracer studies in the surf zone

Many tracer studies have been performed in the surf zone to determine the longshore bed load and near-bed

suspended transport (White, 1998). The sediment transport is determined by qs=rscbeddtrutr, with: cbed=

concentration of bed material (0.6), dtr= thickness of transport layer, utr=migration velocity of centroid of

tracer material. The latter two parameters are estimated from recovered tracer samples. Thickness estimates

require core samples at many locations in the surf zone. The maximum tracer penetration depth is the upper

limit of the transport layer thickness. The transport velocity can be obtained by measuring the distance

moved by the mass centroid of tracer divided by the time between injection and sampling. The mannner in

which these discrete measurements are translated into sand velocities depend on the type of sampling grid.

The most common method is the spatial integration method. It consists of a sampling grid spread over all

three spatial coordinates but which is sampled at points in time. Each of these discrete sampling values

(tracer concentration Ni) is vertically averaged within the bed to obtain a set of layer-averaged concentrations

at each time. The transport velocity is then obtained as: utr=(S(xi/ti)Ni)/SNi, with Ni=layer-averaged tracer

concentration at location xi at time ti.

Conclusions

Particle tracking within a geoscientific context has firm foundation as a field method for the study of

sediment transport processes, with a research history reaching back to the 1900's. It offers a practical method

for the assessment of transport pathways of sediments from silts to cobbles in almost all aquatic

environments, and presents a measurement capability that few other contemporary technologies can provide.

In recent years many specific elements of the method have been improved considerably. The issue of tracer

separation has been resolved through the use of a modern magnetic tracer, and substantial advancements

have arisen in synthetic particle manufacturing methods, sediment capture devices, and analytical methods

(e.g. digital image analysis). The central feature of sediment tracking - sampling - can be improved, and there

is benefit in using approaches adopted in other fields e.g. geostatistics. Used in conjunction with a range of

more traditional methods, particle tracking is a useful tool which provides additional lines of evidence in

sediment transport studies and thus contributes to forming a more detailed understanding of sediment

transport issues. The use of particle tracking in sediment transport studies will be of interest to a variety of

professionals including sediment researchers, coastal managers, engineers and modellers, conservation

agencies, regulatory authorities, and members of the dredging industry.

References

Black, K.S., Athey, S., Wilson, P. and Evans, D., 2006. The use of particle tracking in sediment transportstudies: a review. In: Balson, P., et al., (Eds.) Journal of the Geological Society of London (SpecialIssue) Measuring Sediment Transport on the Continental Shelf. 2006.

Ingle, J.C., 1966. The movement of beach sand. Developments in sedimentology, Vol. 5, 221 ppWhite, T.E., 1998. Status of measurement techniques for coastal sediment transport. Coastal Engineering,

Vol. 35, p. 17-45



7. MEASURING INSTRUMENTS FOR BED MATERIAL SAMPLING

7.1 General aspects

Broadly, there are four methods of bed material sampling:

· grab samplers,

· dredge samplers,

· scoop samplers,

· core samplers.

Grab, dredge and scoop-type samplers can only be used to collect a surface sample of the bed material.

Grabs are applicable when the bed material consists of non-cohesive sandy material, while dredges should be

used in the case of coarse and/or firm bed material. Various grab samplers were evaluated by Sly (1969) for

sampling efficiency, reproducibility and sample protection. Based on these studies, good results can be

obtained using the SHIPEK sampler. A relatively simple sampler is the VAN VEEN grab. Scoop sampling

consists of collecting samples from the top layer of the bed by means of a scoop-type bucket which swings

out of the sampler body. The bucket surrounds and encloses the sediment sample. Good results are obtained

with the US BM-54 (see www.rickly.com). Corers generally produce the least disturbed samplers. In the case

of stratified bed material or deposits only corers should be used. Core sampling consists of driving a tube

into the bed material through the use of gravity, hydrostatic pressure and/or vibration. Free-fall corers can

cause compaction of the vertical structure of the sediment samples, while shock waves generated ahead of

the descending sampler may wash away the fine fraction of the sediment bed. The latter effect can be

minimized by using samplers with openings to create a flow-through system during descent.

Free-fall corers recommended are: The PHLEGER-corer for samples upto 0.5 m in all sediment layers, the

ALPINE gravity corer for samples upto 2.0 m and the BENTHOS gravity corer suitable for samples upto 3.0

m in soft clays, muds or sandy silts.

If the vertical stratification in a core sample is of essential importance, a piston and/or vibration corer should

be used. These devices utilize special equipment to prevent sediment compaction during sampling.

Commercially available is the SENKOWITCH vibrocorer. The actual choice between corers and grabs

should be based on the type of project being evaluated. If the project requires new work or channel

deepening, samples should be collected with a corer. If the project concerns the estimation of maintenance

dredging of an existing channel or harbour, a grab sampler will usually be sufficient since vertical

stratification may be relatively small. The number of samples can be reduced substantially by using sub-

bottom profiling systems (seismic surveying), (Van Oostveen, 1982).

Sample preservation

Ideally, the samples should be analysed directly after collection. If this is not possible, it is recommended to

process the samples within 3 days after collection.

The samples should be stored wet in an airtight container (without air bubbles). Core samples should be

sealed in the core liner and returned to the laboratory in an upright position. In case of chemical analysis the

samples must be stored at 4oC. Chemical agents can be used to reduce bacterial growth.

References

Sly, P.G., 1969. Bottom Sediment Sampling. Proc. 12th Conf.Gr.Lakes Res., pp. 883~898Van Oostveen, R. , 1982. Some Notes on Marine Sub-Bottom Investigations. Delft Hydraulics

Laboratory, S304, Delft, The Netherlands



7.2 Bed material samplers: grab, dredge and scoop samplers

Principle

Grab, dredge and scoop-type samplers are used to collect a bed-surface sample (Figures 1 and 2).

A grab sampler consists of two buckets or jaws which are in an open position during lowering of the

sampler. After contact with the bed the buckets are closed by using a messenger system or by pulling the

hoisting cable. For coarse and/or firm bed material a dredge-type sampler should be used.

Simple and good samplers are the SHIPEK grab sampler and the VAN VEEN grab and dredge samplers.

A scoop-type sampler (Fig. 2) consists of a single scoop-type bucket which swings out of the bottom of the

sampler body. The bucket surrounds and encloses the bed material sample. An advantage of the US BM-54

scoop sampler is its streamlined body enabling sample collection in high-velocity conditions.


SHIPEK grab

sample size: 2 liter

weight: 70 kg

VAN VEEN grab and dredge

sample size: 0.5 liter (small grab)

2 liter (medium grab)

6 liter (large dredge)

weight: 2.5 kg (small grab)

5 kg (medium grab)

20 kg (large dredge)

US BM-54 scoop

sample size: 0.5 liter

weight: 50 kg

Disadvantages

1. collection of a disturbed surface sample,

2. loss of sediment during raising when sampler is not fully closed,

3. generation of shock waves which may wash away fine sediment fraction.



Figure 1

Grab and dredge sampler



Figure 2

US BM54 scoop sampler

Chapter 7: MEASURING INSTRUMENTS BED MATERIAL SAMPLING January 2006

Manual Sediment Transport Measurements Page:7.5

7.3 Bed material samplers: core samplers

Principle

Core sampling consists of driving a tube into the bed material through the use of manpower, gravity,

hydrostatic pressure or vibration.

A simple hand corer can be used in shallow streams, which can be waded or on tidal flats. The lower end of

the sampler contains a cylinder which is pressed into the bed (Figure 1A). A piston with a handle on its upper

end passes through the sampler frame. The piston is retracted when the cylinder is pressed into the bed

material. The suction created by the piston holds the sample in the cylinder.

Box core samples can be taken by using a box corer of about 300 kg lowered to the bed by use of a cable-

winch system. A bed surface core sampler (sand to clay) is taken by mechanical penetration (box is pressed

into the bed mechanically). This device takes a core length of about 0.5 m.

The gravity (or free-fall) corer is allowed to fall freely through the water and is driven into the bed by its

weight (Fig. 1B,C). A one-way valve at the top end of the tube permits the passage of water during the

descent and prevents flushing of the sample during retrieval and raising of the sampler. A core-catcher

generally is present at the inside of the tube just above the cutting edge. Plastic liners generally are used to

minimize the problem of sample extrusion and storage. The core length is limited to 10 core diameters in

sand and 20 core diameters in firm clay. A major disadvantage of gravity corers is the compaction of the

vertical structure of the bed material during sampling.

A Piston corer can be used to reduce the compaction during sampling. This sampler is essentially a gravity

corer, but it has an internal piston which remains at the level of the water-sediment interface when the corer

penetrates into the bed (Figure 2A). The corer is attached to a trip mechanism which is released when a

counter weight hits the bed. The piston creates a slight vacuum above the sample and is supposed to reduce

friction and to prevent compaction.

Buckley et al. (1994) reviewed the problems with piston core sampling. A series of sampling experiments

using a piston corer equipped with data logging instruments has demonstrated that representative

stratigraphic intervals of soft marine sediment may never be obtained with piston corers that use a non-

reference triggering system. Incidents of sediment by-passing and flow-in can occur throughout a single

core, resulting in both foreshortening and stretching of sedimentary units. These problems are difficult to

recognize by standard visual core logging techniques. Most of the problems are initiated by accelerations in

the lowering cable that occur after tripping, with resultant accelerations in the piston, and fluctuations in the

cavity pressure inside the core liner. The non-representative sampling can result in more than 1 m of missing

stratigraphic units below the sediment interface, and as much as 1.3 m of sediment flow-in. These problems

can lead to miscalculations of sediment deposition rates by as much as a factor of 2, and can result in

obtaining samples that are of no value for geotechnical analysis.

Vibration corers are used when core samples with a length upto 10 metres are required in all types of bed

material with exception of rock and stiff clay. The corer is driven into the bed (Fig. 2B) by vibration

equipment mounted on top of the corer. Some vibrocorers use pistons, valves and compressed air to

assist sample retrieval. Sample disturbance is inevitable but this can be restricted to the outer part of the core.

Vibrocorers require a relatively large survey vessel and skilled personnel.


Hand corer

weight: 10 kg

core length: 0.2 m


Box corer

weight: 300 kg

core length: 0.5 m




Gravity corer PHLEGER

weight: 35 kg

core length: 0.60 m


BENTHOS

weight: 250 - 450 kg

core length: 3 - 6 m




VIBRO-CORER SENKOWITCH

Practical operation

1. check electronic and air supply system,

2. install core barrel (with core liner and core catcher) and attach core barrel to the vibrator by means

of closure nut (seal closure nut),

3. open air supply system and check for leakage (use an overpressure of 0.5 Ato),

4. lower frame to the bed,

5. check vertical orientation of corer (maximum angle of deviation =7°),

6. switch on vibro motor and close air supply,

7. release air-pressure in the core liner (gently),

8. regulate pressure-difference between in- and outside of core liner (pressure-difference should be

minimum),

9. check core depth (maximum depth when air pressure remains constant after closing air escape valve),

10. close air escape valve and switch off vibro motor,

11. raise corer (use vibrator if necessary),

12. replace core liner.


dimensions: Corer installation

(frame, vibrator, guiding colums)

3 x 3 x 1 m

vibro motor 0.5 x 0.2 x 0.2 m

air compressor 0.5 x 0.5 x 0.5 m

cable haspel 0.7 x 0.7 x 0.5 m

control panel 0.7 x 0.6 x 0.2 m

weight: corer installation 500 kg

accessories and spare parts 50 kg

vibro motor 20 kg

air compressor 100 kg

cable haspel 100 kg

control panel 60 kg

core length: 2 m


cable length: 70 m

maximum water depth: 100 m

energy: 220 volt (AC), 50 Hz, 660 watt

cycle period: 60 min (between two

measurements)

References

Buckley, D.E., MacKinnon, W.G., Cranston, R.E and Christian, H.A., 1994. Problems with piston coresampling: mechanical and geotechnical diagnosis. Marine geology, Vol. 117, p. 95-106.



Figure 1

Core samplers



Figure 2

Core samplers


























composition.













References





Figure 1



































7.5 Movement of bed material particles

7.5.1 Critical bed-shear stress for initiation of motion

The beginning of movement of bed material particles (especially mixtures of clay, silt and sand) can be

determined by using in-situ erosion flumes and erosion containers.

Houwing and Van Rijn (1998) have used an in-situ erosion flume (ISEF) to determine the critical bed-

shear stress for initiation of motion of sand beds and mixtured beds of clay, silt and sand. ISEF is a

circulating flow system in the vertical plane. It consists of a lower horizontal test section, two bend sections

and an upper section where the flow is generated by a propeller, see Figure 1. The horizontal test section and

the two bend sections have a rectangular cross-section with a height of 0.1 m and a width of 0.2 m. The

bottom part of the test section is open over a length of 0.9 m. The propeller can be rotated at various speeds

by means of an adjustable oil pump system. The flow velocity in the horizontal test section can be measured

using electromagnetic sensors. Sediment concentrations can be measured using optical sensors and pump

samples. The bed-shear stresses derived from measured velocity profiles were calibrated using various beds

of sand particles (of known sizes). The critical bed-shear stress of these sand particles can be determined

from the Shields curve.

ISEF was used to determine the bed strength of mud beds. Figure 2 shows a typical result with increasing

mud concentrations as a function of imposed flow velocity. Since the water volume in the flume and the

sediment concentrations are known, the eroded sediment layer thickness can be derived when the bulk

density of the bed material is known (from separate measurements). The bed-shear stress required to erode

each layer of sediment can be determined from the measured velocities (Houwing, 2000).

Andersen et al. (2002) have used a small-scale perspex tube with a propeller (EROMES) to determine the

critical bed-shear stress of mud bed samples collected in the Hollands Diep, The Netherlands. The field

experiments were carried out on samples taken from the bed by means of a box corer. The instrument

consists of a 100 mm diameter perspex tube which is pushed into the sediment bed sample. The tube is

gently filled with local water and the propeller is placed on top of the tube. An optical sensor is used to

measure the mud concentrations. The propeller revolutions are transferred to bed-shear stress values by use

of a calibration curve based on the onset of erosion of sand particles with known critical bed-shear stresses.

Similar instruments have been used by others (Schünemann and Kühl, 1991; Cornelisse et al. 1997; Gust

and Müller, 1997).

References

Andersen, T.J., Houwing, E.J. and Pejrup, M., 2002. On the erodibility of fine-grained sediments in aninfilling freshwater system, p. 315-328. Fine Sediment Dynamics in the Marine Environment, edited byWinterwerp and Kranenburg, Elseviers Science

Cornelisse, J.M. et al. , 1997. On the development of instruments for in situ erosion measurements, p. 175-186. In: Cohesive Sediments edited by Burt et al, John Wiley, London

Gust, G. and Müller, V., 1997. Interfacial hydrodynamics and entrainment functions of currently usederosion devices, p. 149-174. In: Cohesive Sediment, edited by Burt et al, John Wiley, London

Houwing, E.J., 2000. Sediment dynamics in the pioneer zone in the land reclamation area of the WaddenSea, Groningen, The Netherlands. Dep. of Physical Geography, University of Utrecht, Utrecht, TheNetherlands

Houwing, E.J. and Van Rijn, L.C., 1998. In Situ Erosion Flume (ISEF); determination of bed-shear stressand erosion of a kaolinite bed. Journal of Sea Research, Vol. 39, p. 243-253

Schünemann, M. and Kühl, H., 1991. A device for erosion-measurements on naturally formed muddysediments: the Eromes system Report 91/E/18,28. GKSS, Germany



Figure 1

Sketch of in-situ erosion flume (Houwing and Van Rijn, 1998).

Figure 2

Mud concentrations as function of velocity measured by ISEF (Houwing, 2000)

Figure 3

Sketch of in-situ erosion cylinder (Andersen et al., 2002)



7.5.2 Tracer studies

Increasingly, and necessarily, there is a need to describe sediment (and contaminant) transport pathways on

dynamically variable and spatially distributed scales rather than at single point localities. 'Particle tracking',

or as it is also known 'particle' or 'sediment tracing', providing certain assumptions are satisfied, offers a

practical methodology for the assessment of transport pathways of a variety of sediments across wider

temporal and spatial scales, and is available for silts, sands, granules, pebbles and cobbles. Although not a

new technique, particle tracking has experienced a resurgence of interest and application by geologists,

hydrologists and oceanographers principally as a result of the arrival of new, innovative manufacturing and

measurement technologies. These have overcome previous limitations presented by the method, and have

also provided a foundation for silt tracking that previously did not exist.

Black et al. (2006) have given an overview of tracer studies. Ingle (1966) has produced a classic paper on

tracers for beach sand movement

Historical studies

Historical attempts at tracking sediment have included the use of materials such as pulverised coal, broken

bricks, magnetic concrete and other ferromagnetically marked sediments, painted shingle, and dyed non-

fluorescent hues. By the 1960's research had begun to investigate the use of sand coated with fluorescent

paint, dye or ink and this technique gradually became the predominant and most successful of particle

tagging methods. By the time of Ingle's (1966) now classic paper, there were over 100 reported studies in the

literature employing this approach, albeit many in obscure Russian sources (refer to Ingle's bibliography for

details) and almost exclusively concerning beachface sand transport (notably along the shores of the Black

Sea). Historically, silt tracking has received little attention.

Finally, a variety of miscellaneous approaches to sediment tracking have appeared in literature sources.

These include use of the naturally fluorescent mineral fluorite, labelling of grains with Rare Earth Elements

and use of fluorescent glass beads.

It is essential that the assumptions which underpin tracer application and tracking methodology are fully

identified and tested. The fundamental assumptions are that the tracer must mimic the behaviour of target

sediments adequately (for sand), else must integrate within and be transported via floc aggregates (silts), and

this must remain so for the experimental duration. Finally, that the tracer can be monitored effectively.

Types of tracers

Two principal types of tracer have been utilised in sediment tracking studies. These are:

· Labelled (coated) natural particles,

· Labelled synthetic particles.

In both instances, the label given to particles is commonly referred to as a 'signature'. The majority of

historical studies have used coated natural particles, principally as this was the technology of the day, but

also because it retained use of the natural sediment particles. Using the natural mineral grains was - and still

is - a preferred methodology since it is relatively easy to demonstrate equivalence of hydraulic behaviour

between uncoated (native) and coated grains (Ingle, 1966), and mineral density is not an issue. More

recently, however, the emphasis has shifted to the use of manufactured, labelled synthetic particles.

Most common are the point tracer experiments, which consist of injecting the tracer material onto the bed at

one single point. Sometimes, simultaneous point injections are performed by simultaneously injecting tracer

material at many points along a line.

Coated natural tracers

A number of different signatures have been used historically to label natural sand particles, and these are

radioactivity and fluorescent colour. Although now banned for health and environmental reasons, the

radioactive technology proved eventually to have a limited application due to the cost implications for large

scale studies, and the necessity to process sediment samples immediately to avoid loss of the radioactive

signal. Fluorescent tagging of particles is perhaps the most pervasive tracer tagging methodology employed.

In the past this used to involve application of a fluorescent substance in a colloidal state to sand particles

along with a binding material (e.g. agar or a resin). This process produced particles that were entirely benign,

and therefore in contrast to radiated sand there was no need for special precautions during subsequent



sampling and processing. Fluorescent dyes such as rhodamine (red) or anthracene (yellow-green) have also

been coated to natural sand particles (Ingle, 1966).

