National Academies Press: OpenBook

Measuring and Understanding Coastal Processes (1989)

Chapter: 3. Processes and Measurement Requirements

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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Suggested Citation:"3. Processes and Measurement Requirements." National Research Council. 1989. Measuring and Understanding Coastal Processes. Washington, DC: The National Academies Press. doi: 10.17226/1445.
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Processes and Measurement Requirements Coastal measurement systems designed for sensing the properties of waves and currents are usually categorized into one of two temporal domains: high-frequency and low-frequency. High-frequency wave and current motions are defined here as those with periods of five min- utes or less and include both the gravity: and the infragravity2 por- tions of the surface wave spectrum (see Figure 3-1~. Low-frequency motions are defined here as those having periods greater than five minutes and range from tsunami to tidal or longer oscillation periods. The current velocity measurement techniques can be categorized as Eulerian or Lagrangian, and in situ or remotely sensed. Eulerian measurements are those collected by moored current meters mea- suring velocity at a fixed location. Lagrangian measurements are obtained by a tracer (drifter floats, dye, or tagged particles) follow- ing the current stream over a period of time. In situ measurements are obtained by placing instruments in the ocean. Remote-sensing measurements are those collected by "noncontact" methods from satellites, aircraft, ships, or ground stations, using electromagnetic Gravity waves are those waves with lengths between their crests of a meter or so to a few hundred meters typical of wind-generated waves. 2Infragravity waves have lengths of hundreds of meters to tens of kilometers and are usually driven by gravity waves. 26

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28 radiation. Modern acoustic-Doppler current meters and similar sen- sors that sense current velocity using acoustic waves scattered from particles drifting with the current should be considered a combina- tion of ~ situ and remote-sensing techniques. After the hydrodynamic forces generated by high- and low- frequency waves are deterrn~ned, the engineer and researcher must measure and understand the responses of the sediment particles to these wave and current forces. This chapter addresses the various measurement techniques used for determ~n~g sediment movement on scales from particle-size up to regions and locations from near (within a few centimeters) the seabed to the upper water column. Finally, the engineer must be able to measure the effect of the hydrodynamic forces of the sea on structures in, or at, the wave zone. How structures, in turn, affect both hydrodynamics and sedimentary response also must be gauged if the engineer is to establish adequate design guidelines. This chapter also clears with these fluid structure interactions. HIGH-PREQUENCY WATER MOTIONS Processes High-frequency motions in the coastal environment can be cate- gorized as being wave-induced flows (including gravity and infragrav- ity waves), turbulent flows, and averaged currents (that is, averaging the current in time for more than five minutes). Because Lagrangian measurement techniques generally do not lend themselves as well to high-frequency field measurements, only Eulerian systems are con- sidered here. In contrast to waves, currents are physically difficult to measure with a single instrument except in extremely simple cases. Spatial variation in waves primarily depends on the bathymetry, whereas current variation is influenced by the shape of both the bottom and the shoreline. Also, strong vertical variation in the current velocities can exist due in part to inhomogeneity in the water density caused by salinity and temperature variations, as well as changes caused by the viscous boundary layer effects on turbulence and velocity. Currents modify wave velocities in a complicated fashion when both are varying in time, particularly where the shape of the bottom is also changing as a result of this interaction.

29 TABLE 3-1 Summary of Data Needs for Coastal Engineering 1. SHORE STABILIZATION Sediment Characteristics Grain size distribution Concentration of suspended fraction Near-bed transport rates Beach Characteristics Beach profiles Local areas of deposition or erosion Stabilizing structures Hydrodynamic Characteristics Incident wave heights and steepness Wave direction Velocities of wa~re-dri~ren currents Velocities of other currents Bottom shear stress Turbulence characterization 2. BACKSHORE PROTECTION Sediment Characteristics Grain size distribution Beach Characteristics Beach profiler Longshore sediment transport Crose-shore sediment transport Hydrodynarruc Characteristics Wave direction Wave height and steepness Wave runup Storm surge Tsunarrii runup S. INLET STABILIZATION . Sediment Characteristics Grain size distribution Packing density Inlet Characteristics Bathymetry Net sediment flux Patterns of erosion and deposition Protective structures Hydrodynamic Characteristics Wave height and steepness Wave direction Current velocities Bottom shear stress (wave/current interaction) 4. HARBOR PROTECTION Sediment Characteristics Grain size distribution Harbor Characteristics Bathymetry Shoreline changes Protective structures Patterns of erosion and deposition Hydrodynamic Characteristics Wave height and steepness Wave direction Current velocities Bottom shear stress Measurement Requirements The proper design of a measurement system requires (~) knowI- edge of the expected environmental conditions at the measurement site, (2) appropriate design of the supporting moorings of platforms, and (3) the selection of suitable measurement devices. Each of these components, ant] their interaction, contributes significantly to the quality of the data, and a total system must be considered collec- tively. The requirements for precision and resolution of the data must also be considered. The set of measurement requirements listed in Table 3-1 was established by assessing the engineering needs of each of the four

