Andrew Garcia & Bill Birkemeier

Coastal & Hydraulic LaboratoryUS Army Corps of Engineers

Monitor - to observe systematically;

… a planned series of observations

To establish baseline conditions...

To document episodic or unusual events and theireffects…

To acquire information for planning, design, or remedial efforts…

To access project performance or completionof study goals...

Information !!! - by way of…

Acquiring data…Processing and analyzing…Summarizing, categorizing…Correlating...

Waves, windsWater levels (astronomical tides)CurrentsNearshore bathymetryShoreline evolution

Capital costs… instruments, related hardware,software

Deployment… vessel, divers, expendable supplies

Maintenance… planned and unplanned

Data retrieval… electronic, physical

(continued)

Data processing… analysis, archiving

Data interpretation… plots, figure preparation

Project documentation… field notes, office records

Information, data dissemination… web site

(concluded)

Wave Period

Wav

e H

eigh

t

Wave Dire

ction

The ideal wave:

One Height

One Period

One Direction

example: hurricane swell

Types of instruments...

Surface-piercing probes

Surface-following buoy

Sub-surface pressure sensor

Advantages…Good linearity

Disadvantages…Requires a supporting structure ($$$)Vulnerable to floating debrisSusceptible to bio-foulingNon-directional

• Mooring design is importaqnt to performance

• Can be equipped to transmit data to shore station or satellite

Advantages…Wide range of acceptable sizes (1m-12m dia.)Suitable for very remote and/or deep water locationsResponse characteristics well documentedDirectional/Nondirectional Buoys available

Disadvantages…Not suitable for shallow water application (<15m)Does not track very steep waves wellCan be vulnerable to vandalism and marine mammalsMooring can be vulnerable to vessel traffic

Advantages...Suitable for shallow-water application (<20m)Good survivability during severe stormsVandal resistantNot very susceptible to bio-foulingMultiple sensors can provide directional waves

Disadvantages…Requires supporting structure for deep-waterapplication

Real-time data retrieval only via cable linkSusceptible to trawler traffic

Pressure sensing wave gauge

1. Binary representation of sensing crystal frequency2. Interpret binary string to crystal frequency3. Compute pressure via crystal calibration equation4. Assemble pressure time series5. Compute pressure spectrum (Fourier Analysis)6. Rectify pressure spectrum to sea surface7. Wave data!

Types of instruments…

Rotor or propeller

Electro-magnetic

Acoustic

Advantages…

Can have excellent linearity and directional responseRelatively inexpensiveRelatively low power consumptionWell documented characteristics

Disadvantages…

Point measurementSusceptible to bio-fouling

Advantages…

3-D Measurement at a distanceCan provide directional wave measurementWith software can give turbidity measurementSelf-recording or cable connected to shore

Disadvantages…

Relatively expensiveLarge data files to processData interpretation may require expert aidPower requirements may limit deployment duration

Example ADCP current data:Hurricane Bonnie: offshore mean currents

Top to bottom 1 m/sec flow at -13 m

• Stilling well to damp wave action

• Pressure gauge has to be density and temperature compensated

• Typical 6 min observation

911008 911015 911022 911029 911105-1

0

1

2

Predicted Tide Water Level

0

1 Surge

0

2

4

6

Wave Height

Monthly mean tide variations

1978 1983 1988 1993 1998-150

-100

-50

0

50

100

150

EL

MLW

MSL

MHW

EH

Date

28 Jan 1998

20 cm wave setupWave Setup results from waves pushing water toward the beach.

It is difficult to measure with accuracy.

We used a precise pressure gauge near the beach and compared it to an outer gauge.

Actual setup on the beach would be higher.

• Local observers make daily visual observations

• Advantages:– Low cost, practical– Public involved

• Disadvantages– Subjective observations– Not continuous

• Wave– Height, period, direction– Breaker type

• Wind speed & direction• Foreshore slope• Surf zone width• Longshore Current

– Speed and direction• Rip current• Beach Cusps• Beach Profile (Emery Type)

• Clip board• Hand level• Dye packet or float• Wind meter• LEO form• Optional

– Digital Camera– Survey gear