Ingle (1966) and references therein describe in detail a number of coating application methods. Various dye

formulations and application methodologies may also be found elsewhere. Coating/dyeing sand particles for

beachface dynamics studies is not problematic, with many studies undertaking the coating process using a

cement mixer actually at the field site. Coatings applied in this manner typically have a short life-time,

particularly in the high-energy surf-zone. Ironically, this may prove an advantage in that coloured particles

then do not persist in the nearshore environment for long periods of time. Others provide accounts of the

modern usage of labelled natural sand particles. Finally, exotic substances such as gold and silver and Rare

Earth Elements have been used as coatings in particle tracking studies.

Labelled synthetic tracers

The arrival of new, innovative manufacturing technologies has given rise to the use of labelled, entirely

artificial particles in tracking studies. These particles comprise a carrier substance mixed together with a

commercially available fluorescent dye, pigment or other signature.

Some researchers explored the utility of polystyrene plastic beads embedded with a magnetic powder. The

carrier substance is frequently polymer-based but concerns related to the use of polymer-based particles in

the natural environment, particularly in ecologically sensitive regions have prompted the use of natural

materials. Black et al. have taken synthetic tracers a step further and produced a 'dual signature' tracer,

comprising both fluorescent colour and para-magnetic character. 'Para-magnetic' means that the particles are

attracted by strong permanent or electro-magnets, thereby facilitating a simple and efficient separation from

native sediment, but they are not themselves magnetic. The authors state that 4 spectrally distinctive tracer

colours are available.

Density can be controlled during the manufacturing process to within 5-10% of most common natural

mineral densities (e.g. quartz, feldspar, kaolinite}, but there is limited control on particle shape. Synthetic

particles may be manufactured with confidence across the size range 1 to 5000 microns, appropriate for the

tracking of silt and sand-gravel. Specific size fractions are obtained through sieving. Far stricter quality

assurance testing is usually required to demonstrate hydraulic equivalence of manufactured particles in

comparison to naturally dyed/painted particles.

Miscellaneous methods

Some adopted a rather different approach whilst still making use of natural sediment (sand). Ordinary beach

sand can be made magnetic by heating at 500-900 °C which converts small quantities of iron compounds on

grain surfaces to magnetic oxides. This process, termed 'thermal magnetic enhancement', increases the

magnetic signal of the material by over 300 times, and has been used previously in the terrestrial soil

transport context. The quantity of tracer sand is then measured using a field-portable magnetic susceptibility

sensor supported by laboratory methods. This technique has considerable potential as a tracer method as the

analytical equipment is reasonably cheap, magnetising the material is comparatively simple and -although

not especially straightforward - the analysis is non destructive, allowing scope for re-analysis.

Tracer studies in the surf zone

Many tracer studies have been performed in the surf zone to determine the longshore bed load and near-bed

suspended transport (White, 1998). The sediment transport is determined by qs=rscbeddtrutr, with: cbed=

concentration of bed material (0.6), dtr= thickness of transport layer, utr=migration velocity of centroid of

tracer material. The latter two parameters are estimated from recovered tracer samples. Thickness estimates

require core samples at many locations in the surf zone. The maximum tracer penetration depth is the upper

limit of the transport layer thickness. The transport velocity can be obtained by measuring the distance

moved by the mass centroid of tracer divided by the time between injection and sampling. The mannner in

which these discrete measurements are translated into sand velocities depend on the type of sampling grid.

The most common method is the spatial integration method. It consists of a sampling grid spread over all

three spatial coordinates but which is sampled at points in time. Each of these discrete sampling values

(tracer concentration Ni) is vertically averaged within the bed to obtain a set of layer-averaged concentrations

at each time. The transport velocity is then obtained as: utr=(S(xi/ti)Ni)/SNi, with Ni=layer-averaged tracer

concentration at location xi at time ti.



Conclusions

Particle tracking within a geoscientific context has firm foundation as a field method for the study of

sediment transport processes, with a research history reaching back to the 1900's. It offers a practical method

for the assessment of transport pathways of sediments from silts to cobbles in almost all aquatic

environments, and presents a measurement capability that few other contemporary technologies can provide.

In recent years many specific elements of the method have been improved considerably. The issue of tracer

separation has been resolved through the use of a modern magnetic tracer, and substantial advancements

have arisen in synthetic particle manufacturing methods, sediment capture devices, and analytical methods

(e.g. digital image analysis). The central feature of sediment tracking - sampling - can be improved, and there

is benefit in using approaches adopted in other fields e.g. geostatistics. Used in conjunction with a range of

more traditional methods, particle tracking is a useful tool which provides additional lines of evidence in

sediment transport studies and thus contributes to forming a more detailed understanding of sediment

transport issues. The use of particle tracking in sediment transport studies will be of interest to a variety of

professionals including sediment researchers, coastal managers, engineers and modellers, conservation

agencies, regulatory authorities, and members of the dredging industry.

References

Black, K.S., Athey, S., Wilson, P. and Evans, D., 2006. The use of particle tracking in sediment transportstudies: a review. In: Balson, P., et al., (Eds.) Journal of the Geological Society of London (SpecialIssue) Measuring Sediment Transport on the Continental Shelf. 2006.

Ingle, J.C., 1966. The movement of beach sand. Developments in sedimentology, Vol. 5, 221 ppWhite, T.E., 1998. Status of measurement techniques for coastal sediment transport. Coastal Engineering,

Vol. 35, p. 17-45

CHAPTER 8: LABORATORY AND IN-SITU ANALYSIS OF SAMPLES February 2006


8. LABORATORY AND IN-SITU ANALYSIS OF SAMPLES

Sample analysis usually consists of determining the following parameters:

· sediment concentration,

· sediment compostion,

· sediment density,

· chemical analysis.

Samples for chemical or bioassay analysis should be immediately chilled and stored at 4 oC after collection.

8.1 Sediment concentration

The two most commonly used methods are evaporation and filtration. The filtration method may be

somewhat faster for samples of small concentrations. However, large concentrations tend to clog the filter

material (silt concentrations > 100 mg/l).

8.1.1 Evaporation method

The method consists of:

1. pour the sediment sample over a 50 mm-sieve to separate the silt and sand particles,

2. wash the silt and sand sample with distilled water to remove dissolved solids (salt!),

3. allow the silt and sand sample to settle for 24 hours (or longer),

4. decant (or siphon) the sediment-free fluid from both samples,

5. wash the silt and sand sample into evaporating dishes (pre-weighed),

6. dry the samples in an oven (at 90 °C) until all visible moisture has evaporated,

7. dry the samples in an oven (at 105 °C) for one hour,

8. cool the samples in a dessicator,

9. weigh the samples and dishes (use balance accurate to 0.1 mg),

10. determine the dry sediment weights.

8.1.2 Filtration method

The method consists of:

1. install a pre-weighed (non-hygroscopic) nylon or glass-fiber filter of 0.5 pm (c, see Figure 1)

2. decant sediment-free water of the sample,

3. pour the sediment sample over a 50 mm -sieve (a) suspended in a glass-cylinder (b) to separate the sand

particles,

4. wash the silt particles through the sieve with distilled water to remove salt traces (light brushing may

be necessary),

5. add more distilled water, if necessary,

6. remove filter with silt particles,

7. wash sand fraction over another pre-weighed filter of 0.5 mm (or wash sand fraction in an evaporating

dish),

8. dry filters in an oven (105°C),

9. weigh filters (balance accurate to 0.1 mg),

10. determine dry weight of silt and sand sample.

Remark

Using non-hygroscopic filters, it is not necessary to cool the samples in a dessicator, but the filters should be

weighed directly after drying to prevent absorption of moisture by the sediment materials.



Figure 1

Filtration unit



8.1.3 Units

The common unit for the sediment concentration is milligrams per liter (mg/1) defined as:

dry sediment mass (mg)

c = ____________________________

volume of water-sediment mass (l)

Another unit, frequently used, is parts per million defined as:

dry sediment mass x 106

c = __________________________

mass of water-sediment mass

Figure 1 shows the conversion factor from parts per million to milligrams per liter. The conversion factor is

based on the assumption that the fluid density is 1000 kg/m3 and the sediment density is 2650 kg/m3.

Sometimes, the concentration is expressed as a volume percentage defined as:

volume dry sediment

c = ________________________ x 100%

volume water-sediment mass

Figure 1

Conversion factor PPM to mg/l



8.2 Bed material composition

8.2.1 General aspects

Preparation of samples prior to analysis is of the utmost importance if accurate and reproducible results are

to be obtained. Samples containing clay minerals or organic material are very liable to cracking on drying

and care should always be taken to avoid samples drying out prior to analysis. However, when samples may

have dried out naturally when collected on a mudflat or a riverbank, then the aggregates should be broken

down (hydrogen peroxide treatment).

Organic materials ranging from macroscopic plant and coal to microscopic colloidal humus does affect

average specific weight and greatly affects the particle size and/or fall velocity, if present in sufficient

quantities. Quantitative determination of organic material, usually is recommended if the sample consists of

10% or more of organic material. Complete removal of organic material is necessary for all samples to be

analyzed for particle size or fall velocity when other than native water is used because the organic material

may bind together the sediment particles.

Samples having a size range from pebbles or cobbles down to fine sands will require hand separation of the

largest particles. If possible, the size-distribution of the large particles (cobbles) should be determined in-situ

by manual measurement of the nominal diameter or by means of photographic methods.

8.2.2 Detailed method

The detailed method (see also Figure 1) consists of:

· determination of the content of silt, sand, organic and carbonate (shell) material

· determination of the particle size and/or fall velocity.

For accurate results it is recommended to use two subsamples, one for each analysis (size analysis with dried

samples should be avoided, if possible). The original sample should be mixed thoroughly and two

subsamples should be taken of about 25 grams for silty material and 50 grams for sandy material.

A. Determination of content of silt, sand, organic and carbonate material

1. wash subsample in a 1 liter-beaker, add 1 liter distilled water to dissolve the salt particles (if present),

2. allow sample to settle for 24 hours and decant (or siphon) the sediment-free water,

3. dry the subsample in an oven (evaporation),

4. cool the subsample in a dessicator and weigh the subsample (Wt),

5. add 100 ml distilled water, 25 ml 30%-hydrogen peroxide (H2O2 and 10 ml peptiser and boil the

sample to break down the aggregated sediment lumps (use a large beaker to prevent sample loss

due to boiling water or vigorous reaction),

6. add more hydrogen peroxide (in portions of 25 ml), if necessary, until the reaction is completed to

remove all organic material,

7. allow sample to cool,

8. vibrate sample in an ultra-sonic bath for 5 minutes,

9. add 100 ml distilled water, boil the sample to remove the excess hydrogen peroxide

(completed when frothing (CO2) ceases),

10. add 100 ml 0.1N-hydrochloric acid (HCl) to remove carbonate (Shell) material,

11. add 500 ml distilled water and boil the sample for 15 minutes,

12. add more hydrochloric acid, if necessary (completed when frothing (C02) ceases),

13. allow sample to settle for 24 hours (or longer)

14. decant (or siphon) sediment-free water and add 500 ml distilled water,

15. mix sample thoroughly,

16. allow sample to settle for 24 hours (or longer),

17. decant (or siphon) sediment-free water,



18. dry sample in an oven (105°C) and cool sample in a dessicator and weigh the sample (Wsand silt),

19. determine percentage organic and carbonate material Poc=[(Wt - Wsand silt)/Wt]x 100%,

20. repeat A5 .... A9,

21. wash sample over a 50 mm- mesh sieve suspended in a funnel over a 3-liter beaker to separate

the sand particles (>50 mm), continue washing until only clear water passes through the sieve,

22. dry sand sample (105 °C), cool sand sample in a dessicator and weigh sand sample (Wsand),

23. determine sand content, Psand = Wsand/Wt x 100%,

24. determine silt content, Psilt = 100% - Poc - Psand.

B. Determination of Particle size and/or Fall velocity

1. remove organic and carbonate (shell) material, if present (see A5 ... A17),

2. wash subsample over a 2000 pm- mesh sieve on top of a 50 mm-mesh sieve, both suspended in a

funnel over a 3-liter beaker, to separate the sand and gravel particles. Continue washing until only

clear water passes through the sieves, light brushing of the 50 mm sieve may be necessary. The silt

sample is collected in the 3-liter beaker.

Sand and gravel fraction (>50 mm)

1. combine sand and gravel sample,

2. dry sample in an oven- (at 50°C to prevent cementing of particles),

3. weigh sample,

4. determine size distribution by means of dry sieving (use a mortar and pestle to break down aggregated

sediment lumps, if present),

5. determine a fall velocity distribution by means of settling tests.

Silt fraction (< 50 mm)

1. allow silt sample to settle for 24 hours (or longer),

2. decant (or siphon) sediment-free water,

3. wash silt sample in a 1 liter-beaker,

4. add distilled water to prepare a 1 liter-suspension (add 10 ml peptiser to prevent flocculation, if

necessary),


6. vibrate sample in an ultra-sonic bath for 5 minutes,

7. withdraw a small sample using a 25 ml-pipet,

8. determine silt concentration by filtering, drying and weighing,

9. if the silt concentration is larger than 2000 mg/1, then

a) prepare a 0.5 liter-suspension of about 2000 mg/1 (by splitting and diluting) for the Andreasen-

Eisenwein Pipet-method (Par. 6.5.3.4) or the (Sartorius) Balance Accumulation Tube (see Par.

6.5.3.2), or

b) prepare a 3 liter-suspension of about 2000 mg/1 (by splitting and diluting) for the 25 ml-pipet

method (see Par. 6.5.3.4),

10. if the silt concentration is smaller than 2000 mg/1, then

a) take a subsample (0.5 liter) for the (Sartorius) Balance Accumulation Tube (Par. 6.5.3-2),

b) prepare a 3 liter-suspension (by diluting) for the 200 ml-pipet method (see Par. 6.5.3.4).

References

Guy, H.P., 1969. Laboratory Theory and Methods for Sediment Analysis, Book 5. U.S. GovernmentPrinting Office, Washington, USA

Kiff, P.R., Sedimentation Methods Manual. Hydraulics Research Station Wallingford, EnglandPolhuys, T., 1973. Summary Analysis Chemical-Physical Laboratory (in Dutch). Delft Hydraulics

Laboratory, Delft, The Netherlands



Figure 1

Analysis of bed material sample



8.2.3 Simple method

When the percentage of organic and carbonate (Shell) material is relatively small (< 5%) and the sample

mainly consists of silt or sand particles (>90%), the analysis method can be simplified considerably. Usually,

it is sufficient to determine only the size or fall velocity distribution of the dominant fraction.

Organic material present in a silty sample should always be removed because it may bind together the silt

particles resulting in flocculation.

Procedure


2. take one subsample (25 grams for silty material or 50 grams for sandy material),

3. remove organic material with hydrogen peroxide, (see A5....A9, Par 8.2.2),

4. wash sample over a 2000 mm-sieve on top of a 50 mm-sieve, both suspended in a large funnel over

a 3 liter-beaker to separate the sand and gravel fraction. Continue wet sieving until only clear water

passes through the sieves, light brushing of the 50 mm-sieve may be necessary,

5. combine sand and gravel fraction,

6. allow silt sample to settle for 24 hours and decant (or siphon) sediment-free water.

Sandy sample (>90% sand)

1. dry and weigh coarse fraction (>50 mm),

2. determine size or fall velocity distribution by sieving or settling tests,

3. dry and weigh silt fraction (< 50 mm),

4. determine percentage silt and sand.

Silty sample (>90% silt)

1. dry and weigh coarse fraction (sand and gravel > 50 mm),

2. determine particle fall velocity distribution of silt fraction (see Par 8.2.2)

3. determine total weight of silt fraction (Par 8.2.2),

4. determine percentage silt and sand.

A general measuring sheet is given in Fig.1.

Chemical Reagents

1. Removal of organic material can be established by adding 5 ml of 30%-hydrogen peroxide

(H2O2) for each gram of dry sample in about 100 ml water. Add the hydrogen peroxide in portions of

25 ml until the reaction is completed (no more frothing C02). Boiling (90°C) is necessary to dissolve the

chemical constituents completely and to remove the excess amount of hydrogen peroxide (use a large

beaker to prevent sample loss).

2. Removal of carbonate or shell material (CaCO3) can be established by adding 20 ml 1.0 N-

hydrochloric acid (HCl) for each gram of dry sample in 100 ml water. Add hydrochloric acid until

the reaction is completed (no more frothing C02). Boiling is necessary to dissolve all chemical

constituents completely. The excess amount of HC1 can be removed by washing with distilled water.

3. Dispersing or deflocculating agents (peptisers) may be used for homogenizing suspensions, as follows:

a) 0.5 ml of 1%-Calgon solution (10 grams commercially available Calgon in 1 liter distilled

water) for 1 gram of silt,

b) 5 ml of 10%-Sodium hexametaphosphate solution for 1 gram of silt. After adding a peptiser, the

suspension must be vibrated in an ultra-sonic bath for 5 minutes. When a peptiser is used, a weigh

correction of filtered samples is necessary by determining the weight of the peptiser per unit volume

(dummy test).

4. Aggregated sediment lumps can be broken down by boiling the sample with hydrogen peroxide and a

peptiser being added.



Figure 1

Data sheet



8.3 Suspended sediment composition

8.3.1 General aspects

The physical analysis of suspended sediment samples should be focussed on the determination of the particle

fall velocity distribution because this latter parameter is of essential importance in sedimentation studies.

Therefore, the sedimentation methods (settling tests) must be preferred above the other methods such as

sieving or the Coulter counter. These latter two methods may be used to check the results of the

sedimentation tests.

Figure 1 presents a summary of analysis methods for samples collected in sandy, silty or sandy-silty

environments.

Figure 1

Analysis of suspended sediment samples



8.3.2 Sandy environment

If the quantity of sand available for analysis is too small, single samples collected under similar flow

conditions can be accumulated. For tidal flow conditions sample accumulation may be achieved by defining

a number of flow velocity ranges (0.5-1.0, 1.0-1.5, 1.5-2.0 m/s) and depth ranges (0-0.5h and 0.5h-1h, h =

flow depth). All samples within each range are accumulated and analyzed (see also Figure 1, Par. 8.3.1).

Procedure

1. dry single samples,

2. accumulate single samples, (if necessary),


4. take a representative subsample (splitter),

5. determine fall-velocity distribution (VAT, MAT, BAT, see Par. 6.5.3),

6. determine size distribution by dry sieving (see Par 6.5.2).

8.3.3 Silty environment

In the absence of disruptive forces of turbulence the stability of a silt suspension is mainly controlled by the

magnitude of the electrokinetic potential associated with the particles. This potential can be defined as the

potential between the layer of immobile ions absorbed on the particle surface and the last mobile ion

associated with the particle. Most clay or silt particles have a negative charge. When the electrokinetic

potential is sufficiently high, particles will repel each other, but below a critical potential (as present in a

saline environment) the electrical layer associated with the particle collapses and the material flocculates.

In a natural stream the stability of a silt suspension is controlled by continuous turbulent motion as well as by

the electrokinetic potential; whereas in the laboratory the stability is only controlled by the electrokinetic

potential and artificial turbulence (if applied). It is obvious that the degree of flocculation of sediment

particles resuspended in laboratory conditions cannot be representative for natural conditions.

Other variables that influence the amount of flocculation are: temperature and salinity of the suspension, silt

concentration and the content of organic material. From these considerations it is evident that the

determination of the particle fall velocity distribution should only be done in native water as the settling

medium.

Basically, there are two options:

· settling test directly after collection of the sample using the undisturbed sample (in-situ analysis),

· settling test in the laboratory (sample transfer).

