SEDIMENT SAMPLING IN SAND-BED RIVERS: METHODS AND INSTRUMENTS

Peters J. J. Consulting engineer - Prof. Vrije Universiteit Brussel - Prof. Université Catholique de Louvain

44, Rue Philippe de Champagne, B-1000 Brussels, Belgium Tel +32 2 512 8006, Fax +32 2 502 4644, e-mail jjpeters@skynet.be

Abstract: Understanding the sediment transport processes in sand-bed rivers is a prerequisite for the morphological studies and modelling, and requires sampling of the sand fraction on and close to the riverbed. A limited number of devices are available for these kinds of measurements. This paper discusses the Bed Load Transport Meter Arnhem (BTMA) and the Delft Bottle (DB12), from their original design to the present one. Changes made by the manufacturer for easing the production of the samplers sometimes resulted in less efficiency and even erroneous measurements. Thanks to the feedback from users, improvements have been implemented to both instruments.

Keywords: sediment, samplers, bedload, suspended load, design

1. INTRODUCTION Sediment measurements are difficult to perform in the field, especially bedload sampling, which is seldom measured, usually only calculated. The discrepancies between measured and calculated values are generally high and a most common reaction is to blame the measurements instead of the formulas. Sediment measuring devices and methods are said to be unreliable. Literature about this issue is scarce. Some countries, such as the USA and China, have developed since long an array of samplers, equipment and methods for routine use in their national hydrometric networks or for specific engineering projects. In some other countries, only the suspended sediment is surveyed, with primitive equipment such as simple vertical bottles, or using indirect methods, such as light absorption. For the standards, the practitioner must rely on a few ISO’s only (see references). WMO published an operational manual on sediment measurements (Long Yuqian, 1989). In 1964, Hubbell wrote the first comprehensive overview of bedload apparatus and techniques. Since then, little attempts were made to develop new, original instruments or techniques for measuring bedload and those who need to collect bedload data have difficulties to find clear guidelines for setting up sediment surveys. The aim of this paper is to discuss the BTMA and the Delft Bottle, sand sampling equipment that is existing since long but not frequently utilised, though it is in our view quite useful for collecting sediment data in engineering projects with morphological aspects.

2. PRINCIPLES FOR MEASURING SEDIMENT TRANSPORT RATES No one single instrument allows the direct determination of sediment transport rates for the entire range of sediment sizes. The sediment transport rate – the discharge of solids expressed in volume or weight per unit time – can be measured directly or indirectly.

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Direct measurement is done with samplers. The “transport-rate-samplers” catch the quantity – either volume or weight – of solids passing through a given cross-section during the sampling time. The solid discharge is then obtained directly. The “velocity-concentration-samplers” take a water volume, usually over a short period of time. The concentration of solids is determined in a sediment laboratory and multiplied with the flow velocity measured in the same sampling point to obtain the solid discharge. The solid particles are therefore supposed moving with the same speed as the flow. Indirect measurement methods are based on indicators, parameters that are closely correlated with the sediment concentration (e.g., with the absorption or the scatter of light, with acoustic waves, with laser, etc). The ADCP (Broad-Band Acoustic Current Profiler) – a modern technology in flow measurements using the Doppler effect on ultrasonic signals – may provide some qualitative information on the sediment concentration, however not yet quantitative data. The indirect techniques are not discussed in this paper, only direct measurements with samplers. The selection of methods and equipment for sediment transport measurements is crucial, especially to know which equipment is best suited for collecting a particular information relevant for the engineering problem. Figure 1 defines different kind of sediment load and transport. Starting on the left side: the total sediment load is divided according to its origin in a bed material load and a wash load; on the right side, the total sediment load is separated according to its transport mode, in the bed load and the suspended load.

Figure 1: Definition sketch (according to ISO 4365, 1985)

2.1. WASH LOAD Wash load is composed of very fine sediment particles moving almost as fast as the water. The wash load discharge is obtained by multiplying the sediment concentration with the flow velocity taken at the sampling point. Wash load sampling is not discussed in this paper, as it is usually not relevant for morphological studies in sand-bed rivers. 2.2. BED MATERIAL LOAD Equipment for determining suspended sediment concentration in water sample “Concentration” samplers are designed to catch suspended sediment containing bed material load and wash load. The inertia of the particles larger than silt requires an isokinetic sampling, i.e. the flow entering the sampler having the same speed as the surrounding flow velocity. Series of samplers were conceived, their size, shape and weight adapted to specific field conditions. The Federal Interagency Sedimentation Project (FISP, sponsored by US official agencies, among which U.S. Army Corps of Engineers, U.S. Geological Survey and U.S. Bureau of Reclamation) proposes a long list of sediment equipment. It offers various devices:

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isokinetic suspended load samplers, hand-held or cable-suspended, depth-integrated or point samplers, bottle-samplers, collapsible-bag samplers, etc., and continues developing new ones. Most of the FISP samplers have a fixed-volume, glass or plastic container, usually half-litre-size for the small samplers, one-litre-size for the larger ones. The collapsible-bag samplers were conceived to avoid the disturbing influence produced by the air escaping from the container during filling. The Nanjing Automation Institute of Water Conservancy and Hydrology, Chinese Ministry of Water Resources, has designed a series of these samplers. In the ANX3 series, the heaviest ANX3-750, weighing 750 Kg, was designed for use in large depths and strong currents. Samplers for determining transport rates of sand in suspension A direct determination of the transport rate of suspended sand can be obtained by capturing the solid particles entering a sampler while letting the water passing through and escaping from the sampler body, the same principle as applied on most bedload samplers. The Delft Bottle (DB-12) was designed before 1940 in the Hydraulic Laboratory of Delft (the Netherlands) for measuring suspended sand transport on the Dutch rivers. The instrument has a chamber built as a labyrinth in which the flow loses velocity, so that the particles coarser than 50 to 60 microns are trapped, while the finer ones escape. The sampler body has a profiled shape so that a pressure difference between the nozzle entrance and the exhaust openings equalises almost the head loss in the chamber for a large range of velocities. The sampler has two nozzles, one of 1.9 cm² for the higher velocities and one of 3.8 cm² for the smaller ones. If the transport rate for the particles finer than about 50 µm is needed, another sampler is required. The isokinetic Delft Bottle has a significant advantage over the other suspended sediment samplers as it “filters” large volumes of water. It integrates the sediment concentration over larger times and yields larger sediment catches, giving a good representative average value of the suspended bed-material transport-rate. Bedload samplers Bedload is defined as “the sediment in almost continuous contact with the bed, carried forward by rolling, sliding or hopping” (ISO 772, 1977). Bedload samplers are designed to capture those bed-material particles only rolling or gliding over the riverbed in a thin layer above the bottom. The early trap-type bedload samplers are generally unsatisfactory and the pressure-difference-type is better suited. The selection of size and shape of the bedload sampler depends on the bedload particle’s sizes: box-shape for very coarse material such as rocks, cobbles and pebbles (not discussed in this paper), pressure-difference for gravel and sand. The Bedload Transport Meter Arnhem (BTMA), designed in 1936 for use in the Dutch rivers, is capable of measuring the discharge of particles as small as medium sand. It was calibrated in laboratory conditions, in ETH Zurich, by Meyer-Peter and Müller (1937) who concluded at an efficiency of 70 % (Meyer-Peter & Müller, 1937). The sampler is quite large, and an alternative version was designed in the U.S.A.: the Helley-Smith (Edwards & Glysson, 1988). It is distributed in several versions by the FISP as the US BL-84. The US BL-84 and the BTMA can be used for collecting bedload data for sediment with average size D50 of less than 0.3 mm, though FISP says it should be avoided for sediment with average size D50 of less than 0.5 mm.

3. DISCUSSION OF EXPERIENCES WITH THE SAMPLERS Sediment transport is by essence variable in time and space, mainly because the turbulence and the sediment transport capacity of the flow fluctuate constantly. Moreover, the actual observed transport is the result of the transport capacity and of the availability of the sediment. Local sediment transport rates are linked to the river’s morphology, which governs the spatial distribution of turbulence and power of the flow in the cross-sections. The variability of the sediment transport rates is usually the highest close to the riverbed, because of turbulence.

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Measuring sediment discharge requires a sampling strategy that should be based on the best possible knowledge about the spatial and temporal variability of the sediment transport. Sampling time should therefore be largest when the variability is highest. Bedload is best determined by collecting the weight or volume of sediment that is passing a section in a given time, the largest possible. However, because of the risk for clogging, the time must be limited and sampling repeated several times. Bedload The original BTMA (see Hubbell, 1964, Figure 10) was designed to create the least possible disturbance of the flow. A rigid rectangular entrance is connected to the basket of 0.2-mm mesh by a flexible neck, the system hanging in a large frame (Figure 2). When lowered to the bed, this frame lands first, then the basket and finally the sampler’s mouthpiece. The second version of the sampler had an original leaf spring system. Because of its weight and dimensions, it has to be handled from a davit. The frame is hanged with the tail lower than the front supports, so that it reaches the bed first. This version was used in the Congo River (Peters & Goldberg, 1989), in water depths and with flow velocities much larger than those for which the BTMA was designed, by adding lead weight to the support front plates and tail (Figure 3).

Figure 2: BTMA 2nd version (van Til, 1956) Figure 3: BTMA, with lead weight (Congo 1971)

Figure 4: BTMA sampler 3rd version

In the seventies, the design was improved by replacing the 3-point suspension-cable with a system shown on Figure 4. When the frame is placed on the riverbed and the cable further released, the mouthpiece lands slowly on the bed; the rotating V-shape arms are finally reposing on the frame, so that drag exerted on the sampler by the flow will reduce to a minimum. During measurements on the main rivers of Bangladesh, the front supports were sometimes found covered with lots of sediment, suggesting that they could have been scooping the bed. The manufacturer adapted the design, shaping these supports like skis.