30 application areas or features discussed in Chapter 2 (see Figure 2-1~. The optimum spatial and temporal resolutions are specified for each measurement requirement (see Tables 3-2, 3-3, and 3-4) and existing sensing systems are identified for evaluation of their measurement capability. If the measurement capability of an existing sensing system failed to meet the necessary spatial and temporal resolution requirements, a need was then identified for instrument or system development. Measurement Capabilities Velocity Measurements In Situ A wide variety of physical measurement techniques incorporate current measurements taken at a point. A primary difficulty for nearshore velocity measurements is that oscillatory wave-induced velocities are generally present throughout the water column, super- imposed on steady currents. In the wave, simple impeller (speed) and vane (direction) current meters were found to rectify the oscilIa- tory velocity imposed by the waves, thereby biasing the results (e.g., McCullough, 1978~. Vanes generally do not respond to wave motion rapidly enough to resolve direction accurately, giving Aliases" or noisy directional information. This condition has led to the develop ment of a number of current meters that respond to wave motion and that have been adapted especially for shallow-water measurements. These instruments resolve the wave motion into velocity compo- nents at right angles to each other. Instruments include biaxial rotor vector-measuring current meter (VACM) (Figure 3-2), electro- magnetic current meters (Figure 3-3), forward-scatter or backscatter acoustic-Doppler current profiler (ADCP) (Figure 3-4), and laser ve- locimeter. The EuTerian current meters have a relatively high capital cost per unit, ranging from over $4,000 for a meter to as much as $65,000 for a current profiler system, depending on the data record- ing and internal processing capability. Some of the difficulties and problems encountered with Eulerian measurement systems are bi- ological fouling, corrosion, orientation uncertainties, disturbance of the flow fields by the instrument, bubbles, cavitation, extreme storm waves, and disturbance of sensors by fishermen. Velocity Measurements Remote Sensing Remote-sensing techniques avoid many of the disadvantages of

31 FIGURE 3-2 Davis-Weller Vector Measuring Current Meter, Model 630. SOURCE: EG&G Environmental Equipment Division, Burlington, Mass. FIGURE 3-3 Electromagnetic Current Meter, Model 511. SOURCE: Marsh- McBirney, Inc., Gaithersburg, Md.

32 ~ `11 ~ ~ / / / / FIGURE 3-4 75-KHz Self-Contained Acoustic-Doppler Current Profiler. SOURCE: RD Instruments, San Diego, Calif.

33 in situ measurements and have the advantage of generally providing rapid coverage of a wide area. The disadvantages in most techniques include sensing only of the surface currents (providing no information about the subsurface velocity profiles) and integrating over large areas so that the local gradients are filtered out. One method for the remote sensing of currents relies on the scattering of radar signals from the sea surface. Current speed is d~ term~ned from the Doppler shift (frequency change) ~ the scattered signal after a theoretical shift due to wave speed has been removed. The speed of the current in the direction of the radar beam is deter- mined. Directional components are measured by turning the radar or using multiple radars. Synthetic Aperture Radar (SAR) uses a sin- gle radar mounted on a moving platform (plane or satellite), which allows good definition of currents over large areas. Other remote- sensing techniques are being developed, including optical systems, which offer additional capabilities for the future. Because of the large surface area observed by most of these systems, they are not generally applicable to the nearshore region, where currents change rapidly over short distances. Wave Measurements—In Situ A wide variety of wave-measuring devices for waves based on var- ious physical principles have evolved, including direct, indirect, and remote techniques. Each technique has inherent advantages depend- ing on the application. Desirable characteristics of any wave sensor should include good accuracy, linearity, ruggedness, dependability, and low cost. The utility of any wave sensor is highly dependent on the type of installation for which it can be adapted, such as bottom or pier mountings. The most common direct measurement of the sea surface is by wave staffs (wires that penetrate the sea surface) that measure resistance, capacitance, or inductance. The disadvantages of wave staffs are that they usually require mounting on a rigid structure such as a tower or pier piling. In measuring plunging breakers there is a question as to exactly what is measured, because of multiple interfaces in the plunge portion of the wave. The influence of foam is unknown for direct wave sensors. Other direct measurement techniques include acoustical devices directed up from beneath such as inverted fathometers and lasers and microwaves (infrared) looking down from above; both of these