The best approach is to use the in-situ analysis method as much as possible. Only, when the samples are

collected in a fresh-water environment a laboratory analysis may be considered (see also Figure 1, Par 8.3.1).

As the fall velocity of the silt particles depends on the value of the concentration, salinity and temperature,

it is recommended to collect samples at widely differing values. In tidal conditions this may be achieved

by collecting samples at different stages of the tide and at various depths above the bed.

In-situ settling analysis

This method is extensively described as the field pipet-withdrawal tube and the field bottom-withdrawal tube

(see Par. 6.5.3.4 and 6.5.3.3).

Laboratory settling analysis

The samples should be analyzed as soon as possible after collection and before any decomposition of organic

material (within 3 days). When the samples are analyzed using native water as the settling medium, the

samples should not be treated for removal of organic material. The oxidation of organic material with

hydrogen peroxide results not only in the formation of Carbondioxide (CO2) and water but also in the release

of all ions incorporated in the organic material. Consequently, the flocculating ability of the settling medium

will be changed.



Procedure

1. collect a 3 liter sample (for initial concentrations in the range of 100 to 2000 mg/1) or a 1 liter sample for

an initial concentration larger than 2000 mg/1,

2. store the sample in an airtight bottle away from direct sunlight and as cool as possible,

3. mix the sample thoroughly by turning the bottle upside down (initial agitation by hand stirring may be

necessary, no ultrasonic vibration),

4. take a small subsample with the 25 ml-pipet to determine the initial concentration,

5. select the required pipet method (25 ml, 200 ml or Andreasen-Eisenwein pipet, see Par. 6.5.3.4).

Remarks

1. only native water should be used for the analysis,

2. distilled water and/or deflocculating agents should not be used,

3. organic material should not be removed.

8.3.4 Sandy-silty environment

The best approach is to use an in-situ analysis method for determining the fall velocity distribution of the silt

fraction.

When the sand fraction is sufficiently large, a sedimentation method can be used for the in-situ (or

laboratory) separation of the sand fraction, which is returned to the laboratory for analysis (settling tests).

When a bottle method is used for sample collection, the separation of the sand fraction should be achieved by

wet sieving (50 mm) in the laboratory. Single sand samples can be accumulated before analysis. The silt

sample passing the 50 mm-sieve should not be used for fall-velocity analysis because the wet-sieving process

may have affected the flocculated particles.



8.4 Sediment density

8.4.1 Detailed method

For a small sample (about 1 gram) of fine sediment the most accurate results can be obtained by using a

small bottle (or pycnometer) with a constant volume.

Procedure

1. weigh the clean dry bottle (or pycnometer), Wo,

2. fill the bottle completely with distilled (boiled) water of a known temperature

3. weigh the bottle with water, W1,

4. empty and dry the bottle,

5. add sediment sample (about 1 gram),

6. dry the bottle with sediment (105°C) and weigh, W2,

7. add sufficient distilled water to cover the particles (same water temperature),

8. remove air bubbles by boiling, ultra-sonic vibration or vacuum pump,

9. fill the bottle completely with distilled water,

10. weigh the bottle with water and sediment, W3,

11. determine density of sediment as:

rs =rw(W2 – Wo)/[(W1-Wo)-(W3-W2)],

r w = density of water (see Figure 1).

Temperature

oC oF

Density

(kg/m3)

Kinematic

viscosity

coefficient

(m2/s)

Surface tension

(N/m)

Compression

modulus

(N/m2)

0 32 999.87 1.793 10-6 0.0757 1.994 10-9

1 999.93 1.731 0.0755 2.006

2 999.97 1.673 0.0753 2.017

3 999.99 1.619 0.0751 2.028

4 1000 1.567 0.0749 2.039

6 999.97 1.473 0.0747 2.061

8 999.88 1.386 0.0745 2.083

10 50 999.73 1.309 0.0742 2.105

12 999.52 1.237 0.0740 2.125

14 999.27 1.172 0.0737 2.144

16 998.97 1.112 0.0734 2.161

18 998.62 1.057 0.0731 2.177

20 68 998.23 1.010 0.0728 2.191

25 77 997.07 0.896 0.0720 2.231

30 86 995.68 0.802 0.0712 2.261

35 95 994.04 0.727 0.0704 2.278

40 104 992.25 0.661 0.0696 2.287

45 113 990.24 0.604 0.0689 2.295

50 122 988.07 0.556 0.0683 2.298

60 140 983.24 0.477 0.0661 2.275

70 158 977.81 0.415 0.0643 2.244

80 176 971.83 0.366 0.0626 2.205

90 194 965.34 0.328 0.0607 2.160

100 212 958.38 0.296 0.0589 2.091

Figure 1

Fluid density versus temperature



8.4.2 Simple method

For large sand or gravel samples the method can be simplified by measuring the dry sediment weight and the

volume of the sediment particles by immersion, as follows:

Procedure

1. use a calibrated capillary (accurate to 1 ml),

2. add distilled water with a known temperature,

3. read the water volume, V1,

4. add the sediment sample of a known dry sediment weight, Ws,

5. close capillary with a rubber stop,

6. remove air bubbles by boiling, ultra-sonic vibration or vacuum pump,

7. read the water volume (at same temperature), V2,

8. determine the solid density as:

rs=Ws/(V2- V1)



8.5 Chemical analysis

Rapid chemical sediment characterization (RSC) is often required for ecological and environmental studies

and is defined as utilization of near real-time screening techniques to rapidly delineate extent of

contamination, physical characteristics and/or biological effects.

Possible analyses include:

· Total organic carbon (TOC),

· metals (Pb, Ni, Cu, Zn, Cd, Cr, Fe, Mn, Hg, As),

· nutrients (COD, Total Phosphorus, Total Kjeldahl Nitrogen TKN, Ammonia),

· cyanide

· oil and grease,

· persistent organics (pesticides, insecticides, herbicides, PCB’s, BNA’s, TPH’s, PCDD, PAH’s),

· volatile sulfides,

· oxidation reduction potential (ORP/redox),

· pH.

Rapid sediment characterization tools are laboratory or field transportable tools that provide measurements

of chemical, or biological parameters on a real-time or near real-time basis. A wide variety of tools exist

which are capable of making these types of measurements.

The most common RSC-techniques are:

· X-ray Fluorescence Spectrometry (XRF) for Metals,

· UV Fluorescence Spectroscopy (UVF) for Polycyclic Aromatic Hydrocarbons (PAHs),

· Immimoassay PCBs for pesticides PAHs,

· QwikLite/QwikSed Bioassay for Organic (e.g., PAHs), inorganic (e.g., Metals).

In order to determine if RSC tools are appropriate to assess contamination at a given site several questions

should be asked. For example: What are the goals of the study? What are the contaminants of concern? Are

the contaminants known? What are the action limits? What are the strengths and weaknesses of the analytical

methods being considered? Do instrument detection limits meet action limit requirements?

By asking these questions before sampling is started and considering the advantages and disadvantages of

different techniques, appropriate decisions can be made on how best to implement a technology or suite of

technologies to facilitate the ERA process.

X-ray Fluorescence Spectrometry (XRF): Metals

This technique measures the fluorescence spectrum of x-rays emitted when metal atoms are excited by an x-

ray source. The energy of emitted x-rays reveal the identity of the metals in the sample and the intensity of

emitted x-rays is related to their concentrations. Rapid, multi-element analysis can be performed by XRF. An

XRF spectrometer can analyze a wide range of elements (i.e., sulfur through uranium), with a wide dynamic

range, from parts per million to percent levels, encompassing typical element levels found in soils and

sediments. Detection limits are different for each element. For metals such as Pb, Zn and Cu the detection

limits typically range from 50 ppm to 150 ppm. Field portable XRF (FP-XRF) instruments can be calibrated

using several different methods: 1) internally, using fundamental parameters determined by the

manufacturer, 2) empirically, based on site-specific calibration standards, or 3) using specific normalization

methods. Field portable XRF units provide near real-time measurements with minimal sample handling,

allowing for extensive, semi-quantitative analysis on site.

UV Fluorescence Spectroscopy (UVF): PAHs

This screening method is based on the measurement of fluorescence observed following UV excitation of

organic solvent extracts of sediments. In general, this method is used to measure fluorescent organics

(especially PAHs), though some care must be taken to reduce signals from natural organic compounds (e.g.,



humics) that fluoresce. Because fluorescence measurements are matrix sensitive, it is currently necessary to

make measurements on solvent extracts rather than directly on the wet, solid sediment sample in order to

achieve detection limits appropriate for marine sediment PAH benchmark criteria and typical levels in many

marine sediments. Solvent extraction requires additional time for sample extract analysis, so although

fluorescence is a near real-time measurement, the total time for analysis may be up to half an hour. Solvent

extraction makes it possible to improve detection limits by several orders of magnitude. Detection limits

range from one ppm to five ppm total solid-phase PAH. Many studies have used UVF to assess total PAH

levels in various types of sediment. This technique can be used to determine the presence of chlorophyll,

petroleum hydrocarbons, etc.

Immunoassays: PCBs, PAHs, Pesticides

An immunoassay is a technique for detecting and measuring a target compound through use of an antibody

that binds only to that substance. Quantitation is generally performed by monitoring solution color changes

with a spectrophotometer. The technology can be used to measure concentrations of a variety of organic

contaminants including PCBs, PAHs and organic pesticides. Detection limits range from hundreds of ppb to

low ppm levels.

QwikSed Bioassay

The QwikLite and QwikSed Bioassays measure the inhibition of light emitted by marine bioluminescent

dinoflagellates (e.g., Gonyauloxpolyedra) exposed to a test solution (effluents, elutriates, or sediment pore

waters). Any decrease in light output relative to controls suggests bioavailable contaminants or other

stressors. The bioassays are capable of measuring a response within 24 hours of test setup and can be

conducted for a standard four-day acute test or seven-day chronic test. QwikSed can be used to evaluate

sediment toxicity. If the contaminated sediment is found to be toxic and requires cleanup, QwikSed can be

used to assess the toxicity reduction.

References

Taft, R.A., 2001. Sediment sampling guide and methodologies. Environmental Protection Agency, Ohio,USA



8.6 Laboratory equipment

Instruments

Filtration unit

Nylon or glass-fiber 0.5 mm-filters

Drying-oven

Dessicator

Balance (accurate to 0.1 mg)

Ultra-sonic bath

Magnetic stirrer

Thermometer

Pycnometer or density bottle

Stopwatch

Sample splitter for dry sand sample

Sample splitter for silt suspension (inverted y-tube)

Sieving set (dry)

50 fita stainless steel sieve (wet sieving)

Pipet tube (3 liter) for silt suspension

Andreasen-Eisenwein pipet for silt suspension

Bottom-withdrawal tube for silt suspension

Settling tube for sand particles (VAT or MAT)

25 ml-pipet with fill bulb

Mortar and pestle

Syphon bottle

Evaporating dishes

Graduated bottles, beakers and cylinders (50 ml up to 3000 ml)

Reagents

Distilled water Hydrogen peroxide (30%)

Hydrochloric acid (1.0 N)

Deflocculating agent (Calgon)

Hygroscopic material for dessicator

Chapter 9: IN-SITU MEASUREMENT OF WET BULK DENSITY February 2006


9. IN-SITU MEASUREMENT OF WET BULK DENSITY

9.1 General aspects

In deposition and navigation depth studies of muddy areas the wet (bulk) density defined as the mass of the

water-sediment mixture per unit volume is an important parameter.

The position of the surface of consolidated mudlayers can be determined by means of echo-sounding

instruments. Good penetration can be obtained with 30 kHz-instruments, see Fig. 1A. Higher frequencies

(210 KHz) do not have sufficient energy to penetrate into the bed.

Various methods are available to determine the wet bulk density:

· mechanical core sampler,

· acoustic probe,

· nuclear radiation probe,

· electric conductivity probe,

· vibration transducer probe,

· pressure transducer probe.

Electric conductivity probes and pressure transducer probes are not generally applicable. Electric

conductivity probes are very sensitive to the fluid salinity which should be known beforehand. Pressure and

vibration transducer probes can only be used in unconsolidated fluid muds (low density < 1200 kg/m3).

9.2 Mechanical core sampler

A basic requirement is undisturbed sampling of bed material.

Various mechanical core-samplers are available to take undisturbed bed material samples of the surface

layers (upper 0.5 m of the bed). Most samplers can only be used during low velocity conditions to ensure

vertical penetration of the bed.

After sampling, it is common practice to make slices by a machined ring of the same internal diameter as the

core. The core content is extruded into the ring until it is full of the water-sediment mixture. A thin plate is

then introduced between the ring and the core to isolate the sample.

As the core diameter is known and fixed and the slice thickness is fixed by the ring, the volume can be

calculated. After weighing (and drying) of the sample, the wet and dry density can be determined. The wet

density is defined as:

rwet = (Mw + Ms)/V=p rw + (1-p) rs

in which:

rwet = wet (bulk) density (kg/m3),

Mw = water mass (kg),

Ms = sediment mass (kg),

V = sample volume (m3),

rw = fluid density (kg/ma),

rs = sediment density (kg/m3),

p = porosity factor (-).

Hilton et al (1986) tested several samples from a 75 mm core which were sliced at either 5 or 10 mm

intervals. Each slice was transferred to a calibrated bottle to determine the sample volume. The measured

volumes were found to be systematically larger (10% to 20%) than the volumes calculated from the ring

dimensions. This difference is caused by the centre of the sediment surface bowing upwards slightly when

the edges are just level with the top of the ring. This error was reduced substantially when a sheet of perspex

was put on top of the ring. A small gap was left at one side, to allow air to escape until the sediment almost



touched the lid, after which it was moved to completely cover the ring. The slice was then treated similarly

(determination of volume, wet and dry mass).

The wet (bulk) density of dredged material in a hopper dredger can be determined as:

rwet = rwVd/Vh

in which:

Vd = displaced volume of hopper vessel (m3),

Vh = volume of hopper (m3),

rw = fluid density (kg/m3).

References

Hilton, J., Lishman, J.P. and Millington, A., 1986. A Comparison of Some Rapid Techniques for theMeasurement of Density in Soft Sediments. Sedimentology, Vol. 33, p. 777-781



9.3 Acoustic sensor

The principle is based on measuring the attenuation of the intensity of monochromatic ultra-sonic waves

through the (fluid) mud layer.

The basic equation is:

I = Io exp(-a L rwet)

in which:

I = acoustic intensity measured through water-sediment mixture,

Io = acoustic intensity measured in clear water,

L = path length,

a = absorption coefficient of water-sediment mixture (calibration),

rwet = wet (bulk) density.

Granboulan et al (1987) presented an instrument composed of two piezoelectric transducers each 50 mm in

diameter and placed 150 mm apart. A series of waves are (860 KHz) transmitted between the two

transducers. An envelop sound detector connected to a micro-processor is used to process the received

signal, which is compared with the signal in clear water. Calibration of the instrument is required. The linear

dry density range was from 100 to 500 kg/m3 (wet density of 1050 to 1300 kg/m3) with an inaccuracy of ± 30

kg/m3.

The probe (mass 10 kg) was lowered and raised by a winch for vertical measurements. The probe was also

attached to a submarine vehicle which was towed horizontally through fluid mud layers to determine the

navigation depth.

References

Granboulan, J., Chadmet, M. and Feral, A., 1987. An Ultra-Sonic Probe for Hydrographic Sounding inMuddy Channels. Coast and Port Conference in Developing Countries, Beijing, China



9.4 Nuclear radiation sensor

The principle is based on measuring the attenuation or scattering of the radiation intensity through a water-

sediment mixture.

The basic equation reads as:

I = Io exp(-m L rwet)

in which:

I = intensity measured in water-sediment mixture,

Io = intensity measured in clear water,

L = path length of radiation,

m = absorption coefficient (calibration),

rwet = wet (bulk) density of water-sediment mixture.

The attenuation principle is preferred above the scattering principle because this latter method is more

sensitive to sediment properties, requires more input energy and has a larger vertical measuring volume.

The radiation sources usually are gamma radiation sources: Cs137, Am241 and Cd109. The detector usually is a

scintillation (crystal) probe.

Calibration of the probe is required. The calibration coefficient depends on the distance (L) between the

source and the detector, the radiation source and the absorption coefficient of the sediment particles. The

measuring range (dry density) is 50 to 1000 kg/m3 with an inaccuracy of ± 20 kg/m3. The wet (bulk) density

range is 1020 to 1500 kg/m3, see Fig. 1A.

The Public Works Department of Rotterdam uses a nuclear backscatter probe to determine in-situ density

profiles in the harbour basins. The needle-shaped probe (= 70 kg) has a spatial resolution of 0.15 m being the

distance between the source and the detector. The probe is calibrated in a large container using artificial mud.

In field conditions the probe measures the local density of the bed while it penetrates into the bed (free fall).

A complete density profile over 2 to 3 meter can be obtained in approximately 1 minute. The measurement is

stopped when the inclination of the probe is larger than 10°. Some results are shown in Figure IB.

The GKSS research center Geesthacht in Germany (Elbe estuary) uses a gamma radiation attenuation probe,

see Fig. 2A. The instrument consists of two needle-shaped tubes (diameter = 0.07 m, length = 3 m, weight =

70 kg) . The radiation source is 0.37 - Gbq - Cs137 with a gamma-energy of 662 keV. The detector is a

scintillation crystal placed at a distance of L = 0.25 m from the source. The vertical position of the

instrument is determined by means of a pressure sensor. Inclinometers are used to determine the vertical

inclination of the instrument. The maximum penetration depth in soft mud is approximately 3 to 5 m (see

Fig. 2B). Reproducibility tests show variations of the order of 20 to 50 kg/m3, see Fig. 2C.

References

Hellema, J.A., 1980. Density Measurements of Mud in Europoort (in Dutch). De Ingenieur no. 39,Amsterdam, The Netherlands

Hellema, J.A., 1979. In-situ Density Measurements (in Dutch). Dir. Tidal Rivers, Rijkswaterstaat,Dordrecht, The Netherlands

Von Fanger, H.U., Bossow, E. and Kuhn, H., 1985. Eine Gammasonde zur Schlickdichte-Bestimmung DieKüste, Heft 42



Figure 1

Nuclear radiation sensor



Figure 2

Nuclear radiation sensor

CHAPTER 10: BED LEVEL DETECTION February 2006


10. INSTRUMENTS FOR BED LEVEL DETECTION

10. 1 Introduction

The management of rivers, estuaries and coastal seas always involves the production of bathymetric maps for

evaluation of navigationable depths, shoaling and erosion volumes, etc. Hence, accurate measuring

instruments for bed level detection are required. Herein, the following methods and accuracy involved are

discussed:

1. mechanical bed level detection in combination with DGPS;

2. acoustic bed level detectors (single and multi-beam echo sounders);

3. optical bed level detection.

10.2 Mechanical bed level detection in combination with DGPS

In coastal environments the bed level soundings are often performed by use of a vehicle moving through the

surf zone. Rijkwaterstaat uses the WESP in combination with DGPS. The CRAB vehicle (see Figure 1;

www.frc.usace.army.mil) is in use at the Duck site (USA). The WESP is an approximately 15 m high

amphibious 3-wheel vehicle, which can be used for bed level soundings in the surf zone in depths up to -6 m

with waves upto 2 m. It is equipped with a DGPS positioning system (De Hilster, 1998). Small vehicles with

DGPS can be used on the dry beach.