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Figure 5: BTMA 4th version, modified frame of the BTMA (Eijkelkamp, 2001)

Other changes were recommended to and implemented by the manufacturer for new BTMA samplers delivered to Mexico in 2001. The design of the tap closing the end of the basket needs still improvement because it is difficult to recover the sample completely.

Near-Bed Load For morphological studies in sand-bed rivers, it is important to determine the strong gradient of sand transport rates close to the riverbed. The Delft Bottle placed on a frame (DB-2) is well suited for this purpose, as it can sample at precise altitudes in the first half a meter above the bottom. The original frame design was robust, allowing an easy and precise positioning of the intake nozzle in seven positions, every 5 cm between 0.05 and 0.35 m from the bottom (Figure 5). The equipment was procured in its original version in 1958 for the Congo River and used intensively from 1968 till 1988. A new version of the Delft Bottle sampler was acquired in the early eighties. The manufacturer Van Essen had meanwhile changed the design of the frame (Figure 6), reducing the number of nozzle positions to five, now at 0.10, 0.20, 0.30, 0.40 and 0.50 m from the bed. Extending sampling over 0.50 m from the bottom was a significant improvement. However, a nozzle position at 0.05 m was missing for determining the gradient close to the bed. The system for fitting the bottle to the frame was less precise and wrong operation made possible errors on the nozzle positioning. At one occasion, the observed error amounted to 3 cm in elevation, so that the lowest sample was taken occasionally at 2 cm from the bottom while the sampler body placed in its theoretical 0.05 m position for the nozzle, producing a significant error in the determination of the vertical distribution of near-bed sand transport. A similar error was found in Bangladesh on the Jamuna-Brahmapoutra River, using the same version of the Delft Bottle sampler (Figure 8). The present manufacturer Eijkelkamp took into account these – and other – shortcomings and delivered in 2001 a new version of the sampler for a project in Mexico (Figure 9).

Figure 6: DB2 sampler 1st version (van Til, 1956) Figure 7: DB2 2nd version

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Figure 8: DB2 2nd version (on Brahmapoutra) Figure 9: DB2 3rd version (Eijkelkamp, 2001)

Contacts between the users and the manufacturer continue to improve further this quite valuable instrument.

Suspended load for sand fraction The design of the Delft Bottle for measuring the suspended sand transport was also changed since its conception. It is less well balanced, though this has little effect on the sampling (Figures 10 & 11). The reason for the new design is to have one same sampler-body that can be fitted either on the frame or suspended from a cable.

Strategy for sand transport measurements Information about sand gauging strategies is lacking and measurements are usually executed by technicians, without a prior assessment by the engineer of the optimal procedure for collecting the sediment data. Field data are needed because of the poor reliability of sediment transport formula, in general. However, field data collected with a wrong sampling strategy are evenly bad. It is therefore important to have those engineers involved in morphological studies participating closely in the setup of the measurement campaign. A preliminary campaign to study and identify the relevant sediment transport processes is crucial, and needs to be performed before the data collection. Sediment data are not only for feeding models, rather to understand the processes and field investigation in sand-bed rivers may help solving engineering problems even without any modelling (Peters & Wens, 1991). From the experiences discussed, it can be concluded that there is a need for setting up clear guidelines for the design of samplers and for their use under various field conditions.

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Figure 10: DB1 1st version (Congo, 1968) Figure 11: DB1 present design (Mexico, 2002)

4. CONCLUSIONS Our experience about sand transport measurements in relation to morphological aspects of engineering projects confirmed the usefulness of transport-rate samplers such as the BTMA and the Delft Bottle. This experience over more than 30 years shows how the manufacturers changed the sampler’s design, sometimes improving the instrument, sometimes creating difficulties for its use. A feedback from the users to the manufacturer is crucial to avoid the production of unsuited instruments.

ACKNOWLEDGEMENTS The experience on which this paper relies was gathered in several projects in Africa, Asia and Latin America: the Project for Improving Navigation Conditions in the Inner Congo River Delta (supported by the Belgian Cooperation from 1967 till 1988), in the Brahmapoutra/Ganges/Meghna Delta in Bangladesh (Flood Action Plan, River Survey Project N° 24,1991-1998, co-financed by the European Commission, COAID Directorate), and in the Grijalva-Mezcalapa River in Mexico (Project for Modernising the Monitoring Network of the Hydrological Cycle, co-funded by the World Bank with technical Assistance of the World Meteorological Organisation, sediment component starting in 1998). The excellent collaboration with M. A.G. Eijkelkamp, Managing Director and A. Eijkelkamp, of Eijkelkamp Agriresearch Equipment for revising the design of the BTMA and DB12 samplers is acknowledged.

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