34 techniques are based on measuring the tone for a pulse to travel from the sensor, be reflected off the surface, and return. Acoustical and electromagnetic methods have the advantage that they do not disturb the surface. The disadvantage of the inverted fathometer is that the speed of sound is a function of temperature and salinity, which can vary temporally and spatially over the vertical. Steep waves may not provide a good acoustic return rn~rroring exact surface configuration because of side-Iobe reflection problems. The inverted fathometer does not work in the highly turbulent region near the surf zone due to scattering of the sound by turbulence and bubbles. Because of beam spreading, the sampled surface area may be wide and as a result, the signal return provides an area average depth; this may be an advantage or disadvantage depending on the mtencled use of the device. The inverted fathometer does offer the advantage that it can be mounted on the bottom, generally a much easier type of mounting than that required for a laser beam or microwave, which must be mounted from a structure looking down onto the surface. Pressure sensors that provide indirect measurements of the wave surface generally have the advantage of ease of installation, durabil- ity, and low vulnerability to environmental forces. For this reason and because pressure sensor technology is highly advanced, they are a popular means of measuring the waves. The water column above the pressure sensor acts as a hydraulic filter, partially filtering out the high-frequency components of the wave spectrum. The wave spectrum is increasingly filtered as the depth of the sensor increases. A practical deep-water limit is approximately 20 m for placement of the pressure sensors to determine sea and swell waves. Some un- certainty exists concerning the performance of the sensors when the waves are quite steep (Forristall, 1982~. Nearshore direction characteristics are typically either measured locally in shallow water with "slope arrays" or measured in deep wa- ter with buoys and then transformed to the nearshore location. The important problem in predicting littoral sediment transport is that an accuracy of 1-3° is required to obtain good estimates. This means that the deep-water waves must be resolved to within 2-5° to obtain the desired nearshore directional accuracy after transformation. Wave directional buoys are used primarily in deep water to mea- sure the heave, pitch, and roll with accelerometers and an inclinome- ter. A second type of "orbit following" directional buoy, developed by ENDECO (Brainard and Gardner, 1982) infers the surface slope due to the forcing by the horizontal motion of water particles. The

35 TABLE 3-2 Assessment of Needs for Measurements of High-Frequency Motions Measurement Accuracy Measurement Objective Requirements Technique Capability Need Currents Longshore +2 cm/e Electromagnetic I 2, 6, 7 cross-shore 5 Hz Acoustic 1-2O Mechanical ; Turbulence +1 cm/e Acoustic III 2, 4, 6, 7 high- Hot film frequency Water level Tides +5 cm Pressure, I 7 Mechanical Storm surge is cm Pressure I 7 Wave setup +5 cm Pressure III 7 Wave runup +5 cm Photo/Video I 4, 6, 7 Electrical Wa~recharac- 0-15 m +5% Many I 6, 7 teristice Wave direction O Slope array III 1, 2, 6, 7 deep water >lOm 2-50 Remote sensing shallow water 1-3 Breaker Photographic II 2, 6, 7 characteristics Meteorological Wind velocity 5% Many I 7, 8 Wind direction 1056 Many I 7, 8 LEGENDS: Need: 1 Major development 2 Improve information detail 3 Improve physics 4 Improve efficiency 5 Improve tuning 6 Special data needed 7 Verification needed 8. None Capability: I Good II Adequate III Possible but not satisfactory IV None buoys have their own individual dynamic response functions, and the mooring also can influence the dynarn~c response of the buoys. Therefore, the dynamic response of the combined system of the buoy and mooring system must be known so that proper calculation of

36 TABLE 3-3 Assessment of Needs for Measurements of Low-Frequency Motions Measurement Accuracy Measurement Objective Requirements Techniques Capability Need Currents Offshore 3 cm/e EMC\I, ATTCM, (10-20 m depth) VACM, AD CM III 3, 4, 5, 6 Nearshore 3 cm/s (same as above) II 3, 4, 5 Inlets 3 cm/e (same as above) III 3, 4, 5, 6 Radiation stress 10% Slope array II 5 Water level Offshore 10 cm relative Pressure sensor, II 5, 6, 7, 8 - Remote altimeter Nearshore 10 cm absolute Pressure sensor, I 5, 6, 7, 8 Tide gauge Back~hore 20 cm absolute Pressure sensor, I 5, 6, 7, 8 Float gauge Runup 0.3 m Contact sensors, II 3, 4, 5 Photogrammetry Wave setup 10 cm Slope array III 5, 6 Direction spec. 10%, 25° Buoys, SAR, PUV, II 3, 4, 6 Slope array Meteorological Wind velocity 5%, 10° Many I 6, 7, 8 Barometric 1 mb Many I 6, 7 pressure Morphological Bathymetry 5% or 0.5 m Fathometer, I 9 (large-ecale) Precise leveling Bathymetry 0.3m Fathometer, III 2, 8 (small-ecale) Precise leveling, Side-scan surface displacement can be made. Buoys have not been used as reli- ~bly in shallow water as in deep water because of the demand placed on shallow-water surface moorings (large displacements, steep waves, etch. Although buoys provide some directional wave information in deep water, these buoys do not give the high-resolution direction information often required for coastal engineering (see Table 3-2 to 3-43. More accurate, inexpensive inertial compasses based on optical interferometry should be available soon. These solid-state devices may give improved accuracy and reliability to the directional buoy measurements but will probably not improve the directional resolu- tion.