Accuracy of WESP surveys

The measuring accuracy of the WESP depends on:

· the location of the DGPS antenna,

· the accuracy of the DGPS system, and

· the accuracy of the attitude (tilt) sensor.

The antenna is situated in the middle of the wheelbase of the WESP. The bed level at the location of the

WESP is calculated straight down from the antenna in the middle of the wheelbase. This means that some

morphology wavelengths are measured partially or not all. Theoretical considerations suggest that this error

source especially affects morphological features with a wavelength smaller than about 10 times the

wheelbase. For the bars at Egmond, the errors in the vertical co-ordinate because of the location of the

antenna are estimated at about 3 cm at the inner bar and about 1 cm at the outer bar.

The vertical accuracy of the applied DGPS is estimated as 0.05 to 0.07 m. A small difference of about 0.01 m

should be added because of non-constant difference of the ellipsoid and geoid in the study area.

At present, the tilt of the WESP over a sloping sea bed is not accounted for in the computations of the

horizontal and vertical co-ordinates. Typical bed slopes at Egmond vary between zero to six degrees. The

latter may cause a height error of about 0.08 m. The squat of the wheels into the sand has not been taken into

account due to a lack of knowledge on this subject.

Overall, it is fair to say that the WESP survey accuracy is 0.10 to 0.15 m, or less, depending on the precise

settings of the DGPS, the bed slope and the degree of compaction of the bed under the weight of the wheels.

This error does not account for small unresolved bed forms with wave lengths of O(1 m) and heights of O

(0.1 m).

Accuracy of bed level detection at dry beach

Beach level soundings are often done by DGPS receivers mounted on small vehicles. The DGPS receiver

used during the COAST3D studies at the Egmond (Van Rijn et al, 2002) and Teignmouth sites in 1998 and

1999 was mounted on a small trailer pulled by a 4-wheel vehicle. The topographic surveys performed with

DGPS have theoretically a centimetric precision (say 0.03 m) both in planimetry and in elevation.

Observations of the scattering of the coordinates of the control points obtained during the different surveys

can also give an idea of the precision which may be achieved in practice. During the main experiment at the

Teignmouth site in 1999, for example, the maximum deviation was usually less than 0.05 m for all the



available control points. The difference between the obtained coordinates and the values provided by HR

Wallingford is also less than 0.05 m. An important condition to achieve precise measurements is that the

control points are distributed all around the surveyed zone.

Some additional errors may also occur during the beach survey. The actual difference in elevation between

the beach and the reference level of the DGPS is measured before every survey but may vary during the

experiment. The trailer indeed penetrates more or less (say 0.02 m) in the soil depending on its resistance

characteristics, which influences this parameter. Overall, the vertical accuracy is about 0.05 m on relatively

flat and smooth areas and about 0.10 m on steep sloping faces of bars (no tilt correction).

10.3 Acoustic bed level detection (Echo-sounding instruments)

Single beam echo sounder

The most common system for measuring water depth is the single-beam echo sounder (Figure 2;

www.dosits.org). This sonar instrument uses a transducer that is usually mounted on the bottom of a ship.

Sound pulses (usually 210 KHz for surface detection) are sent from the transducer straight down into the

water. The sound reflects off the seafloor and returns to the transducer. Acoustic penetration into the bed

increases with decreasing frequency (usually 10 to 15 KHz for subsurface detection). The time the sound

takes to travel to the bottom and back is used to calculate the distance to the seafloor. Water depth is

estimated by using the speed of sound through the water (approximately 1500 meters per second) and a

simple calculation: distance = speed x time/2. The product is divided by 2 because the measured time is the

round-trip time (from the transducer to the seafloor and back to the transducer). The faster the sound pulses

return to the transducer from the ocean floor, the shallower the water depth is and the higher the elevation of

the sea floor. The sound pulses are sent out regularly as the ship moves along the surface, which produces a

line showing the depth of the ocean beneath the ship. This continuous depth data is used to create bathymetry

maps of the survey area.

Gallagher et al. (1996) have studied the performance of a sonar altimeter in the surf zone near Scripps and

Duck (USA). The altimeter consisted of a 2.54-cm diameter transducer (manufactured by Panametrics;

www.panametrics-ndt.com) and its electronics housed in a 7-cm diameter, 35-cm long PVC tube. The

transducer beam width of approximately 3.4o results in a 6-cm diameter footprint at a range of 1 m. A 1 MHz

acoustic pulse (duration of 10 msec) is transmitted 25 times per second, with return echos detected after each

pulse. The minimum detectable distance to the seafloor is about 20 cm. The maximum range is about 180 to

250 cm in turbid, bubbly water. The strength of the bottom reflection depends on the sediment concentration

in the water column and the concentration of air bubbles in the water (surf zone). It is found that an altimeter

that uses a fixed threshold to detect the bottom echo is ineffective in the surf zone with relatively high

sediment concentrations. An automatic gain control (AGC) algorithm was used to adjust the instrument gain

(adjustment of subsequent pulses) to maintain an approximately constant peak voltage regardless of

attenuation and scattering in the water column. A threshold voltage for detecting the bottom was set just

below this constant level, and the travel time of the first return above the threshold was used to calculate the

distance to the seafloor (dependence of sound speed on water temperature, measured with a co-located

sensor, was accounted for in post-processing). The altimeter (placed at 90 cm above a smooth metal plate)

was tested in a laboratory tank. The mean distance to the bottom measured by the altimeter was within the

few mm accuracy of an independent distance estimate and had a resolution (scatter) of ±1 mm. The scatter

increased to ±2 mm when a level bed of sand grains covered the bottom plate, probably caused by the uneven

scattering surface of the grains. The scatter increased to about ±8 mm in the case of a rippled bed. Field tests

were conducted in the surf zone near Scripps (USA) with a water depth of about 3 m. Altimeter estimates of

the distance to the undisturbed sand bottom had about ±20 mm scatter and about ±5 mm scatter when a

smooth metal plate was placed on the sand bed. After removal of the plate, the scatter was ±10 mm possibly

owing to the compaction of the sand during placement of the plate. Sand was released from a container,

suspending sediment in the water column between the altimeter and the plate. There were many false returns

from the water column, but the bottom location was still detectable. The use of AGC was essential; a co-

located altimeter used without AGC did not perform well. During turbid conditions when the bottom was

more difficult to locate, 33% of the AGC altimeter estimates were accurate, compared to only 1% of the non-



AGC altimeter estimates. The scatter range of the measured bottom distance of the AGC altimeter was found

to be about ±30 mm during a 1.2 days field test with a mobile sediment bed. It was found that as many as

70% of the sonar returns can be erroneous in the surf zone. To routinely process the raw data of 2 Hz sonar

returns, an algorithm was developed that provides accurate estimates of the bottom locations every 32

seconds. Each 256-s record is subdivided into eight 32-s records and a histogram with 5 mm-wide bins

(within 20 cm of the maximum of the 256-s histogram) is constructed. The maxima of the 32-s histograms

provide estimates of the distance to the seabed every 32 s. The algorithm only failed during the most

energetic conditions in the surf zone. A field deployment of 16 altimeters near Duck (USA) demonstrated

that the sensors were robust over a period of 3 months (summer fall 1994). The accuracy of the bed

soundings was found to be about ±30 mm.

Multi-beam echo sounder

Multibeam bathymetry sonar (Figure 2) is the relatively recent successor to single-beam echo sounding.

About 30 years ago, a new technology has been developed that uses many beams of sound at the same time

to cover a large fan-shaped area of the ocean floor rather than just the small patch of seafloor that echo

sounders cover. These multibeam systems can have more than 100 transducers, arranged in precise

geometrical patterns, sending out a swath of sound that covers a distance on either side of the ship that is

equal to about two times the water depth. All of the signals that are sent out reach the seafloor and return at

slightly different times. These signals are received and converted to water depths by computers, and then

automatically plotted as bathymetric maps. The data acquired by multibeam systems are much more complex

than single-beam surveys; this means higher resolution is possible, but also that more involved signal

processing is necessary in order to interpret the data. Multibeam systems produce high-resolution bathymetry

data throughout the survey area. Since they acquire dense sounding data both along the ship's track and

between the track lines, they can provide 100% coverage of the seafloor. Multibeam bathymetry sonar is

used to locate topographical features on the seafloor such as sediment ridges, rock outcrops, shipwrecks, and

underwater cables. Ships also use this technology to avoid areas that would endanger their vessels or gear, to

find fishing grounds, and to precisely map the seafloor. Objects as small as one meter long can be located

with this technology. Multibeam bathymetry sonar is a valuable tool for scientists hoping to learn more about

seafloor habitats in the hopes of conserving them.

Bed form profiler

An acoustic ripple profiler has been developed by Bell and Thorne (1997). This instrument measures bed

forms using a 2-MHz transducer mounted on a rotating assembly. The transducer rotates in a vertical plane

so that a horizontal transect along the bed of length of approximately 3 to 4 m is measured. The strong

backscattered signal from the bed particles is used to measure the bed form profiles. Profiles can be obtained

approximately every minute, providing information on the evolution of the bed over time. The length and

height of the sand ripples can be determined from the data to within 0.005 m, together with information

pertaining to ripple migration speeds (and estimates of bed load transport).

Side/sector scanning sonar

One of the best systems for imaging large areas of the ocean floor is side scan sonar (Figure 3A), either ship-

mounted or bottom-mounted. The basic concept is much the same as the basic echo sounder; however, side

scan sonar instruments are towed behind ships and often called towfish or tow vehicles. This technology uses

a specially shaped acoustic beam, which pulses out 90 degrees from the path that it is towed, and also out to

each side. Each pulse provides a detailed image of a narrow strip directly below and to either side of the

instrument. The topography of the ocean floor and underwater objects reflect back the sound energy to

hydrophones on the tow vehicle. These reflections are amplified, processed and displayed as images. Some

of the sound that is emitted by side scan sonar is absorbed by the seafloor; the rest is reflected or scattered

back in different amounts, which leads to different images of the seafloor. For example, hard objects such as

rocks and metal will reflect strong signals while softer features such as mud absorb sonar energy and produce

lighter acoustic returns.



Side scan sonar technology provides high resolution, almost photographic quality imagery of the seafloor. It

is commonly used in industry to locate pipeline or cable routes, and to search out small but specific objects

that need to be found, such as shipwrecks, mines, or downed aircraft. Side scan sonar is sensitive enough to

measure features smaller than 10 cm (less than 4 inches) on the seafloor. It is also good to use when accurate

maps of large sections of seabed are needed.

Betteridge et al. (2003) have used a bottom-mounted side scan sonar, which is a 2-MHZ high-resolution

sector scanning sonar system, producing images (one scan every minute) of the seabed over a 5 m radius.

The system uses a small fan beam acoustic transducer mounted on a stepper motor.

Dolphin et al. (2005) have used a side scan sonar to determine the bed form patterns (See Figure 3B) at an

offshore site near Noordwijk in the North Sea.

Seismic reflection and refraction

Seismic reflection (Figure 4) uses a stronger sound signal and lower sound frequencies than echosounding.

The sound pulse is often sent from an airgun towed behind the ship. An airgun uses the sudden release of

compressed air to form bubbles. The bubble formation produces a loud sound. The sound from the airgun

travels down to the seafloor. Some of the sound reflects off the seafloor but some of the sound penetrates the

seafloor. The sound that penetrates the seafloor may also reflect off layers of material within the seafloor.

The reflected sounds travel back up to the surface. The ship also tows a number of hydrophones (called a

towed array or streamer) which detects the reflected sound signal when it reaches the surface. The time it

takes the sound to return to the ship can be used to find the thickness of the layers in the seafloor and their

position (sloped, level, etc). It also gives some information about the composition of the layers.

Sub-bottom profiling systems are used to identify and characterize fluid mud layers or layers of sediment or

rock under the seafloor. Acoustic penetration into the bed increases with decreasing frequency (usually 10 to

15 KHz for subsurface detection). The technique used is similar to a simple echo sounder. A transducer emits

a sound pulse vertically downwards towards the seafloor, and a receiver records the return of the pulse once

it has been reflected off the seafloor. Parts of the sound pulse will penetrate the seafloor and be reflected off

of the different sub-bottom layers. The data that is obtained using this system provides information on these

sub-floor sediment layers. Sub-bottom profiling systems utilize the principle of seismic reflection.

Seismic refraction (Figure 5) gives more information about the layers. Sound pulses that enter the seafloor

are both reflected and refracted (or bent) as they pass into different layers. The refracted sound pulse follows

a complex path. With seismic refraction, the density of the layers can be determined. Seismic reflection and

refraction can also be done with an instrument on the seafloor called an Ocean Bottom Seismometer (OBS).

This instrument is placed on the seafloor and uses sound from artificial and natural sources. A seismic survey

may make use of both shipboard measurements and measurements from an array of ocean bottom

seismometers.

Accuracy of echo soundings from ship surveys

The accuracy of echo soundings from ship surveys has been evaluated by Rijkswaterstaat, The Netherlands

(Westlake et al., 1996).

Survey errors can be divided into systematic and stochastic errors. The former affect the whole survey data

set, whereas the latter are random and cancel out from survey to survey (e.g., difference plots). The accuracy

of ship surveys is therefore essentially related to systematic errors. For a nearshore nourishment site, the

following error sources were considered to be of most importance:

· determination of the water level at the moment of sounding,

· setting ‘zero’ on the echo sounder (= depth of the transducer below the water surface),

· squat of the ship (= decreased water level around a moving ship, only of importance in shallow water,

say, < 6 m),

· ship specific characteristics, such as the ship’s weight,



· the surf riders effect (= effect of waves on survey accuracy), and

· the accuracy of the positioning in the horizontal plane (x,y).

For the nourishment site, the total effect of these errors in water depths less than 6 m was -0.01 to -0.25 m, or

in water deeper than 6 m the errors are -0.05 to +0.10 m. The larger error in shallower water is caused by the

ship’s squat. Negative errors indicate that the true depth is underestimated by the survey, whereas a positive

notation means that the depth is overestimated.

10.4 Optical bed level detection

Rijkswaterstaat (2004) has tested the use of an optical pole (ASM-IV) for measuring the bed levels in the

surf zone near the beach of Petten (The Netherlands). This instrument consists of a steel pole (diameter of 32

or 40 mm; lengths of 1.8, 2.4 and 2.9 m), which can be driven into the bed. The pole is supplied with many

infra-red light sources/receivers (backscattering sensors) at spacings of 10 mm (100 sensors per meter;

sampling volume of 0.5 cm3).

The instrument measures:

· vertical distribution of the turbidity levels in the water column;

· transition from water column to bed based on the scattering of light from the suspended particles and the

bed material particles;

· transition from water column to air (if pole end is above the water surface).

Additional sensors on the pole are: tilt meter (two directions), pressure sensor and temperature sensor. The

electronic equipment is placed in the upper end of the pole. Enery supply by batteries is sufficient for in-situ

operation of about 2 months (10 measurements every 5 minutes).

The pole can be used for bed level detection of beaches, sedimentation in harbour basins, etc.

Comparison of bed levels in the shallow surf zone measured with the optical system (ASM-IV) and other

techniques shows systematic undersampling of the ASM-IV of about 50 mm due to the generation of a small

scour hole around the pole at the transition from water column to the bed. Other errors are due to positioning

on the pole (horizontal and vertical).

The pole functioned well in the shallow surf zone over a winter period of about 6 months. Problems are:

accumulation of debris at the foot of the pole and biological fouling of the sensors (reducing the sensitivity

for turbidity measurements). The standard software cannot detect when the bed is out of the range of the

sensors. The sensitivity of the sensors is variable (difference of 30% between individual sensors), which

implies that each sensor has to be calibrated to determine with sufficient accuracy the turbidity values in the

water column.

The ASM-IV is concluded to be a robust instrument for optical bed level detection in the coastal

environment.

10.5 Conclusions

The following conclusions are given:

· bed levels soundings using acoustic sensors mounted on frames, tripods and poles have a vertical

inaccuracy of about 0.03 m;

· bed levels soundings using optical sensors mounted in a pole driven into the bed have a vertical

inaccuracy of about -0.05 m (systematic error related to small scour hole at the transition for water

column to bed);

· bed level soundings performed by means of a ship-mounted echo sounder have a vertical inaccuracy of

about 0.1 m to 0.15 m in depths larger than about 6 m due to tide level corrections, ship-induced motions

and wave-induced motions; the inaccuray may be as large as 0.25 m in shallow depths (smaller than 6 m)

due to relatively large ship-induced motions;

· bed level soundings performed by means of a DGPS system mounted on the WESP vehicle have a

vertical inaccuracy of about 0.1 m to 0.15 m in depths smaller than 6 m;



· beach level soundings performed by use of a DGPS receiver mounted on a small vehicle moving over the

beach have a horizontal inaccuracy of about 0.05 m and a vertical inaccuracy of about 0.05 m on

relatively flat and smooth areas and about 0.1 m on steep sloping faces of bars (without tilt correction).

References

Bell, P.S. and Thorne, P.D., 1997. Measurements of sea bed ripple evolution in an estuarine environmentusing a high resolution acoustic sand ripple profiling system. Proc. of Oceans 1997. Halifax, NovaScotia, 6-9 October 1997, MTS/IEEE. IEEE Oceanic Engineering., Piscataway, NJ., p. 339-343

Betteridge, K.F.E., Williams, J.J., Thorne, P.D. and Bell, P.S., 2003. Acoustic instrumentation formeasuring near-bed sediment processes and hydrodynamics. Journal of Experimental Marine Biologyand Ecology, 285-286, p. 105-118

De Hilster, N., 1998. Measuring Accuracy WESP. Dir. Noord-Holland, Rijkswaterstaat, The NetherlandsDolphin, T.J., Grasmeijer, B.T. and Vincent, C.E., 2005. Sand suspension due to waves and tidal flow over

short and long wave ripples, and flat beds on the dutch shoreface. Paper T. In: Sandpit edited by VanRijn et al. 2005 (www.aquapublications.nl)

Gallagher, E.L. et al., 1996. Performance of a sonar altimeter in the nearshore. Marine Geology, Vol. 133,p. 241-248

Rijkswaterstaat/RIKZ., 2004. ASM-IV instrument. Report W2004.108. RIKZ, The Haque, The NetherlandsVan Rijn, L.C. et al., 2002. COAST3D-Egmond. ISBN90-800356-5-3 (www.aquapublications.nl)Westlake, S.J. et al., 1996. Accuracy of NOURTEC bathymetric surveys. Report NOURTEC project, RIKZ,

Rijkswaterstaat, The Netherlands



Figure 1

Crab vehicle at Duck site (USA)

Figure 2

Single beam echo sounder (left) and multi-beam echo sounder (right), (www.dosits.org)



Figure 3A

Side scan sonar (www.dosits.org)



Figure 3B

Bedform examples.A: Location of sonar echoes and shadows from tripod structure;B: tidal ripples (TR);C: short wave ripples (SWR);D: 3D long wave ripples (3D-LWR);E: 2D long wave ripples (2D-LWR);F: flat bed.