37 TABLE S-S (Continued) Measurement Objective Accuracy Measurement Requirements Technique(~) Capability Need Natural topography Structures Vegetation Water properties Temperature Salinity 0.3 m elevation 1096 horizontal 10% area 0.1°C 0.1 ppt Precise leveling Precise leveling, Photos Photos CTD CTD I 8 I ~ II 8 7, 8 7, 8 ACRONYMS: EMCM electromagnetic current meter ATOM acoustic tra~rel-time current meter VACM ~rector-averaging current meter AD CM acoustic-Doppler current meter LEGENDS: Need: 1 Major development needed 2 Improve information detail 3 Improve physics 4 Improve efficiency Capability: I Good II Adequate III Possible, but not satisfactory IV None SAR synthetic aperture radar PUV pressure sensor combined with current meter CTD conductivity (salinity)- temperature-depth meter 5 Improve tuning 6 Special data needed 7 Verification needed 8 None Slope arrays are designed to measure the directional wave char- acteristics in shallow water using three or more bottom-mounted pressure sensors (Higgins et al., 1981~. Although the directional spectrum resolution is the same as a pitch and roll buoy, the wave momentum flux is well estimated. A measurement system compara- ble to the slope array is provided by using a pressure sensor and a two-component electromagnetic velocity sensor (PUV) (Grosskopf et al., 1983~. An intercomparison of directional measuring capabilities for the buoys, slope arrays, and PUV was accomplished during the

38 TABLE S-4 Assessment of Needs for Measurements of Fluid/Sediment Interaction Measurement Accuracy Measurement Objective Requirement Technique Capa- Selected bility Need Reference Sediment characteristics Grain size silt-gra~rel Bottom sampler I 7 1 ~ Remote sensing III 1 2 Particle density 2-6 g/cm3 Bottom sampler I 7 Bulk density 1-4 g/cm Bottom sampler II 6 Nuclear density probe II Suspended 0-300 g/L Optical II 6 sediment Acoustic III 1 6 concentration Water samplers I 7 6 Remote III 2 7 Bed load Samplers, traps II ~ 8 Remote IV 1 9 Sea level Wave charac- 0-10 m +5% Pressure, staff, teristice photo/video I 7, 6 Wave riders I 7, 6 Seismic III 2 Other III 1 Wave direction 2-5O Slope array III 1 Remote Wave runup is cm Photo/video I 7 Breakercharac- Photographic I 7 teristice Tides is cm Pressure I 7 Mechanical I 7 Storm surge +10 cm Pressure I 7 Currents Nearshore is cm/e Remote III 1 circulation (large-ecale) Longshore, iS cm/e Electromagnetic II 7 crose-shore 10°, 5 Hs Acoustic II 7 Mechanical II 7 Boundary Al cm/e shear stress low-ftaquency Current profile II 7 high-frequency IV 1 direct i IV 1 Reynolds stress II! 7 Radiation stress +10%, 1 Hs

39 TABLE S-4 (Continued) Measurement Accuracy Measurement Capa- Objecti~re Requirement Technique Selected 4ility Need Reference Morphology Beach profiles small scale 1-5 cm Hand sunrey I 7 10 medium scale Kilo cm Mobile sunrey I 7 11 large scale i50 cm or 5% Photographic/ III 2 12 Remote Bed profiles i0.5 cm Acoustic III 2 IS Scour at i30 cm Side-ecan sonar III 2 structures REFERENCES: 1 Gable, 1980 2 Schuman and Rea, 1981 Won and Smite, 1986 3 U.S. Dept. of the Interior, 1957 Anonymous, 1978 Barts et al., 1978 Downing et al., 1981 5 Huntley, 1982 Kraus, 1987 6 Nielsen, 1984 Inman et al., 1980 7 Collins and Pattiaratchi, 1984 Thomas, 1980 LEGENDS: Capability: I Good II Adequate III Possible, but not satisfactory IV None 8 White and Inman, 1987 Drapeau and Long, 1985 Downing, 1981 Salkield et al., 1981 9 Lowe, 1987 10 Aubrey and Seymour, 1987 11 Seymour et al., 1979 Clausner et al., 1986 12 Fraser, 1985 Kasischke et al., 1983 Smite and Won, 1987 13 Sallinger et al., 1986 Need: 1 Major development (new instrument) 2 Improve information detail 3 Improve reliability 4 Improve durability 5 Improve installation and use 6 More sensors, or lower data cost 7 None

40 ARSLOE3 (Vincent and Lichy, 1982) exper~rnent, and all methods gave similar results. Recent developments in radiation stress (momentum flux) mea- surements include using differential pressure sensors to make direct measurements of the surface slope (Bodge and Dean, 1984) and us- ing acoustic travel time to infer velocity gradients (Guza, personal communications) . Wave Measurements Remote Sensing Several techniques are available for remote sensing of waves and the sea surface, and more are under development. Remote sensing offers the advantage that the sensors can be mounted on movable platforms such as airplanes or satellites, and large areas can be mea- sured rapidly. Other advantages are freedom from shore connection and the ability to sample extreme wind and wave conditions. The key disadvantage presented by using remote sensing of sea surface condi- tions is poor temporal coverage; observations of a series of events at an area are difficult to obtain by aircraft-mounted sensing. Remote-sensing techniques are based on the scattering of elec- tromagnetic radiation from the ocean surface, either coherently or incoherently. Early techniques utilized photography to determine surface slopes from sun-glint patterns. Stereoscopic photography was used to determine directional wave spectra, although this method proved to be cumbersome. Present image-analysis techniques make stereophotogrammetry a viable technique to estimate the directional spectrum. Wave heights (but not directions) over the ocean are measured operationally using an altimeter aboard the Navy satellite GEOSAT. The altimeter measures wave height from the shape of the reflected radar pulse. NASA's plans for the next decade include a dedicated wave altimeter mission, TOPEX/POSElDON, proposed for launch in 1991. The airborne Surface Contour Radar (SCR), developed jointly by NASA and the Naval Research Laboratory (NRL), was designed 3ARSLoE: Atlantic Remote Sensing Land-Ocean Experiment for sensor intercomparison, sensor development tests, and data wave-model verification; October 6 to November 30, 1980, onshore Duck, North Carolina. The tests were organized by the U.S. Army Corps of Engineers Coastal Engineering Research Center and the National Ocean Survey of NOAA.

41 to measure the directional wave spectrum (Walsh et al., 1986~. An oscillating mirror scans a 0.85° x 1.2° pencil beam laterally to mea- sure the sea surface elevation at 51 evenly spaced points within a swath approximately half the aircraft's altitude. Ground truth com- parisons were made by a variety of sIope-measuring buoys during the ARST~OE experiment, and reasonable comparisons were obtained (Walsh et al., 1986~. Synthetic Aperture Radar (SAR) has been operated from both aircraft and satellites. The imaging mechanism is complex, but the primary signal is that due to scattering from the cent~rneter-scale ocean waves. This signal can be accentuated at the crest of the long wave and may become the principal image for large waves (Harger, 1986~. Thus, the SAR requires the presence of short waves of a few centimeters that result when winds are greater than approximately 3 m/s. Due to present resolution limitations, only significant wave heights greater than ~ m and wave lengths exceeding 25 m can be measured. The SEASAT satellite system obtained a resolution of approx- imately 25-40 m using a 23-cm wavelength SAR to acquire radar images of the ocean surface in swaths 100 km wide and varying in length from 300 to 3,000 km. Vesecky et al. (1986) compared the results to intensive ship, buoy, and aircraft wave measurements dur- ing the Joint Air-Sea Interaction (JASIN) experiment. Comparisons between SAR and buoy estimates of wave length and direction agree to within about +14 percent and ~t10°. Correlations with buoy mea- surements suggest that significant wave height could be estimated to about Ott m. SAR data from European and Japanese SAR-equipped satellites, to be launched in 1989 and 1991 respectively, will also provide valuable wave information. During the space shuttle oceanographic mission in 1984, Shuttle Imaging Radar-B (SIR-B) SAR obtained images of the ocean surface. These were compared to those obtained from two aircraft scanning radars, the SCR and the continually scanning Radar Ocean-Wave Spectrometer (ROWS) (Bear et al., 1986~. The ROWS obtains a high-resolution slope spectrum. A surface elevation spectrum can be obtained from all three systems. High-frequency (HF) radars have been used to measure ocean waves (Barrick, 1982~. These systems use polarized electromagnetic waves that are scattered from the ocean surface wave component in-line with the radar signal. Huang (1982) reports accuracies of +8 percent for wave height greater than 1 m and +7° for direction. Full

42 directional information would require at least two radars looking in different directions. Promising radar systems for coastal applications are broad-beam surface-wave radars. These systems can yield the same direction e] information as heav~pitch-roll buoys. An example is CERC's opera tional Coastal Imaging Radar System (CIRS), which is a shor~based X-band radar. The system is designed to obtain long-term statistics of wave direction and wave length, but does not provide wave height. It can operate unattended in aD weather. Data-can be collected up to 5.5 km off shore. Modal wave direction can be resolved within +3° and wave length to 10 percent (COE, 1984~. Surface-scanning acoustic-Doppler sonar technology has demon- strated the ability to measure surface directional wave spectra (P~nke! and Smith, 1987~. Doppler sonars mounted on the floating instru- ment platform FLIP have been used to scatter 7~kHz sound from the underside of the sea surface at range intervals of from 60 to 1,400 m. Complete wave directional information has been obtained from a single location using a pair of sonars aimed at right angles. Although some aspects of this technique are not fully understood, the observed motions are consistent with linear wave theory. This technology al- lows a single-point instrument in the open ocean to resolve wave direction an wed as an array of conventional measuring devices 1,000 m ~ length. Measurement Needs Cost of installation, recording, and analysts can easily exceed the cost of the instrument; these cost considerations should be a part of instrument design and selection. The availability and experience in the use of rrucroprocessors and other technologies can allow for preprocessing or analysis of the data in situ. New expanded stor- age media such as bubble memories, optical discs, and high-density digital tape allow for larger and more reliable recording capability. Satellites for telemetry links such as system ARGOS and improved ground telemetry can be used to obtain real-time data. Cable tech- nology for ciata links to shore using fiber optics is rapidly advancing. Waves The National Research Council-sponsored symposium and work- shop on wave measurement technology (NRC, 1982), which was men- tioned earlier, assessed the needs, status, and future directions to be