A

E

B

FDC

0 4 m 0 1.5 m

0 4 m0 1.75 m

0 4 m0 2.6 m



Figure 4

Seismic reflection, (www.dosits.org)

Figure 5

Seismic refraction, (www.dosits.org)

ANNEX A: MEASURING INSTRUMENTS FOR FLUID VELOCITY,PRESSURE AND WAVE HEIGHT

February 2006

Manual Sediment Transport Measurements Page 1

ANNEX A: MEASURING INSTRUMENTS FOR FLUID VELOCITY, PRESSURE AND

WAVE HEIGHT

CONTENTS

A1 Introduction

A2 Velocity sensors

A2.1 Velocities and bed-shear stresses, instrument characteristics and accuracies

A2.2 Electro-Magnetic Velocitymeter (EMV)

A2.3 Acoustic Doppler Velocitymeter (ADV)

A2.4 Acoustic Doppler Current Profiler (ADCP, UVP)

A2.5 Phased Array Doppler Sonar (PADS)

A2.6 Coherent Doppler Velocity Profiler (CDVP) and Cross-Correlation Velocity Profiler (CCVP)

A3 Comparison of measured velocities

A3.1 Electro-magnetic Velocitymeter (EMV) and Laser Doppler Velocitymeter

A3.2 Acoustic Doppler Velocitymeter (ASTM) and Electro-Magnetic Velocitymeter (EMV)


A3.4 Ultra-sonic Velocity Profiler (UPV) and Particle Image Velocitymeter (PIV)

A4 Fluid pressure and wave height instruments

A4.1 General instrument characteristics, accuracies and selection criteria

A5 Comparison of measured wave heights

A5.1 Pressure sensor and capacity wire

A5.2 Pressure sensor and surface following wave gauge

A5.3 Pressure sensors

A5.4 Velocity sensor, fluid pressure sensor and capacity wires

A5.5 Pressure sensor and resistance wave staff

A5.6 Accelerometer and DGPS on wave rider bouy


February 2006


A1 Introduction

Field measurements of sediment transport in rivers and estuaries generally involve the use of instruments for

measuring sediment concentrations and fluid velocities. In addition, instruments for measuring wave-induced

fluid pressure (wave height), orbital velocities and wave height are required in coastal conditions.

Field measurements in rivers and estuaries usually are carried out by using survey vessels and instruments

suspended from a davit and winch on board of the vessel.

In coastal conditions these types of measurements cannot be performed when surface waves are present and

therefore the use of stand-alone tripods with electronic equipment and data storage is required. The

electronic equipment commonly consist of :

· electromagnetic velocity sensors,

· acoustic velocity sensors (point sensors and profilers),

· optical sediment concentration point sensors,

· acoustic sediment concentration point sensors and profilers,

· optical particle tracking sensors (size and fall velocity),

· acoustic bed level sensors (altimeters, single/multi beam echo sounders; bed profilers, side scan sonar),

· data storage discs.

The use of instruments for measuring physical parameters inevitably involve the problem of the accuracy of

the measured parameters.

The measurement errors are related to:

· the physical size of the instrument including supports, cables, housing for electronics, etc.;

· the measurement principle including electronic instability, drift, offset, calibration procedure, sampling

size and applicability and validity ranges of the instrument concerned;

· the conversion principle including assumptions of applied theories (for example: conversion from fluid

pressure to wave height; errors in position of pressure sensor above bed).

Information of the measurement errors involved can be obtained by comparing instruments based on

different measurement principles under controlled conditions. Recently several studies focussing on

hydrodynamics of wave motion in the large scale wave tanks of Delft Hydraulics (The Netherlands) and of

the ‘Forschungszentrum Küste’ in Hannover (Germany) have been carried out. Various types of instruments

have been used to measure fluid velocities and pressures during the experiments in the wave tanks. In

addition data sets from various field experiments are used to evaluate the performance of the instruments

considered.


February 2006


A2 Velocity sensors

A2.1 Velocities and bed-shear stresses, instrument characteristics and accuracies

Velocities and bed-shear stresses

Velocities, turbulence and shear-stresses play a fundamental role in the nature of sediment transport,

particularly in the near-bed region. Bed-shear stress controlls the entrainment of sediment into the flow and

hence the erosion and deposition of sediments at the bed as well as their transport in the water column.

Recent advances in instrumentation have greatly expanded the sophistication with which near-bed velocities,

turbulence and bed-shear stress can be measured in rivers, estuaries and coastal seas (Kim et al., 2000).

Until recently, eloctro-magnetic velocitymeters (EMV) were among the best instrumentation available for

studying the structure of the bottom boundary layer where sediment transport takes place.

The EMV’s are robust, resistant to fouling, moderately intrusive, and reasonably inexpensive, but they also

suffer from severe limitations including offset drift, limited frequency response and relatively large sampling

volume.

Within the last few years, acoustic instruments have become increasingly available for coastal conditions.

These instruments are also reasonably robust, resistant to fouling, and increasingly affordable. In addition,

acoustic instruments are less intrusive, have better frequency responses and smaller sampling volumes.

Examples are, the Acoustic Doppler Current profiler (ADCP), Ultrasonic Velocity Profiler (UVP) and the

Acoustic Doppler Velocitymeter (ADV).

Bed-shear stresses can be determined from the measured velocity data using the following methods (Kim et

al., 2000):

· Logarithmic profile method (LP): the vertical distribution of the near-bed velocities is assumed to be

logarithmic with the bed-shear stress (tb) and zero-velocity level (zo) as basic parameters which can be

derived by regression analysis from the velocity data (at least 5 sensors in the lowest 1 m of the water

column);

· Covariance method (COV): the bed-shear stress (tb=-r<U/W/>) is assumed to be equal to the time-

averaged value (<..>) of the horizontal (U/) and vertical velocity (W/) fluctuations (turbulent

fluctuations);

· Turbulent Kinetic Energy method (TKE): the bed-shear stress is related to the turbulent energy (tb=rc1E

with E=0.5(<U/U/>+<V/V/>+<W/W/>), with c1=0.21), (also used: tb=rc2<W/W/> with c2=0.9);

· Velocity Spectrum method (VS); the bed-shear stress is derived from the spectral characteristics of the

instantaneous velocity data.

Kim et al. (2000) have evaluated these four methods. Their conclusions are:

· The optimum sensor height of the ADV is problematic. Too close to the bottom, the velocity data suffer

from drastically increased Doppler noise because of increased acoustic signal scatter associated with

near-bed suspended sediments. Reasonable results were obtained using a sensor height of about 0.15 m

above the bed.

· The LP method is sensitive, because it normally requires measurements at several heights up to a 1 meter

above the bed. The LP method generally gives the largest estimates of the bed shear stress.

· The COV method is considered to give unbiased estimates of bottom stress as long as the sensor is

within the constant stress layer near the bed but sufficiently far from the bed to avoid noise problems.

· The TKE method is considered to be the most consistent and exhibits the least variability; c1 is found to

be c1=0.21 and c2=0.9.

· The VS method requires the sensor location to be somewhat further away from the bed to have an

adequate separation of production and dissipation scales. The bed-shear stresses are systematically

underestimated (10%).


February 2006


Instrument characteristics and accuracies

Various measuring principles can be used to determine the flow velocities in laboratory and in field

conditions, as follows (see Goldstein, 1983):

· Mechanical sensors (propellermeters);

· Differential pressure sensors based on the pressure difference between dynamic (stagnation) and static

pressure (Pitot-tube principle for time-averaged, piezo-electric sensors for instantaneous values,

membrane strain-gage sensors for instantaneous values);

· Thermal anemometers based on the principle of measuring the changes in heat transfer from a small,

electrically heated sensor (wire) in the fluid flow (hot-film and hot-wire meters);

· Electro-Magnetic Velocitymeters (EMV) based on the principle that a conducting fluid will generate a

voltage proportional to the flow velocity as it passes through a magnetic field generated by the

submerged sensor;

· Laser-Doppler Velocitymeters (LDV) based on frequency shift of optical signal scattered from particles

in the flow (velocity of particles not that of the flow), applicable in field using fibre glass optics;

· Acoustic-Doppler Velocitymeters (ADV, ADCP, UVP, PADS) based on frequency shift of (ultrasonic)

acoustic signal scattered from particles in the flow (velocity of particles not that of the flow), also based

on frequency shift of travel time of acoustic signals applied in upstream and downstream direction, also

used as depth-profilers (Acoustic Doppler Current profiler ADCP for full water depth or Ultrasonic

Velocity profiler UVP for near-bed region),

Herein, the attention is focussed on the commercially-available instruments for measuring the instantaneous

flow velocity in field conditions.

The following most popular instruments are:

· Mechanical propellermeters for rivers and estuaries without surface waves (www.valeport.co.uk),

· Electro-Magnetic Velocitymeters (EMV) for instantaneous velocities in coastal conditions,

· Acoustis-Doppler Velocitymeters (ADV, ADCP, UVP, PADS) for instantaneous velocities in coastal

conditions.

Based on detailed comparisons of measured velocities using various sensors (see Section A3), it may be

concluded that:

· peak orbital velocities of EMV and ADV may have an uncertainty of maximum 15%; ADV yields peak

orbital velocities which are systematically smaller (about 15%) than those of EMV;

· EMV sensors are not very accurate at very low velocities (<0.05 m/s) due to offset values;

· ADV sensors are not accurate at low velocities (<0.3 m/s) due to insufficient sediment particles in the

measuring volume;

· time-averaged velocities smaller than 0.05 m/s may have an inaccuracy of maximum 100%;

· time-averaged velocities in the range of 0.15 to 0.3 m/s may have an inaccuracy of maximum 30%;

· time-averaged velocities larger than 0.5 m/s are assumed to have an inaccuracy of maximum 15%.

The inaccuracies of Acoustical Doppler Velocitymeters (ADV) are caused by:

· air bubbles in the water (breaking wave conditions);

· insufficient suspended matter in the water during low velocities; insufficient signal detection;

· velocity of suspended matter is measured and not fluid velocity;

· practical working range for velocity range is 0.3 to 2 m/s.

The inaccuracies of Electromagnetic Velocitymeters (EMV) are caused by:

· offset values (zero drift stability);

· air bubbles in the water (breaking wave conditions);

· wearing and fouling;

· practical working range for velocity range is 0.03 to 2 m/s.


February 2006


References

Goldstein, 1983. Fluid Mechanics Measurements, Springer VerlagKim, S.C., Friedrichs, C.T., Maa, J.P.Y. and Wright, L.D., 2000. Estimating bottom stress in tidal

boundary layer from Acoustic Doppler Velocitymeter data. Journal of Hydraulic Engineering, ASCE,Vol. 126, No. 6, p. 399-406


February 2006


A2.2 Electro-magnetic Velocitymeter (EMV)

These instruments are based on the principle that a conducting fluid will generate a voltage proportional to

the flow velocity as it passes through the magnetic field created by the sensor.

Commercially available instruments are manufactured by:

· Delft Hydraulics (www.delfthydraulics.com),

· Marsh-McBirney (www.marsh-mcbirney.com),

· InterOcean S4 (www.interoceansystems.com),

· Valeport (www.valeport.co.uk)

· HS engineers (www.hs-engineers.com).

The EMV (E40 field instrument; Figure A2.2.1) of Delft Hydraulics has the following features:

· electromagnetic, bi-axial, 4-quadrant, ellipsoid 14 x 40 mm, fully immersible (max. 30 m; cable

lengths up to 100 m);

· usable in clean and dirty liquids, including slurries;

· velocity range up to 5 m/s, accuracy 0.01 m/s, 1% of measured value provided that tilt angle < 10°;

negligible effect for tilt angles < 10°; uncertain/too large for tilt angle >10°;

· temperature effect: medium: 1 mm/s per °C; ambient: 0.3 mm/s per °C;

· conductivity effect: 0.02 % of reading per mS/cm;

· towing tank calibration (ISO 3455);

· calibrated and set to meet specifications;

· up to 100 meter cable between probe and signal processor (optional);

· < 0.5 cm/s zero-stability;

· ellipsoid type sensors for high spatial resolution and minimum disturbance;

· low electrical interference susceptibility by transmitter/converter isolation;

· high abrasive resistance, robust and reliable.

Main characteristics of Electro Magnetic Velocitymeters (EMV) are:

· robust and easy to use,

· simple electronics,

· velocity in 2 horizontal of 2 vertical directions,

· offset values (not accurate at very low velocities),

· relatively large sensor head (>0.03 m); relatively large measuring volume and limited frequency

response (not suitable for high-frequency turbulence measurements).

The S4 sensors are manufactured by InterOcean Systems, Inc. The applicability range is 0 to 350 cm/s

with resolution of 0.2 cm/s. The accuracy is about ±2% of the reading. The sensor head is relatively large.

The S4 performance will be affected by breaking waves in the surf zone, where considerable aeration is

produced around the instrument. The effect will be a 'noisy' signal, which will be apparent on a time-series

plot.

The electromagnetic current meters of Marsh McBirney (type 512 OEM) have a resolution 0.2 cm/s and

the precision is better than 2 cm/s.


February 2006


Figure A2.2.1

EMV (E40) of Delft Hydraulics


February 2006



Basically, the ADV measures the velocity of particles (sediments) at a point in the water column from the

Doppler shift in frequency of the emitted and received acoustic signals (without calibration) in 2 or 3-

directions, depending on the sensor arrangement. The system includes three modules: sensor, signal

conditioning module and signal processing module. The measurement probe consists of four ultrasonic

transducers: a transmit transducer located at the bottom end of the stem and three receive transducers, slanted

about 30o from the axis of the transmit transducer and pointed at the sampling volume, which is located

about 0.1 m below the probed tip. Hence, the flow velocity in the sampling volume is not disturbed by the

presence of the probe.

The acoustic frequency is of the order of 10 MHz. The accuracy is of the order of ±1% of the reading, if

sufficient particles are present in the measurement volume. Often, insufficient particles are present at low

velocity conditions resulting in loss of signal and rather inaccurate velocities. An important source of error is

the presence of air bubbles in the water column (breaking wave conditions in surf zone).

ADV’s are commercially available velocitymeters. Velocity measurements can be integrated with

temperature, salinity and pressure measurements in one instrument housing (field instrument).

Typical applications are:

· bottom boundary layer velocities,

· wave orbital velocities,

· turbulent velocities near the bed (in range between 0.15 to 1 m above bed).

General instrument characteristics are:

· small measuring volume sensor (order of 5 to 10 mm),

· minimum flow interference,

· inaccuracy of about 1% of measured value (zero offset),

· capable of measuring turbulent fluctuations in 3 dimensions (sampling rate in range of 1 to 25 Hz),

· vulnerable sensor in field conditions,

· background noise levels are problematic close to bed (<0.15 m) due to high suspended sediment

concentrations close to bed; turbulent parameters are not acurate close to bed

Figure A2.3.1 shows the 3D Vectrino-ADV of NORTEK-instruments (www.nortekusa.com).


February 2006


Figure A2.3.1

Acoustic Doppler Velocitymeter of NORTEK (www.nortekusa.com)


February 2006


A2.4 Acoustic Doppler Current Profiler (ADCP, UVP)

ADCP instruments are being used as:

1) bottom-mounted (big-size upward-looking for velocity profiles over the water column; or small-size

downward-looking for near-bed velocity profiles),

2) ship-mounted (big-size downward-looking).

The ADCP profiler measures the current profile in water using Acoustic Doppler technology. It is designed for

stationary and non-stationairy (ship’s hull mounted) applications. It can be deployed on the bottom, on a

mooring rig, on a buoy or on any other fixed structure. It is a complete instrument and includes all the parts

required for a self-contained deployment with data stored to an internal data logger. Typical applications include

coastal studies, online monitoring and scientific studies in rivers, lakes, estuaries and tidal channels.

The use of Doppler sonar to measure ocean currents is by now well established, and is documented in the

RDI publication: Acoustic Doppler Current Profilers, Principles of Operation (RD Instruments, 1989).Conventional acoustic Doppler current profilers (ADCP’s) typically use a Janus configuration consisting of

four acoustic beams, paired in orthogonal planes, where each beam is inclined at a fixed angle to the

vertical (usually 20 or 30 degrees). The sonar measures the component of velocity projected along the

beam axis, averaged over a range cell whose along-beam length is roughly half that of the acoustic pulse.

Since the mean current is assumed horizontally uniform over the beams, its components can be recovered

by subtracting the measured velocity from opposing beams. This procedure is relatively insensitive to

contamination by vertical currents and/or unknown instrument tilts (RD Instruments, 1989).

The big-size ADCP of RD-Instruments (www.rdinstruments.com) is based on a patented 4 acoustic beam design.

RD’s patented broadband signal processing delivers very low noise data resulting in unparalled fine track

resolution.

The big-size ADCP (Aquadopp profiler) of NORTEK Instruments (www.nortekusa.com) uses three acoustic

beams slanted at 25° to accurately measure the current profile in a user selectable number of cells. The internal

tilt and compass sensors measure the current direction and the high-resolution pressure sensor gives the depth -

and the tidal elevation if the system is fixed mounted. The standard 5 MB recorder and internal alkaline

batteries are typically sufficient for a 2-4 month deployment.

Similar instruments are also available from SONTEK instruments (www.sontek.com).

The small-size Ultrasonic Velocity Profiler (UVP-DUO Monitor) of MET-FLOW instruments

(www.met-flow.com) can be used for near-bed velocity profiling from a frame or tripod placed on the bed. A

single transducer (frequencies of 0.5 to 8 MHz) can measure velocity profiles in water along a single line.

Since measurements and its evaluation is very fast, the UVP-DUO can scan through 20 transducers

simultaneously and still retain time resolution. If the transducers form a grid with crossing measurement

lines, the flow mapping function can be used. In each crossing point, two velocity vectors projections are

known, and UVP-DUO software can compute 2D velocity vectors there. With 20 transducers, the flow field

with up to 100 local velocity vectors can be mapped.

General instrument characteristics are:

· velocity range up to 5 m/s,

· big-size (with transducer/sensor heads of 0.1 to 0.2 m) 300 to 1200 KHz instruments for depth ranges

from 10 to 100 m,

· ping rate of order 2 Hz,

· vertical resolution of 0.25 to 1 m (bins or cells) for depth ranges from 10 to 100 m,

· number of vertical bins up to 100,

· velocity accuracy of the order of 1% of the water velocity (say +/-0.02 m/s) relative to ADCP velocity

(vessel velocity),


February 2006


· velocity error (including boat velocity error) of bin-averaged and time-averaged values (1 minute) is of

the order of +/- 0.05 m/s,

· instrument size of 0.2 m,

· big-size ADCP is not suitable for bottom boundary layer with relative large velocity gradients,

· big-size ADCP is not suitable for turbulent velocity fluctuations (bin measurement volume is too large),

· small-size (pencil-type transducer/sensor with diameter of about 1 to 2 cm, length of about 15 cm) UVP

for near-bed velocities has spatial resolution in the range of 0.5 to 5 mm, sampling rates of 10 to 3000

Hz, profiling range up to 3 m (various sensors available).

The overall accuracy of a downward-looking ship mounted ADCP can be determined as: accuracy = sqrt(p2

+ boat variance) cm/s. The boat variance can be estimated as: 25(cm/s)2 for typical DGPS speed input to

software. The velocity precision p= +/-2 cm/s resulting in an overall error of +/-5 cm/s (1.0 m cells; 1

minute time-averaging).

The boat variance will depend on weather conditions; the rougher the weather the more swinging around the

boat will do and the greater the amount of wave-induced velocity that must be smoothed out. No moving

vessel ADCP surveys should be conducted in truly bad weather.

References

RD Instruments, 1989. Acoustic Doppler Current Profilers, Principles of Operation(www.rdinstruments.com)


February 2006


A2.5 Phased Array Doppler Sonar (PADS)

Phased-array Doppler sonars (PADS) have been used by Smith (2001) to probe an area several hundred

meters on a side with 8-m spatial resolution, sampling every second or less with under 2 cm/s rms velocity

error per sample. Estimates from two systems were combined to produce horizontal velocity vectors.