43 pursued. Operational and research needs were described in that NRC report. Five major research areas for wave data were described and, for the most part, these areas have been aggressively pursued In the intervening seven years as noted (m italics): I. Properties of a sequence of large waves need to be studied, both in the field and theoretically. This phenomenon has been identi- fied as a primary mechanism for damage to structures. The workshop participants recommended investigation of the nonlinear aspects of wave interaction. This area of research has been aggressively pursued (see e.g., Eigar et al., 1985J, but not all aspects of the problem still need to be solved. 2. Understanding is lacking of wave interaction particularly the nonlinear aspects of waves with currents, bottom bathymetry, and winds. Progress has been made in the understanding of the weakly nonlinear evolution of waves due to shoaling (see e.g., Freilich and Guza, 1984), but our knowledge still needs to be greatly improved. 3. Progress has been made on refraction/diffraction models, but a fully operational mode} ~ not available and field verification data are lacking. This assessment is still true, although significant im- provements have been made (see discussion in Chapter 4, wave- induced currents). 4. There ~ a need to improve understanding and ability to measure extreme events and the effects of such events on structures and the nearshore environment. This is still tree' as [ong-term wave records are needed to establish reliable wave statistics. 5. Improvement is needled in wave directionality measurement and analysis, both in situ and remote. This is still a requirement today see Chapter 6: Conclusions and Recommendations. Velocity A need has been identified by this committee for a high-frequency turbulence measurement capability for application ~ sediment-laden or aerated waters that occur within the surf zone. Laser- and acoustic-Doppler measuring devices offer nonintrusive methods for measuring near the bed but need to be adapted for in situ applica- tion in the surf zone. These instruments offer promising are" for technology advances, as do improved recording and analysm systems. At the 1978 IEEE conference on current measurements, sum groups made specific recommendations, most of which still apply today. One recommendation was that the engineering community

44 sanction standardized testing methods and procedures. The idea of some sort of government-sponsored central testing and calibration capability, perhaps open to all who have a need, was proposed as a cost-effective service to the nation. It was agreed that there is a need for both hardware and software ~standards" applied to the measurement of currents. The need for standards and calibration facilities is exemplified by the continuing controversy over the capabilities of electromag- r~etic flow meters the standard instrument used in the surf zone for the last decade (Aubrey and Trowbridge, 1985, 1988; Guza, 1988; Hamblin et al., 1987; Doering and Bowen, 1987~. The remote sensing of waves and currents is an area that requires more development and holds promise for obtaining simultaneous measurements over a broad area. A question was raised in 1978, and is still valid today, as to who should underwrite the cost of research and development of new ideas in current-measurement technology. Should government or in- dustry bear the cost of instrument development? A problem for manufacturers is the small volume in a specialized market. Man- ufacturers continually must incorporate technological advances or risk being noncompetitive. A new current-meter concept can quickly make older meters obsolete. [OW-FREQUENCY WATER MOTIONS This section addresses long-period phenomena, at time scales of five minutes to years or decades. Classifications of such phenomena are shown here: Classification Tsunami4 Harbor seiche Shelf wave Astronomical tide Storm surge Seasonal sea level Range of Period 5 minutes to 1 hour 1 to 10 minutes minutes to hours semi-diurnal to annual hours to days 1 to 12 months 4A tsunami is a long wave generated by vertical movement of the ocean Boor caused by an offshore earthquake. In the past, tsunamis were often referred to as "tidal waves although they have nothing to do with astronomical tides.

45 Long-term sea level Ground-water intrusion years to decades years to decades Concerns with such phenomena mclude navigation of mIets to harbors (tidal currents), backshore and harbor protection (tsunamis, storm surges, tides, and long-term changes in sea level), and berthing of vessels (tidal range and harbor seiches). This report does not address the inner harbor wave motions as these are not considered normally to be a shoreline, high-energy measurement concern. Tides are global and ever-present, whereas tsunamis are of local origin and rare In occurrence but can have significant impact on shorelines and backshores. Storm surges, while also event-related, are as frequent as the intense storms that cause them and are of major concern to localities on the Atlantic and Gulf of Mexico coasts. Pacific coastal regions of the United States are less affected by storm surges because the narrow continental shelf inhibits surge. Measurements of the strength and tracks of storms are needed for use both in design of sea walls and other protective works and ~ evaluation of risk to coastal communities. Shelf waves excited by longshore winds, or seasonal and long-term changes in sea level or land level, compound such considerations. Measurements Requirements The most predictable of the low-frequency water motion phe- nomena are the astronomical tides, at least for those coastal loca- tions where long-term tide data are available. Storm surge prediction (either In real time or in the hindcast mode) demands accurate wind velocity information characterizing the storm in space and time. It also demands accurate bathymetric data near shore, and land elevations in the backshore if overland flooding is an unportant con- sideration. Numerical models for storm surges need water-level data during storm events and adequate astronomical tide data to verify the models. Longshore current data, particularly for currents at key locations such as inlets, are valuable, if not indispensable, for refining the mode} algorithm for computing the friction between the water and the seabed. Existing water-level data for verification and possible improve- ment of mode! performance are generally not adequate for two rea- sons: lack of a sufficient number of tide gauges within backwater regions and lack of combination of tide gauges and wave gauges in regions onshore of the breaker zone during storm events. The

46 water-level evolution during a storm depends on the integrated ef- fect of the wind stress and atmospheric pressure, and indirectly on the w~nd-induced surface waves that produce a low-frequency water- leve} anomaly near shore (referred to as "wave setups. The next- generation storm surge models need to allow for this phenomenon using computationally efficient numerical models of waves. Verifica- tion of such mode! upgrading requires water-level and wave data to infer the contribution of wave setup to the total rme in sea level near shore. Bottom pressure gauge data at the continental shelf break also would be useful in fine-tuning the outer boundary conditions that are used in present surge models. Another area for improvement ~ coastal flooding surge mod- els is the effect on the flow caused by vegetation and other natural or constructed obstructions. The effect of vegetation on wind-wave propagation over flooded land ~ addressed by Camfield (1977) and in a report of the National Research Council (NRC, 19773. Verification of the proposed methodologies and their extension to low-frequency flow impedance is largely lacking because of insufficient data on pro- totype conditions. The requirement is for strategically placed wave and water-level gauges on both sides of such natural obstructions during storm events. Although such information is highly site spe- cific, it is a necessary step in the development of models that might be applied to different generic classes of natural and constructed obstructions. Photogrammetric techniques (using photography in surveying) together with in situ surveys would be useful in attempt- ing to develop a measure of the extent and effect of such overland morphological features. In the design of coastal protection structures or in the evaluation of risk of flooding for coastal communities, the data on high-water- leve! events at any particular location are usually inadequate to determine the probability that a water-level anomaly will exceed a given value in a given period of time (e.g., 50 years). In the absence of data for coastal regions where storm surges constitute the main threat (combined with tide), the historical behavior of storms can be employed together with surge models to give reasonably mean- ingful estimates of such probability (NRC, 1983; Office of Chief of Engineers, 1986~. Good storm data for the region are needed to vali- date the storm surge mode! used to simulate flooding. Evaluation of risk for coastal communities threatened by tsunamis is much more difficult because of the lack of adequate data on seabed motions as- sociated with the seismic events that cause tsunamis. Seismic data

47 themselves yield quantitative information on the location and mag- nitude of the event, but are not sufficient to determine whether a tsunami has been generated (Murty, 1977; Van Dorn, 1984~. Data required for proper characterization of the source are those derived from benthic (dee~water) pressure gauges such an those in the Pa- cific Ocean (Gonzales et al., 1987; Eble, 1988), well away from the influence of continental shelf, and In harbors, where most tide gauges are located. Long-term changes in sea level, whether due to a change in the land level (subsidence, uplift) or cInnatic conditions, are important in designing to mitigate possible flooding scenarios over time spans of several decades (NRC, 1987~. The long-term change not only adds directly to water elevations due to surge, but also affects such things as wave cInnate and seawater intrusion into aquifers. The measurement requirement ~ for accurate long-term tide gauge data where sea-level changes are of concern. Satellite-based measurement systems such as Very long Baseline Interferometry (VBl) and Differ- ential Global Positioning Systems (DGPS) show great promise for measuring relative sea-level changes accurately and rapidly (Carter et al., 1986). Measurement objectives related to engineering needs are sum- mar~zed ~ Table 3-3, and specific accuracy requirements for each measurement objective are given. Measurement Capabilities The capability for most of the required measurements exists as shown in Table 3-3 (capability I, II, and IIT). For water level, both stilling-well tide gauges and bottom-mounted pressure gauges are generally adequate to measure sea-le~re} variation with time periods of a few minutes or more (capability ~ and IT). However, for variations with periods of months or longer, the accuracy of pressure gauges may be inadequate due to instrument drift. If sea-level changes are to be corrected for barometric pressure or water density, these data must also be available. Currents associated with low-frequency phenomena can be mea- sured by a variety of means including tracking of drifters, in situ cur- rent meters, and remote acoustic-Doppler systems. The Lagrangian tracking methods are well suited for measurements offshore during normal weather conditions. However, for measurements during storm