Two "Phased Array Doppler Sonars" (PADS) were deployed as part of "SandyDuck" in September and

October, 1997 at the Field Research Facility (FRF) of the US Army Corps of Engineers. Looking shoreward

from the 6-m depth contour, they probed a total area about 400 m alongshore by 350 m cross-shore. Over

the smaller region probed by both systems, perhaps 200 m by 300 m, horizontal velocity vectors are fully

resolved. In the outer corners, only one component is resolved; however these 1-component estimates still

provide useful information, particularly concerning wave propagation.

An acoustic signal from a bottom-mounted sonar is projected in a wide horizontal fan, radiating outward in

the water from the instrument package and filling the water column in shallow water. The sound scatters off

particles in the water (especially bubbles) and off the bottom. Some backscattered sound returns to the

sonar, where the signal is received on an array, beamformed into returns from various directions, and

analyzed for frequency shift versus direction and elapsed time since transmission. For direct-path

transmission and return, the time-delay since transmission translates to distance from the sonar. The

frequency shift of the backscattered signal (Doppler shift) is proportional to the radial component of the

velocity of scatterers at the sample volume. The systems used by Smith (2001) were operated at 190 KHz

and 225 KHz center frequencies, with 11 repeats of two different 13 bit Barker codes to spread the signal

over 15.6 KHz bandwidth. The measurements are resolved to 7.6 m (range) by 6 degrees (bearing), with

new estimates produced every 0.75 seconds, pair-averaged to 1.5 second sample rate. The resulting velocity

"radials" have rms error levels of order 1.5 cm/s. By combining the radial velocities from two such

devices located some 300 m apart, both horizontal components of velocity can be estimated on a grid

several hundred meters on a side.

Vertical location (elevation angle) is not resolved; the ~22° vertical beam-width takes in the whole water

column, and the effective location of the measurements is dictated by the centroid of scatterers. In the

frequency range considered here (175 KHz to 240 kHz), microbubbles are efficient scatterers. These are

produced copiously by breaking waves, and in general dominate the backscatter even outside the surfzone

when the wind exceeds 5 m/s or so. In deep water, bubble densities vary by orders of magnitude over

moderate horizontal distances (depending strongly on windspeed), and have a mean vertical distribution

approximated by an exponential distribution with a depth scale of order 1 to 2 m, depending weakly on

windspeed. In shallow water this distribution may be different affecting the effective depth of the

measurement. In particular, the shallower the water is, the larger the fraction of bottom backscatter, so the

stronger the bias is toward zero Doppler shift in the estimates.

There are several aspects of the nearshore environment that distinguish it acoustically from deep water:

(1) The bottom backscatters sound that competes with the signal from scatterers in the water volume. The

received signal attributable to bottom backscatter varies with water depth, and can also vary slowly in

time (presumably as bottom roughness characteristics evolve). In general, this becomes significant when

the wind and waves are weak, when few bubbles are generated.

(2) Plunging breakers can produce a "wall" of bubbles so dense it is acoustically impenetrable. This limits

the shoreward extent of measurements, confining the sample area to outside the active surfzone.

(3) There are large variations in the scatterer content of the water, on scales of meters to tens of meters

(e.g., water advecting offshore in "rip currents" that is full of bubbles from the surfzone).

(4) Advection of water from inlets can also lead to variations in stratification and in particle content of the

water.

Direct comparisons have been made with other current measurements made during the multi-investigator

field experiment “Sandy Duck,” sponsored by the Office of Naval Research (USA), which took place in the


February 2006


autumn of 1997 off the coast of Duck, North Carolina, USA. The coherences between PADS and in-situ

current measurements are high (Figure A2.5.1), but the amplitude of the sonar response is generally low. To

explore this further, a simplified model of wave shoaling has been developed, permitting estimates of wave-

frequency velocity variances from point measurements to be extrapolated over the whole field of view of

PADS for comparison. The resulting time–space movies of sonar response are consistent with quasi-steady

acoustic backscatter intensity from the bottom competing with a variable backscatter level from the water

volume. The latter may arise, for example, from intermittent injection of bubbles by breaking waves,

producing patches of high or low acoustic response that advect with the mean flow. Once this competition is

calibrated via the surface wave variance comparison, instantaneous measured total backscatter intensities can

be compared with an estimated bottom backscatter level (which is updated on a longer timescale, appropriate

to evolution of the water depth or bottom roughness) to provide corrected sonar estimates over the region.

Figure A2.5.1 shows that the PADS velocities and the current meters are in reasonable agreement. However,

while the correlations are high, the scaling factor relating the two kinds of data can vary from one run to

another. The PADS estimates are systematically low; scaling factors up to 3.7 are needed to match the

current meter magnitudes. Further, the scaling adjustment at the deeper site is always smaller than at

shallower sites. This suggests that the cause may be interference from bottom backscatter (having zero or

near-zero Doppler shift), which has increasing effect as the water depth decreases.

The correlations and scale factors were investigated as functions of frequency as well (cross-spectra and

transfer functions). Within the limits of the resolved wave field, these appear to be uniform across frequency.

This suggests that such wave-frequency comparisons can be used to "calibrate" the PADS estimates

independent of frequency.

References

Smith, J.A., 2001. The use of phased array Doppler sonars near shore. Journal of Atmospheric and OceanicTechnology, 2001: Vol. 19, No. 5, p. 725–737.


February 2006


Figure A2.5.1

Sandy Duck experimental site, showing the area covered by the two "Phased Array Doppler Sonars" (PADS).The circles show locations of current measurements made by other investigators.The thin dark arrows indicate velocity estimates from 2.5-minute-averaged PADS data at 16:40 UTC, 10/13/97,during stratified conditions. The longer arrows correspond to velocities near 12 cm/s. Vertical profiles areavailable at x=1000, y=460.The currents 1.5 m below the surface (thick dark arrow) correspond closely to the PADS estimates.Currents closer to the bottom (thick gray arrows centered in some of the circles) do not correspond to PADS estimates.North is about 20° clockwise of left.The location is the Field Research Facility of the US Army Corps of Engineers, near Duck, North Carolina.


February 2006


A2.6 Coherent Doppler Velocity Profiler (CDVP) and Cross-Correlation Velocity Profler (CCVP)

In principle, the backscattered signal from suspended particles in the flow can be utilised to determine the

velocities of the particles (Thorne and Hanes, 2002).

The two techniques are:

· coherent Doppler method,

· cross-correlation method.

The coherent Doppler method is based upon pulse-to-pulse phase coherence between consecutive

transmissions to measure the radial component of the velocity along the beam axis. This instrument uses

Doppler shift to obtain the Doppler flow velocity. The Doppler frequency is obtained from the pulse-to

pulse coherence (phase coherence). Averages over pulse pairs are taken.

The correlation method employs a pair of horizontal separated transducers directed vertically downward and

cross-correlation of the backscattered signals from the transucer pairs is used to obtain the velocity.

Unlike the coherent Doppler system, the correlation method is incoherent, as it is the signal intensity that is

used. The basic requirement is that there are fluctuations in the suspension field, which have spatial scales

greater than the distance between the transducers and that these fluctuations can be cross-correlated. Small-

scale turbulent fluctuations cannot be measured.

Both methods (used as profiling or point-sensors) are in a developing stage of research.

References

Thorn, P.D. and Hanes, D.M., 2002. A review of acoustic measurements of small-scale sediment processes.Continental Shelf Research, Vol. 22, p. 603-632


February 2006


A3 Comparison of measured velocities

A3.1 Electro-Magnetic Velocitymeter (EMV) and Laser Doppler Velocitymeter (LDV)

A disc-type (diameter of about 0.04 m) Electro-magnetic Velocitymeter (EMV) manufactured by Delfts

Hydraulics has been used to measure the orbital velocities in the Large Oscillating Water Tunnel (LOWT) of

Delft Hydraulics (Walstra et al., 1998). The EMV consisted of a 2-axis, 4 cm diameter, ellipsoid probe with

an inaccuracy of 0.01 m/s (± 1% of the measured value) and a zero stability (offset) of less than 5 mm/s. The

EMV measures the water velocity along two perpendicular horizontal axes. The sampling frequency

generally is 2 Hz. The EMV was calibrated at Delft Hydraulics before and after the experiments by towing

the EMV at different constant speeds through a tank. The calibration was performed in the range 0-2.5 m/s.

The calibration curves are linear with a high correlation coefficient (r2 » 0.99).

The EMV was positioned at about 0.03 m above the top of the sand bed in the wave tunnel. In the present

experiments the x-axis of the EMV was oriented parallel to the wave tunnel. Comparison of the calibration

results performed before and after the wave tunnel experiments did not show significant changes in

calibration.

The oscillating water tunnel has the shape of a vertical U-tube with a long rectangular test section (length=

14 m, width= 0.3m, height= 1.1 m). The oscillating water motion is generated by the motion of a pistion

operated in one of the vertical legs of the U-tube. The range of velocity amplitudes is 0.2 to 1.8 m/s with

periods of 4 to 15 s. The standard instrument to measure the fluid velocities in the vertical plane (horizontal

and vertical velocities) in the water tunnel is a forward scattering Laser Doppler velocitymeter (LDV). This

latter instrument is a high-precision instrument with an velocity accuracy of 0.001 mm/s. The height and

length of the measurement volume are approximately 0.22 mm; the lateral width is 6.5 mm (perpendicular to

main orbital motion). The standard range of the velocities is 0.001 to 2 m/s. No calibration is required. The

LDV is mounted in a movable frame standing over the tunnel; the laser beams penetrate through the glass

windows of the tunnel. The measurement position of the LDA was about 0.2 m above the top of the sand bed

present in the tunnel.

The EMV velocity signals have been compared with the velocity signals from the LDV for two tests B7-3

and E2-2.

The basic characteristics of these tests are:

· B7-3; Umax,forward= 1 m/s, Umax,backward= 0.5 m/s and period= 6.5 s, plane sand bed;

· E2-2; Umax,forward= 1.8 m/s, Umax,backward= 1.3 m/s and period= 7.2 s, plane sand bed.

Results of these comparisons are given in Figures A3.1.1 and A3.1.2 It can be observed that the general trend

of the LDV signal is quite well followed by the EMV signal. Discrepancies are as follows:

· Test B7-3; the peak forward velocity of the EMV is about 15% smaller than that of the LDV; the peak

backward velocity of the EMV is about 10% smaller than that of the LDV;

· Test E2-2; the peak forward velocity of the EMV is about 10% to 15% smaller than that of the LDV; the

peak backward velocity of the EMV is about 5% to 10% larger than that of the LDV.

References

Walstra, D.J.R., Van Rijn, L.C., Aarninkhof, S.G.J., 1998. Sand transport at the middle and lowershoreface of the Dutch coast. Report Z2378, Delft Hydraulics,The Netherlands.


February 2006


Velocity

B7-3

-1.20

-0.80

-0.40

0.00

0.40

0.80

1.20

11:24:00 11:24:10 11:24:20 11:24:30 11:24:40 11:24:50 11:25:00

time (hh:mm)

velo

cit

y (

m/s

)

Laser on z = 0.20 m

EMF on z = 0.03 m

Figure A.3.1.1

Orbital velocity in water tunnel measured by Laser Doppler Velocitymeter and by Electro-Magnetic Velocitymeter; Test B7-3

V e lo c it y

E 2 - 2

- 2 . 5

- 2

- 1 . 5

- 1

- 0 . 5

0

0 . 5

1

1 . 5

2

0 9 : 2 1 : 0 0 0 9 : 2 1 : 1 0 0 9 : 2 1 : 2 0 0 9 : 2 1 : 3 0 0 9 : 2 1 : 4 0 0 9 : 2 1 : 5 0 0 9 : 2 2 : 0 0

t im e ( h h : m m : s s )

ve

loc

ity

(m

/s)

L a s e r a t z = 0 . 2 0 m

E M F a t z = 0 . 0 3 m

Figure A.3.1.2

Orbital velocity in water tunnel measured by Laser Doppler Velocity meter and by Electro-MagneticVelocitymeter; Test E2-2


February 2006


A3.2 Acoustic Doppler Velocitymeter (ASTM) and Electro-Magnetic Velocitymeter (EMV)

A tripod (of University of Utrecht) with various instruments (velocitymeters and pressure sensors) was

deployed in the wave tank during experiments on hydrodynamic and sand transport processes (De Boer et

al., 1997a,b). The tripod was deployed on a horizontal sand bed placed on the bottom of the large-scale

Deltaflume (length= 200 m, height= 7 m, width= 5 m) of Delft Hydraulics. The water depth was about 4.5 m.

The acoustic velocitymeters (ASTM or ADV) were mounted in the middle of the tripod. The ASTM

instrument consists of five acoustic velocitymeters arranged in a vertical array between about 0.1 m and 1 m

above the sand bed. The ASTM is attached to a movable arm for accurate positioning of the sensors with

respect to the local sand bed. Basically, the ASTM measures the horizontal velocity of the sand particles

from the Doppler shift in frequency of the emitted and received acoustic signals. The measurement volume is

about 0.2 m (horizontally) from the transducers. Irregular waves with a peak period of 5 s were generated in

the tank.

The velocity signals have also been measured by various Electro-magnetic velocitymeters (EMV) attached to

the wall of the wave tank and attached to the tripod. The EMV measures the fluid velocities.

Figure A3.2.1 shows an example of simultaneously recorded velocity signals of the ASTM and an EMV

attached to the wall (at the same height as the ASTM sensor). It can be observed that the velocity signal of

the ASTM and the velocity signal of the EMV are quite similar with the exception of the peak orbital

velocities. Peak values measured with the ASTM are on average 10% to 20% smaller compared to the peak

values measured with the EMV on the wall.

Figure A3.2.2 shows an example of simultaneously recorded velocity signals of the ASTM and an EMV

attached to the tripod (at the same height as the ASTM sensor). Peak values measured with the ASTM are on

average 10% to 25% smaller compared to the peak values measured with the EMV on the tripod.

The differences between the ASTM and EMV results can be attributed to:

· influence of the wall on the velocities measured near the wall,

· disturbance of the flow field caused by the ASTM transducers,

· disturbance of the flow field caused by the tripod in which the ASTM and EMV are mounted,

· the different measurement principles (measuring sediment velocity by ASTM versus water velocity by

EMV).

Comparison of time-averaged velocities (in the range of -0.05 to 0.05 m/s) derived from the instantaneous

velocity signals of the ASTM and EMV shows relatively large differences.

Figure A3.2.3 shows time-averaged velocities of the ASTM, time-averaged velocities of the EMV

instrument attached to the tripod and time-averaged velocities of the EMV instrument attached to the wall, in

case of irregular waves with a significant wave height of 1.0 m. Near the bed the time-averaged values

measured with the ASTM are onshore-directed, while the EMV velocities tend to be offshore-directed. At

higher elevations above the bed (z > 0.25 m) the ASTM velocities are offshore-directed, while the EMV

velocities are onshore-directed. In case of a larger significant wave height (Figure A3.2.4) the same tendency

was found for the ASTM velocities and the EMV velocities measured near the tripod. The EMV velocities

measured near the wall were offshore-directed at each elevation above the bed.

It is concluded that the relatively small time-averaged velocities (<0.05 m/s) of the orbital motion near the

bed derived from the EMV or the ASTM velocity signals are rather inaccurate (up to 100%) due to

uncertainties of 10% to 20% in the peak values of the orbital motion. The accuracy of the time-averaged

velocities will increase if relatively strong external currents (tide, wind or wave-driven) are present

(superimposed on the wave motion).

Similar measurements have been performed in the surf zone of Egmond (The Netherlands). The ASTM and

EMV sensors were mounted on a trailer positioned in the surf zone close to a minitripod equipped with an

EMV (De Boer et al., 1997a). The time-averaged velocities were in the range of 0.15 to 0.3 m/s and showed

maximum differences of about 30%. The time-averaged velocities based on the EMV were generally larger


February 2006


than those based on the ASTM. This latter instrument becomes inaccurate in conditions with relatively small

velocities, when there is not sufficient sand in suspension for accurate signal detection.

The effect of breaking waves on the performance of the EMV has been studied in a small-scale wave-current

flume by Grasmeijer and Sies (1996). The vertical distribution of the time-averaged velocities was rather

irregular in tests with breaking wave conditions. Local time-averaged velocities are sometimes 30% smaller

than expected according to the trend of the velocity profile over the water depth.

References

De Boer, A.G., Grasmeijer, B. T. and Kroon, A., 1997a. Hydrodynamics, sediment transport and beachmorphology measured with the CRIS and a video system. Report R97-10, Dep. of Physical Geography,University of Utrecht.

De Boer, A.G., Grasmeijer, B. T. and Kroon, A., 1997b. Instruments and methods for measuring thehydrodynamics, sediment transport and beach morphology in the coastal zone. Report R97-21, Dep. ofPhysical Geography, University of Utrecht.

Grasmeijer, B.T. and Sies, E.M., 1996. Sediment concentrations and transport in case of irregularbreaking waves and currents over plane and barred profiles. M.Sc. Thesis Delft University ofTechnology. Report H2466. Delft Hydraulics


February 2006


-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

10:43:00 10:43:20 10:43:40 10:44:00 10:44:20 10:44:40 10:45:00

time (hh:mm:ss)

velo

cit

y (

m/s

)

USTM attached to frame in centre of flume

EMF attached to flume wall

Figure A3.2.1

Example of simultaneously recorded time-series of velocity measured with ASTM (or USTM) and velocitymeasured with EMV attached to the wall at approximately 0.25 m above the bed level

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

10:43:00 10:43:20 10:43:40 10:44:00 10:44:20 10:44:40 10:45:00

time (hh:mm:ss)

velo

cit

y (

m/s

)

USTM attached to frame in centre of flume

EMF attached to frame in centre of flume

Figure A3.2.2

Example of simultaneously recorded time-series of velocity measured with ASTM (or USTM) and velocitymeasured with EMV attached to the tripod (frame) at approximately 0.25 m above the bed level


February 2006


0.0

0.5

1.0

1.5

2.0

2.5

-0.075 -0.050 -0.025 0.000 0.025 0.050 0.075

time-averaged velocity (m/s)

heig

ht

ab

ove b

ed

(m

)

USTM

EMF attached to frame


Irregular waves:

Hs = 1.0 m

Tp = 5.0 s

h = 5.0 m

D50 = 0.375 mm

Figure A3.2.3

Time-averaged velocities measured with ASTM, EMV attached to tripod (frame) and EMV attached to wall;irregular waves: Hs = 1.0 m

0.0

0.5

1.0

1.5

2.0

2.5

-0.075 -0.050 -0.025 0.000 0.025 0.050 0.075

time-averaged velocity (m/s)

heig

ht

ab

ove b

ed

(m

)

USTM

EMF attached to frame


Irregular waves:

Hs = 1.25 m

Tp = 5.0 s

h = 5.0 m

D50 = 0.375 mm

Figure A3.2.4

Time-averaged velocities measured with ASTM, EMV attached to tripod (frame) and EMV attached to wall;irregular waves: Hs = 1.25 m


February 2006


A3.3 Acoustic Doppler Velocitymeters (ADV)

Nortek Instruments has published a technical note (Note 0.14) with information on the comparison of two

ADV’s (NORTEK-Vector and SONTEK-ADVO).

Data were collected during an experiment conducted by Northwest Research, Inc. (NWRA,

www.nwra.com) on the open coast near Ocean Shores, WA. The experiment was a field test of a sensor

package developed by NWRA and sponsored by the Office of Naval Research. The NWRA sensor package

includes a Sontek ADVO. NWRA's sensor package, and its normal placement, are optimized to measure

surface gravity wave directional spectra at frequencies below 0.05 Hz. The NWRA ADVO is configured to

run in continuous mode and to accumulate data in the ADVO registers. The sensor package's imbedded

computer reads and resets these registers based on timing generated by a precise ovenized clock.

Data were collected with both instruments separated by a distance of about 2 m alongshore. At low tide, the

sand bed was dry, and at high tide, the water level was about 2.2 m above the sand. The sampling volume

for both instruments was about 0.5 m above the sand. During the experiment, the instruments were

inundated by the tide five times, each time being under water for about 6 hours.

The Vector measured heading, pitch and roll and recorded velocity data in east, north and up coordinates.

The Vector recorded 13-minutes bursts of 8 Hz data every hour. The Vector was set for a maximum vertical

velocity of 0.6 m/s and horizontal velocity of 2.1 m/s (nominal range 1 m/s).

The ADVO data were collected in coordinates relative to the beach (beach-normal and beach-parallel), and

were corrected for pitch and roll in post processing. The ADVO data were rotated by 180° for comparison

with the Vector data. ADVO data were collected continuously during each of the tidal inundations. The

ADVO’s were sampled at 2 Hz and 4 Hz during the first four inundations, and at 8 Hz during the fifth

inundation. The detailed comparisons all come from the fifth inundation (hours 10-15 on June 1, 2000).

The ADVO’s were set for a maximum vertical velocity of 0.75 m/s and horizontal velocity of 3 m/s

(nominal range 2 m/s).

Data processing consisted of aligning the data in time and identifying and removing data spikes. Time

alignment was necessary because the clocks were independent of each other and because no effort was made

to synchronize them. Velocimeter data commonly has spikes. Spikes were removed by taking the first

difference in the velocity data, and iteratively rejecting outliers exceeding 6 standard deviations. Resulting

gaps were filled by interpolation.

It is concluded that both the Vector and ADVO collected nearly identical data of velocity time series

(velocity range of 0 to 1 m/s) in the surf zone (see Figure A3.3.1). The Vector data had fewer spikes and a

lower noise floor. It does not appear that the spikes and noise affect the data at frequencies below around 1

Hz.

Various ADV-sensors were severely tested in the surf zone of Duck (USA) by Towbridge and Elgar

(2001). The velocity measurements were acquired between 25 August and 21 November of 1997 on a sandy

Atlantic beach near Duck, North Carolina, at the U.S. Army Corps of Engineers Field Research Facility. An

array of five upward-looking Sontek acoustic Doppler velocitymeters (ADVs) was mounted on a low-profile

frame (See Figure A3.3.2). The ADV’s measure the three-dimensional velocity vector in a sample volume

with a spatial scale of approximately 0.01 m (e.g., Voulgaris and Trowbridge 1998) and perform well in

the surf zone. The array included three of the field version of SONTEK's acoustic Doppler velocimeter

(ADVF) and two of the more rugged ocean version (ADVO), one of which was fitted with pressure, tem-

perature, pitch, and roll sensors, as well as compass. The ADVOs shared a common logger that sampled

the two sensors simultaneously at 7 Hz in one burst of 25 minutes each hour. The ADVFs shared a separate

common logger that sampled the three sensors simultaneously at 25 Hz in one burst of 10 minutes each

hour. The ADV frame was approximately 300 m from the shoreline and 300 m north of the Field Research

Facility.


February 2006


The ADVs occupied an on-offshore line along the frame's northern edge, upstream of the frame itself,

relative to the predominantly southerly wind- and wave-driven flows. The bathymetry near the measure-

ment site is approximately uniform in the alongshore direction on scales of kilometers, although there is a

cross-shore channel approximately 100 m wide and 1 m deep beneath the pier.

Other instrumentation included 12 Setra pressure sensors making up "compact arrays" 6 and 7 (CA6 and

CA7), centered approximately 40 m onshore and 80 m offshore of the ADV frame, respectively.

Two Marsh-McBirney two-axis electromagnetic current meters, located near the centers of CA6 and CA7; a

Solent sonic anemometer, located on a mast at the end of the FRF pier approximately 20 m above the

water surface; and a SONTEK 3-MHz acoustic Doppler profiler (ADP), located approximately 170 m

north of the ADV frame. The pressure sensors and two-axis velocitymeters were sampled nearly

continuously at 2 Hz, the sonic anemometer was sampled continuously at 21 Hz, and the ADP sampled

continuously in 0.25-m range bins at 25 Hz.

The ADV array experienced several problems. Estimates of wave angles indicate that the head of the off-

shore ADVF began rotating intermittently immediately after deployment. Approximately six days after

deployment, the offshore ADVO began malfunctioning, and intermittently produced invalid data,

characterized by intervals with velocities of precisely zero or poor correlation with velocities measured by

the other ADVO. The ADVO measurements of acoustic backscatter intensity are clipped (i.e., do not exceed

a fixed maximum value) during strong flows, which precludes use of this measurement to infer sediment

concentration. Approximately ten days after deployment, the two onshore ADVFs were destroyed by large

waves and strong currents. Divers reported that a scour hole with a depth of roughly 0.2 m formed

beneath the ADV frame within a few days of deployment and that sand worms colonized the frame during

the final two weeks, terminating useful data on approximately 27 October (yearday 299).

Voulgaris and Towbridge (1998) used ADV and Laser Doppler Velocitymeter (LDV) in a flume to study

the accuracy of the ADV instrument. Simultaneous measurements of open-channel flow were undertaken in

a 17-m flume using an ADV and a Laser Doppler Velocimeter (LDV). Flow velocity records obtained by

both instruments are used for estimating the true ("ground truth") flow characteristics and the noise

variances encountered during the experimental runs. The measured values are compared with estimates of

the true flow characteristics and values of variance ((w'2), (w'2)) and covariance ({u'w')) predicted by semi-

empirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean

velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at

distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations

greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the

semiempirical models. Closer to the boundary, the measured Reynolds stresses deviate from those

predicted by the model, probably due to the size of the ADV sample volume. Turbulence spectra computed

using the ADV records agree with theoretical spectra after corrections are applied for the spatial averaging

due to the size of the sample volume and a noise floor. The noise variance in ADV velocity records

consists of two terms. One is related to the electronic circuitry of the sensor and its ability to resolve phase

differences, whereas the second is flow related. The latter noise component dominates at rapid flows. The

error in flow measurements due to the former noise term depends on sensor velocity range setting and

ranges from ±0.95 to ±3.0 mm/s. Noise due to shear within the sample volume and to Doppler

broadening is primarily a function of the turbulence dissipation parameter. Noise variances calculated

using spectral analysis and the results of the ground truthing technique are compared with theoretical

estimates of noise.

The analysis of the data collected in conjunction with an LD V sensor showed the following.

· Mean flows measured with the ADV agree within 1% with those measured by the LDV (rms error 5.6

mm/s). This difference is attributed to uncertainties invertically aligning the two sensors. The high

accuracy and the inherent property of zero-drift free velocity measurements make the ADV suitable

for accurate measurements of mean flow even at positions close to the boundary (z= 0.75 cm).

· ADV-measured Reynolds stress values were under-estimated by only 1%, even without any

correction for noise. The accuracy of these measurements was also confirmed by the ability of the

sensor to describe the vertical variation of the Reynolds stress according to existing models for open

channel flow.


February 2006


· Turbulence intensity of the vertical component can be resolved accurately by the sensor, while the

intensity of the downstream components suffers from a high noise term that is an inescapable

feature of the geometry of the ADV.

· Good agreement was found between bottom shear velocities calculated using the Reynolds stress and

logarithmic profile methods.

· Spectral estimations of the horizontal and vertical flow components agree with theoretical spectral

estimations based on Kolmogorov's model if correction is applied for 1) viscous dissipation effects,

2) production effects, 3) spatial averaging of the sample volume, and 4) measurement noise variance.

· The noise variance along the measuring beam of the ADV signal consists of an electronics-related

component (ability of the sensor to resolve the phase of a pair of pulses) and a flow-related

component. The latter appears to dominate at high flow rates, and it has been shown to be primarily a

function of the turbulence dissipation factor. Noise variances estimated using the spectra of the signals

were smaller than noise variances derived using the ground truthing technique but in good agreement

with the theory. The latter is suggested as a first-order approximation for the prediction of noise

levels.

Delft Hydraulics (1996) compared the ADV (SONTEK) and LDV above a flat sand bed (heights of 0.1 and

0.15 m) in an oscillating water tunnel with a mobile sand bed (sheet flow layer; 0.13 mm sand bed). The

sampling volumes of the ADV and LDV were positioned at exactly the same location. Time-averaged

velocities and turbulent velocity fluctuations (<u/u/>, <w/w/> and <u/w/) were measured. The results are

shown in Figures A3.3.3 to A3.3.5.

The time-averaged and the oscillatory velocity components of the ADV and LDV are in close agreement.

The turbulent parameters show relatively large deviations. The horizontal turbulent intensities measured by

ADV are about twice as large as those of the LDV, while the vertical turbulent intensities are about twice as

small. Differences are caused by the differences in sampling volumes (0.2 mm for LDV and 5 mm for ADV)

and background instrument noise, which is relatively large for the ADV. The velocity spectrum of ADV

shows an almost horizontal line above a frequency of 1 Hz, which seems to indicate that fluctuations above 1

Hz are influenced by instrument noise. Basically, the instrument noise should be uniformly spread over the

frequency domain (white noise).

The working of the ADV close to the mobile sand bed was also tested. This was done by positioning the

ADV at 8 different levels, varying from around zero (initial bed level) to about 50 mm above the initial bed

level. The results reveal that the ADV does not work properly close to the bed, because the velocities do not

decrease sufficiently back to zero very close to the bed (1 mm above bed) in conditions with high sand

concentrations (in the sheet flow layer).

Williams et al. (2003) report good agreement between turbulent parameters measured with various types of

ADVs in field conditions with mobile bed forms. Errors are within 10%.

References

Delft Hydraulics (1996). Net sand transport rates and transport mechanisms of fine sand in combinedwave-current sheet flow conditions. Report H2462, Part IV, Delft Hydraulics, Delft, The Netherlands

NORTEK Technical Note 0.14. Surf Zone Observations with a Nortek Vector Velocitymeter and a SontekADVO. Nortek Instruments (www.nortekusa.com)

Towbridge, J.H. and Elgar, S., 2001. Turbulence measurements in the surf zone. Journal of PhysicalOceanography, Vol. 31, p. 2403-2417

Voulgaris, G. and Towbridge, J.H., 1998. Evaluation of the Acoustic Doppler Velocitymeter for turbulencemeasurements. Journal of Atmospheric and Oceanic technology, Vol. 15, p. 272-289

Williams, J.J., Bell, P.S. and Thorne, P.D., 2003. Field measurements of flow fields and sediment transportabove mobile bed forms. Journal of Geophysical Research, Vol. 108, No. C4, p. 6.1-6.35


February 2006


Figure A3.3.1

Measured velocity signals by two Acoustic Doppler Velocitymeters in surf zone over a time period of 80seconds in surf zone (USA), (Nortek Technical Note 0.14)Upper panel: East horizontal velocityMiddle panel 1: North horizontal velocityMiddle panel 2: Verical velocityBottom panel: Pressure

Figure A3.3.2

Array of near-bottom acoustic Doppler current meters deployed in about 4.5 m water depth approxi-mately 300 m from the shoreline.ADVF and ADVO are SONTEK field and ocean probes, respectively (Towbridge and Elgar, 2001).


February 2006


Figure A3.3.3

Time-averaged velocity at 0.15 m above bed

Figure A3.3.4

Turbulent parameters at 0.15 m above bed

Figure A3.3.5

Spectrum of instantaneous velocity at 0.15 m above the bed


February 2006


A3.4 Ultra-sonic Velocity Profiler (UPV) and Particle Image Velocitymeter (PIV)

The UVP and PIV (O’Donoghue, 2005) were compared above a rippled sand bed in the oscillating

water tunnel of the University of Aberdeen (Scotland). The measurements were made at separate times

during separate experiments. The ripples were slightly different between the 2 measurements. The results are

shown in Figure A3.4.1.

The UVP was used to measure the velocities in sheet flow very close to the bed (typically down to z=0, i.e.

the bed level corresponding to no flow). It is difficult to compare these velocity measurements with

measurements using other instruments because other instruments that can make the measurements in the

presence of such high concentrations are not available. The resolution and accuracy of UVP measurements

depends on the choice of transducer frequency and on the settings used.

References

O’Donoghue, T. 2005. Personal Communication, University of Aberdeen, Scotland

Figure A3.4.1

Velocities measured close to bed in oscillating water tunnel of University of Aberdeen (Scotland) using UVPand PIV


February 2006


A4 Fluid pressure and wave height instruments

A4.1 General instrument characteristics, accuracies and selection criteria


Water level fluctuations in deeper water generally are measured by pressure sensors or by wave bouys.

Water level fluctuations can also be measured by capacitance wires/rods attached to poles jetted into the bed.

Bottom-mounted applications can, in principle, also be used to determine the instantaneous wave height by

using the horizontal velocity measured in the near-surface region and linear wave theory.

Herein, the attention is focussed on the use of commercially available wave bouys and fluid pressure sensors

which are commonly used in stand-alone tripods and wave rider bouys for deep water.

Directional wave rider bouys

The wave rider bouy (hull diameters up to 1 m) is a spherical bouy, which measures wave height and

direction (Figure 4.1.1). The wave height measurement is based on the principle of measuring vertical

accelerations. The direction measurement is based on the translational principle which means that horizontal

motions instead of wave slopes are measured. This type of measurement is independent on bouy roll motions

and therefore a relatively small spherical bouy can be used. A single point vertical mooring (with 30 m

rubbercord) ensures sufficient symmetrical horizontal bouy response, also for small motions at low

frequencies.

The bouys generally contains:

· pitch-roll sensors,

· accelero-meters in three directions,

· compass, temperature sensors, GPS position (1 cm vertical, 10 m precision horizontal),

· software for wave spectrum parameters.

The frequency resolution is of the order of 0.01 Hz in the range of 0.025 to 1 Hz (wave periods of 1 to 40 s).

GPS precision is not only determined by the intrinsic GPS signal tracking measurement but also by external

factors such as the varying number of satellites in view and atmospheric variations. Wave rider bouys with

DGPS on it allow the measurement of very accurate wave heights.

Datawell provides commercially available wave rider bouys (www.datawell.nl).

Datawell has analyzed the measurement results of three bouys with diameters of 0.4, 0.7 and 0.9 m in North

Sea conditions with significant wave height of about Hs=0.75 m and peak wave period of Tp=5 s. The

significant wave height measured by bouys of different sizes agreed within 0.04 m, peak wave directions

agreed within 6 degrees. Generally, the true significant wave height is slightly underestimated (missing

energy contributions; cut-off frequency of about 1 Hz) by about 1 %.

PC software is commercially available for data analysis.

Fluid pressure sensors

Various types of pressure sensors are commercially available. Generally, piezo-electric transducers are used.

Piezo-electric materials such as quartz crystals produce an electric field under deformation by pressure

forces. For example, the 2100-A pressure sensors of Paroscientific (www.paroscientific.com) have a

precision of 0.015 % i.e. 1 cm for a maximum range of 0-70 m (0-100 psi), linear calibration curve. The

resolution is 0.0015 % (0.1 cm). The instrument offset can be determined in the laboratory prior to

deployment and taken into account by the calibration curve.

Sources of errors in water depth and wave height derived from pressure sensors are: application of linear

wave theory, variations of the water density, variations in the height of the sensor above the local bed and

variations in barometric (air) pressure. Afterwards, barometric (air) pressure (measured in the field), can be

taken into account (accurate to ±1 cm). The atmospheric pressure influence can also be eliminated by fitting


February 2006


a linear function between the atmospheric pressure and the height recorded by the pressure sensors above

water.

The largest contribution to the inaccuracy of the total water depth is assumed to be caused by the unknown

height of the sensor above the bed related to (sometimes quite large) erosion/sedimentation of the local bed.

Bird (1993) has made a review of the available literature on the accuracy of wave height measurements

based on pressure sensors. Based on analysis of results from various field sites, he has concluded that a

modern well designed pressure transducer system in combination with proper analysis techniques can give

estimates of surface wave height to within +/-10%.

Acoustic Doppler Current Profilers (ADCP)

ADCP instruments can also be used for wave field measurements (see Figure 4.1.2 and Wave User’s

Guide of RD-Instruments). The basic principle behind wave measurement is that the wave orbital ve-

locities below the surface can be measured by the highly accurate ADCP. The ADCP (with 2 Hz data

recording and waves upgrade of software) measures the subsurface orbital velocities created by the wave

field. This raw data is averaged to create a mean current profile, and is accumulated into time series for

waves processing. Each time series of data is called a burst. From this burst, velocity power spectra,

directional spectra, and mean water levels are calculated.

The ADCP should be bottom mounted, upward facing (within 5 degrees of the vertical) with a pressure

sensor for measuring tide and mean water depth. Time series of velocities are accumulated, and from

these time series, velocity power spectra are calculated. To get a surface height spectrum, the velocity

spectrum is translated to surface displacement using linear wave kinematics. The depth of each bin

measured and the total water depth are used to calculate this translation. To calculate directional spectra,

phase information must be preserved. Each bin in each beam is considered an independent sensor in an

array. The cross-spectrum is then calculated between each sensor and every other sensor in the array. The

result is a cross-spectral matrix that contains phase information in the path between each sensor and every

other sensor at each frequency band. The cross-spectrum at a particular frequency is linearly related to the

directional spectrum at a particular frequency. By inverting this forward relation, the directional spectrum

can be determined.

The measurement of oscillatory flow is more complicated than measuring current velocities. At any instant

of time, the wave velocity varies across the array. As a result, except for waves that are highly coherent

during their passage from one beam to another, it is not possible to separate the measured along-beam

velocities into their horizontal and vertical components. However, the wave field is statistically steady in

time and homogeneous in space, so that the cross-spectra of velocities measured at various range cells

(either between different beams or along each beam) depend on wave direction. This fact allows to apply

array-processing techniques to estimate the frequency-direction spectrum of the waves. In other words,

each depth cell of the ADCP can be considered an independent sensor that makes a measurement of one

component of the wave field velocity. The ensemble of depth cells along the four beams constitutes an

array of sensors from which magnitude and directional information about the wave field can be determined.

The ADCP can use its profiling ability (bins and beams) as an array of sensors. Because the ADCP can

profile the water volume all the way to the surface, it can be mounted in much deeper water than a

traditional pressure sensor. Higher frequency waves attenuate more quickly with depth below the surface.

The ADCP can measure much higher frequency waves than a pressure sensor and do so in deeper water,

because it can make measurements higher up in the water column. Additionally, the ADCP has many

independent sensors (bins-beams) so even when sampling at a 2Hz sample rate the data is as quiet as if it

had been sampled at 200Hz by a single point meter (example uses 25 bins and 4 beams).

To achieve the best possible solution for wave height spectrum, the height spectrum and the noise

spectrum are fit to the bin-beam data using a least squares fit. In addition to the orbital velocity technique


February 2006


for measuring wave spectra, the ADCP can measure wave height spectra from its pressure sensor (with

frequency/depth limitations) and from echo ranging the surface. Within the frequency range of the pressure

sensor the pressure height spectrum is an old reliable reference for data comparison. The surface track

measurement of wave height is reliable most but not all of the time. The advantage of the surface track

derived height spectrum is that it is a direct measurement of the surface and can measure wave energy at

very high frequencies (higher than 0.9 Hz in some installations). Having three completely independent

measures of wave height spectrum that all agree very closely is a solid argument for data quality.

Inaccuracy related to the application of linear wave theory

Time series of near-bed pressure (for a pressure sensor in a tripod standing on the bed) are routinely

converted to the sea surface elevation with a depth correction using linear theory. The pressure at a distance zabove the bed, pz, is related to the pressure at the sea surface elevation, psse, as

pz/psse=cosh(kz)/cosh(kh) for z³0 m

and

pz/psse=ekz/cosh(kh) for z<0 m

Here, k is the frequency dependent wave number and h is the total water depth. The accuracy of the wave

height computed from a time series of the sea surface elevation based on linear wave theory, will depend on

(among other factors):

· the use of linear wave theory itself,

· the inaccuracy in z (and thus h, of the order of 5% to 10%),

· the maximum frequency that can be applied.

The estimated inaccuracy of water depth is about 5 to 15 cm, occasionally up to 50 cm on steep slopes. Test

computations indicate that the uncertainty in water depth does not affect the computed wave height by more

than about 10% (Ruessink, 1999). For accurate wave height measurements is recommended to install an

acoustic altimeter (echo sounding instrument) on each tripod for continuous bed level readings.

The maximum frequency that can be used is often referred to as the cut-off frequency, fc. Above fc,

amplification factors between pz and psse become unrealistically large. The choice of fc is related to the ratio

pz/psse, although there is no general consensus on the minimum value of this ratio to which the depth

conversion using linear wave theory may be applied. However, a value of pz/psse = 0.1 seems to be most

commonly used. When analyzing a large data set, one could choose to compute fc for each burst.

Alternatively, one could also define a fixed fc to be used for all bursts and all positions. This latter option is

most often used. The choice of fc is then defined by the sensor located closest to the bed in deepest water.

The value fc = 0.33 Hz can be adopted, based on a bottom-mounted array of pressure sensors in 6 to 8 m

water depth. Ruessink (1999) re-evaluated this choice for tripods deployed in much shallower depth,

typically 1 to 4 m. He found that, based on pz/psse = 0.1, fc for these tripods could be raised to 0.4 Hz.

However, the increase in wave height caused by increasing fc from 0.33 to 0.4 Hz was small, generally

between 5 to 10%.

Obviously, the inaccuracy in wave height will increase if a considerably amount of energy is located above

fc, but cannot be reconstructed with a pressure sensor and linear wave theory. Thus, fc should be well above

typical peak frequencies that can be expected in the field. By changing fc from 0.33 to 0.4 Hz, the increase in

wave height is about 5% to 10%.


February 2006


Overall inaccuracies

Based on detailed comparisons of wave height measurements (see A5), the following results are given:

· rms and significant wave height in non-breaking conditions derived from direct measurement of surface

elevation by use of capacity wires or derived from pressure sensors using linear wave theory may have an

uncertainty of maximum 10%;

· rms and significant wave height in breaking conditions (surf zone) derived from direct measurement of

surface elevation by use of capacity wires or derived from pressure sensors using linear wave theory may

have an uncertainty of maximum 15%;

· significant wave heights derived from pressure sensor measurements (using linear wave theory) are

systematically somewhat smaller (maximum about 15%) than those derived from direct wave elevation

(capacity/resistance wires) measurements;

· wave heights derived from direct water surface elevation measurements and from pressure data using

linear wave theory are in reasonably good agreement (within 15%) suggesting that local nonlinearity

effects are not extremely strong;

· the energy density spectra of a pressure transducer system do not show frequencies higher than about 0.4

Hz, which is the cut-off frequency in the filter method used in the transformation of the data from

pressure to water elevation levels; thus the pressure transducer system is not accurate for relatively small

waves with frequencies larger than about 0.4 (wave periods<2.5 s); large errors may occur when

relatively small waves (<0.5 m) are of importance within the wave spectrum;

· wave height parameters of wave rider bouys in deep water have an accuracy of the order of 5% to 10%;

the peak wave period parameters may be inaccurate when longer and shorter waves of similar energy

intensity coexist.

The inaccuracy of the water depth derived from pressure sensors is strongly dependent on the inaccuracy of

the vertical position of the sensor above the bed; this latter parameter should be measured continuously

(acoustic depth sounder) in conditions with rather large bed level changes. The inaccuracy of the wave

height is much less affected by inaccuracies of the position of the pressure sensor (except in shallow water).

References

Bird, P.A.D., 1993. Measurement and analysis of sea waves near a reflective structure. Doc. Thesis. Schoolof Civil and Structural Engineering, Faculty of Technology, University of Plymouth, England.

Datawell notes. Wave rider bouys. Datawell, Haarlem, The Netherlands (www.datawell.nl)RD Instruments, 2001. Wave User’s Guide. Report P/N 957-6148.00, USARuessink, B.G., 1999. Data report 2.5D experiment Egmond aan Zee. Dep. of Physical Geography,

University of Utrecht, The Netherlands


February 2006


Figure 4.1.1

Wave rider bouy

Figure 4.1.2

ADCP for wave field measurements


February 2006


A5 Comparison of measured wave heights

A5.1 Pressure sensor and capacity wire

The experiments have been carried out in the ‘Grosser Wellenkanal’ (GWK; length= 300m, depth= 7 m,

width= 5 m) in Hannover, Germany. Irregular waves with a peak period of 6 s have been generated in a

water depth of 3.5 m (Grasmeijer, 2000).

A tripod (of University of Utrecht) with various instruments (velocity meters and pressure sensors) was

deployed on a sand bed (d50= 0.23 mm) in the wave tank during these experiments. The water depth above

the sand bed was about 3.5 m. The pressure sensor was at 0.6 m above the sand bed.

The standard instrument to determine the wave height is a capacity wire attached to the wall of the wave tank

at about 5 m seaward of the tripod location. Furthermore, the measurement period of the capacity wire was

about 6 minutes longer than that of the pressure sensor.

Data are available for one wave height: Hs=1.25 m, Tp= 6 s.

The computed wave heights based on the pressure sensor (cut-off frequency of 0.4 Hz; linear wave theory)

are within 5% to 10% of the wave heights based on the capacity wire system, see Table 5.1.1 below.

Parameter H1/3

(m)

H1/10

(m)

Hrms

(m)

Hm0

(m)

Capacitance wire 1.23 1.55 0.87 1.21

Pressure sensor with correction

based on linear wave theory

1.22 1.43 0.91 1.27

Table 5.1.1

Comparison of wave heights

References

Grasmeijer, B.T., 2000. Large wave flume experiments Hannover. Department of Physical Geography,University of Utrecht.


February 2006


A5.2 Pressure sensor and surface following wave gauge

A tripod (of University of Utrecht) with various instruments (velocity meters and pressure sensors) was

deployed on a horizontal sand bed in the large-scale Deltaflume of Delft Hydraulics during experiments on

hydrodynamic and sand transport processes (Chung and Grasmeijer, 1999). The water depth above the

sand bed was about 4.5 m. The pressure sensor was at 2 m above the bottom of the sand bed. Irregular waves

with a peak period of 5 s were generated in the tank.

The standard instrument used by Delfts Hydraulics to determine the wave height is a water surface following

gauge, which is operated from a (movable) bridge over the tank.

Data are available for two wave heights: Hs= 1 and 1.25 m, Tp= 5 s.

Analysis showed that a cut-off frequency of 0.5 Hz is an appropriate value to be used for converting the

pressure time series to surface elevation (see Figures 5.2.1 and 5.2.2). Using a cut-off frequency of 0.4 Hz,

the computed wave heights are about 2% smaller.

The computed H1/3-wave heights based on the pressure sensor (cut-off frequency of 0.5 Hz) are about 5% to

10 % smaller than those according to the surface following gauge system, see Table 5.2.1 below.

Test Hs=1 m, Tp= 5 s

Type of instrument H1/3

(m)

Hrms

(m)

Hm0

(m)

pressure sensor with correction

based on linear theory

0.94 0.67 1.00

surface following wave gauge 1 1.03 0.73 1.03


Test Hs=1.25 m, Tp= 5 s

Type of instrument H1/3 Hrms Hm0

pressure sensor with correction

based on linear theory

1.20 0.86 1.27



Table 5.2.1


References

Chung, D.H. and Grasmeijer, B.T., 1999. Analysis of sand transport under regular and irregular waves inlarge-scale wave flume. Report R99-05, Department of Physical Geography, University of Utrecht.


February 2006


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

frequency (Hz)

sp

ectr

al d

en

sit

y (

m2/H

z)

pressure sensor with correction, cut-off at 1 Hz

surface following wave gauge 1


Figure 5.2.1

Energy density spectrum measured in the wave tank with a pressure sensor (location x = 118 m) and twosurface following wave gauges (location x = 118 and 120); pressure time series is converted to surfaceelevation based on linear wave theory with a cut-off frequency of 1.0 Hz.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

frequency (Hz)

sp

ectr

al d

en

sit

y (

m2/H

z)

pressure sensor with correction, cut-off at 0.5 Hz



Figure 5.2.2

Energy density spectrum measured in the wave tank with a pressure sensor (location x = 118 m) and twosurface following wave gauges (location x = 118 and 120); pressure time series is converted to surfaceelevation based on linear wave theory with a cut-off frequency of 0.5 Hz.


February 2006


A5.3 Pressure sensors

Field measurements have been carried out in the surf zone of Egmond beach (The Netherlands) within the

COAST3D project (April-May, 1998 and October-November, 1998). The Egmond site is located in the

central part of the Dutch North Sea coast and consists of a sandy beach (about 0.3 mm sand). The local

morphology is 2.5 dimensional exhibiting two longshore bars intersected by local rip channels; the bars are

aligned parallel to the shore most of the time, but crescentic bar forms do also occur. The wave climate is

dominated by sea waves with a mean annual significant offshore wave height of about 1.1 m. The tidal range

varies between 1.4 m (neap) and 2 m (spring). The tidal peak currents in the offshore zone are about 0.5 m/s;

the flood current to north is slightly larger than the ebb current to south. Wave height measurements

(pressure sensor) have been carried out at various locations (1A, 1B, 1C and 1D; around the crest of the inner

bar) along the main transect using stand-alone tripods of the University of Utrecht (Grasmeijer, 2002).

Wave heights have also been measured by use of a pressure sensor at the CRIS trailer connected to the

WESP (of Rijkswaterstaat). The WESP is an approximately 15 m high amphibious 3-wheel vehicle; the

CRIS is a 3.5 m square and 2.5 m high trailer. The instruments on the CRIS are attached to a movable arm at

the seaward end of the CRIS. The CRIS was positioned as close as possible (within 10 m) to the tripod

locations 1A and 1B. The measurements have been carried out in water depths varying between 1 and 5 m.

The wave heights (H1/3) were in the range up to 1.35 m (peak periods of 5 to 10 s). Relative wave heights

(Hs/h) were as large as 0.4. Breaking waves were present during most measurements. Time series of the

wave heights measured at tripod locations 1A and 1B and at the CRIS location are given in Figure 5.3.1.

Analysis of the data shows that the significant wave height at the tripod locations 1A and 1B is on average

about 10% larger than at the CRIS location (neglecting wave heights smaller than 0.5 m). This shows that the

results of pressure sensors on different nearby stations give comparable values; the relatively small

deviations are most probably caused variations in local hydrodynamics and bathymetry.

References

Grasmeijer, B.T., 2002. Proces-based cross-shore modelling of barred beaches. Doctoral Thesis,Department of Physical Geography, University of Utrecht, Utrecht, The Netherlands.


February 2006


0

0.5

1

1.5

2

2.5

4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300

sig

nif

ica

nt

wa

ve

he

igh

t (m

) measured with UU tripod (item 1a)

measured with CRIS

measured with UU tripod (item 1b)

0

2

4

6

8

10

12

4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300

sig

nif

ica

nt

wa

ve

pe

rio

d (

s)

measured with UU tripod (item 1a)

measured with CRIS

-90

-70

-50

-30

-10

10

30

50

70

90

4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300

time (hours)

wa

ve

dir

ec

tio

n (

de

gre

es

).

measured with UU tripod (item 1a)

measured with CRIS

Figure 5.3.1

Significant wave height, wave period and wave direction measured at tripod locations 1A, 1B and at CRISlocation, surf zone of Egmond, The Netherlands


February 2006


A5.4 Velocity sensor, fluid pressure sensor and capacity wire

Guza and Thornton (1980) used pressure sensors (Stathem temperature-compensated, dynamic range of

912-2316 or 912-3720 g/cm2), electro-magnetic velocitymeters (Marsh-McBirney, spherical, diameter of

0.04 m, three-pole output filter at 4 Hz) and wave staffs (dual resistance wires) to measure the

hydrodynamics at Torrey Pines Beach, San Diego, California in the USA. All instruments were mounted on

pipes which had been fluidized in the bed. Data were retrieved from the sensors by telemetering the data to

the shore. Velocity and pressure spectra measured by the instruments have been related to sea surface

elevation, using linear wave theory. The significant wave height is defined as Hs=4(s2)0.5 with s2 = total

variance of the surface elevation with frequencies between 0.05 and 0.3 Hz for a burst of 34 minutes. The

data represent eight different days with rather different incident wave conditions varying from narrow

banded to very broad banded spectra.

Guza and Thornton show plots of the ratio Hs,u/Hs,p and Hs,u/Hs,h as a function of water depth (between 0.5

and 6 m) at the measurement stations involved; Hs,u= significant wave height derived from measured

velocity signal using linear theory, Hs,p= significant wave height derived from measured pressure signal

using linear theory, Hs,h= significant wave height derived from measured surface elevation (wave staffs).

Analysis of the results show the following features:

· the Hs,u/Hs,p and Hs,u/Hs,h ratios usually show a discrepancy less than 10% for non breaking wave

conditions in shallow and deeper water (0.5 to 6 m);

· the ratios have as much as 20% disparity for breaking wave conditions near the breakpoint of the waves;

· the significant wave heights based on direct water surface elevation measurements are systematically

larger than those derived from the velocity and pressure measurements using linear wave theory;

· the relatively good agreement between Hs derived from direct measurement of surface elevation and Hs

based on velocity or pressure data using linear wave theory suggests that local nonlinearity effects are not

extremely strong (across all frequency bands);

· the results are valid for Hs-values in the range between 0.2 and 1.3 m in depth between 0.5 and 6 m.

References

Guza, R.T. and Thornton, E.B., 1980. Local and shoaled comparisons of sea surface elevations, pressuresand velocities. Journal of Geophysical Research, Vol. 85, No. C3, p. 1524-1530.


February 2006


A5.5 Pressure sensor and resistance wave staff

Two measurement systems for wave heights (Ilic, 1994) owned by the University of Plymouth and

University of Brighton have been compared using data collected at the field site Felpham (UK) in water

depths of 0.5 to 3.5 m (due to tidal variation). Relatively low waves have been measured (Hmo is 0.3 to 0.7 m

in depths of about 3.5 m; and 0.3 to 0.4 m in depths of about 1 m). The measurement systems consist of:

· Plymouth pressure transducer system: the system consists of six transducers and a data recorder unit,

which are held in position on the sea bed in specially fabricated supports; the output signal is in the range

of 4 to 20 mA corresponding to a pressure range of 0 to 4 bar; sampling frequency was 2 Hz.; sampling

cycle was 11 minutes in every three hour period;

· Brighton resistance wave staff system: the wave staff system measures instantaneous water surface

elevation directly and simultaneously at four sensor positions; each sensor consists of 6 metre resistive

device mounted on an aluminium scaffold held vertically from the sea bed, on a triangular base frame; a

cable is used to connect the sensor array to a base station on the beach; all wave data with frequencies less

than 0.75 Hz are sampled; sampling frequency was 4 Hz.

Analysis of the data show the following results (see also Table 5.5.1 below):

· the mean water depths based on the transducer system are systematically larger than those based on the

wire system; the maximum difference was about 15% for depths smaller than 1 m and about 5% for

depths larger than 1 m; variations in atmospheric pressure were not taken into account and the wire

system was subject to a small drift in the zero voltage offset related to temperature variations;

· the energy density spectra of the transducer system do not show frequencies higher than 0.4 Hz, which is

the cut-off frequency in the filter method used in the transformation of the data from pressure to water

elevation levels; thus the pressure transducer system is not accurate for relatively small waves with

periods smaller than about 2.5 s; large errors may occur when relatively small waves (<0.5 m) are of

importance within the wave spectrum;

· for the spectra of relatively low waves with clear defined peaks, the Brighton system (after filtering)

generally yields the highest total spectral energy density values (maximum difference of 15% in mo-

values; less than 10% in Hmo= 4(mo)0.5);

· for broad-banded spectra of relatively low waves without clear peaks (18 june 14.26), the Plymouth

pressure transducer system yields the largest mo-values (mo-value is about 40% larger than that of filtered

Brighton wave staff data; Hmo is 20% larger);

· the application of various filter methods (Welch-window, Hanning-window or Cosine-bell window) to

produce energy spectra resulted in differences of about 5% for the same dataset;

· the Hmo derived from both instruments (after filtering) show differences of 10% for peaked spectra and up

to 20% for broad-banded spectra; the Hm0 of the unfiltered wave staff is about 25% larger than the Hmo of

the pressure transducer for relatively small waves (<1 m), which is caused by the presence of waves with

frequencies larger than 0.4 Hz (cut-off frequency for pressure transducer signal).

Date Mean

water

depth (m)

Hmo,wave staff

(m)

Hmo,pressure transducer (m)

unfiltered time

series

filtered using

spectral

windows

filtered using cut-off

frequency and spectral

windows

11 june 17.26 3.55 0.33 0.27 0.25

18 june 14.26 1.07 0.35 0.25 0.31

18 june 11.26 3.38 0.74 0.64 0.59

14 june 11.26 0.82 0.42 0.32 0.32

Table 5.5.1



February 2006


References

Ilic, S., 1994. Comparison of two measurement systems. Internal report No. SCSE 94-002. School of Civiland Structural Engineering, University of Plymouth, England


February 2006


A5.6 Accelerometer and DGPS on wave rider bouy

Datawell deployed a directional wave rider bouy (MKII) with wave sensors (1 Hz data) and a GPS system

(1Hz data) at a station in the North Sea in 1999 for a period of 2 months with the aim to verify the

performance of the GPS algorithm with reference to the accelerometer-based bouy measurements.

Using the data from both methods, a set of directional wave parameters was computed in the time and

frequency domain every 10 minutes over 512 samples. The measured significant wave heights (derived from

spectral data) are in the range of 0.7 to 3.5 m.

Comparison of significant wave heights based on both methods show deviations of smaller than 5% of the

measured values (Hsig in range of 0.7 to 3.5 m), see Figure 5.6.1.

The maximum measured wave height (Hm) shows deviations up to 10%.

Comparison of zero-upcrossing wave periods (Tz) show deviations up to 10%.

Comparison of peak wave periods (Tp) show deviations up to 50%. If longer and shorter waves of nearly

equal intensity coexist, then alternating peak periods may result in considerable scatter.

Comparison of wave directions (Q) show deviations up to 10%.

References

Datawell information notes. Datawell, Haarlem, The Netherlands (www.datawell.nl)


February 2006


Figure 5.6.1

Comparison of wave rider bouy data, North Sea, 1999 (from Datawell)

Documents

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