48 conditions, the existing capability is marginal in terms of rugged- ness, reliability, installation, and portability. For measurements in channels, the bottom-mounted acoustic-Doppler current-meter shy tems appear to have the most promise (Lhermitte, 1981~. ~ theory, acoustic tomography could be a useful technique. However, practical limitations may preclude the use of acoustic tomography in shallow water as there are many unanswered questions concerning the sta- bility of acoustic paths and the signal processing required to resolve currents in shallow water. Electromagnetic and acoustic techniques have been rated, therefore, as capability level IT! passable, but not yet satisfactory. Modeling of low-frequency motions requires much data for validation of the models, including the shape of the bottom contours. Not only is large-scale bathymetry required for modeling needs, but also small scale features (such as bedforms), primarily for modeling bottom frictional terms. Nearshore bathymetry data can be obtained by fathometer (acoustic travel-time measurement); however, precision leveling is required to provide a known elevation or bench mark for reference. The existing capability (leve} I) is quite adequate to determine the general bottom shape, but the capability for determining small-scale features relevant to bottom roughness is only marginally adequate (capability level IlI). Bed forms (periodic or quasi-regular changes in the elevation of the sea floor) having length scales from centimeters to hundreds of meters and amplitudes of centimeters to meters are diffi- cult or impossible to measure during storms, when they are changing rapidly. Existing technology does not permit these measurements to be made during high-energy events; nominal development may permit improved estimates of the time scales of motion of some of the larger bed-form features (see the section on fluid/sediment inter- actions, later in this chapter). Clearly, improved modeling capability verified by observations would provide the best means for simulating the effects of bed-form changes in the near future. Measurement Needs The major needs for quantifying Tow-frequency motions in the nearshore and backshore regions are adequate coverage and strategic placement of sensing devices for water level, waves, and currents dur- ing normal tidal regimes and during storm- or earthquake-induced anomalous events. "Strategic placement" means that which will al- low inferences to be drawn in terms of such phenomena as wave setup,

49 sea-level differentials across inland flooded areas containing extensive and variable vegetation, and volume flow into or out of harbors or estuaries. Acquisition of such data during event-related phenomena such as storms and tsunamis requires real-time capability, including the possibility of placement of portable sensing equipment. The de- velopment needs are for suitably rugged yet reliable sensing devices for currents and waves under storm conditions. Table ~3 summarizes these needs. F[UID/SED=ENT INTERACTIONS One of the ultimate goals of coastal engineering research is to un- derstand and to predict shoreline stability and morphological changes in response to the variety of processes that occur in the coastal en- vironment. The engineer must understand the processes to be able to undertake projects that address such concerns as beach nour- ishment, sedimentation associated with coastal structures, erosion- accretion patterns along exposed coasts or in the vicinity of mIets or navigational channels, and sediment response associated with dredg- ing activities. The coastal engineering data needs listed in Table 3-1 reflect the breadth of problems in which sedimentation plays an important role. From a physical point of view, sedimentary processes are the re- sult of fluid/sediment interactions, or more specifically, the response of sediment particles to the forces produced by shoaling waves, tides, coastal currents, and winds. Sedimentary processes are among the most important but leant understood aspects of the coastal environ- ment. In studying sediment transport phenomena, quantifying the total sediment movement under a variety of conditions is the ultimate goal. A distinction is often made between bed-ioad transport those grains sliding, robing, or moving within several grain diameters of the seabed and suspended-Ioad transport, those grains suspended by fluid turbulence. In extremely high-transport situations, grain-i grain collmions, rather than fluid turbulence, become the dominant suspending mechanism, and a grain-dispersed layer is maintained within 10 to 15 cm of the seabed. For this document, the distinctions between these transport modes are not critical, and distinction is made only between suspended and near-bed transport. Nearshore sedimentation research generally has been limited by technology and the inability to monitor sediment transport on time

50 and space scales commensurate with the physical causes. As a result, progress in sedimentology has lagged behind advances in our under- st~nding of the other physical mechanisms operating in the nearshore environment. At the same time, as conceptual and theoretical mod- eling of nearshore sedunent response has continued to evolve, the need for appropriate field measurements has continued to grow. Fielc! investigations of various aspects of fluid/sediment interac- tions encompass a wide range of efforts. Thus, a broad demand is placed on instrumentation and sensor capabilities. A review of re- cent literature (e.g., Greenwood and Davis, 1984; Edge, 1985; Kraus, 1987) suggests that, within the classifications shown earlier in Figure 2-1, sediment-related field studies fall into three broad categories: regional, site specific, and process oriented. The regional category of field investigation includes broad-scale or reconnaissance-level investigations that, for example, require infor- mation on (1) regional patterns of circulation, suspended sediment, bed morphology, and tongshore bar geometry; and (2) noncorrelated parameters such as wave climatology, sediment accumulation vol- umes, and shoreline changes. Because of the large area involved, remote-sensing devices would be appropriate to these investigations. Typical examples include use of time-lapse photography to map spa- tial and temporal changes in longshore bar morphology relative to wave conditions (Holman and Lippman, 1987~; correlation of Land- Sat images for detecting the nearshore surficial suspended-sediment concentration field with various physical processes thought to cause resuspension (Fedosh, 1987~; and comparison of h~starical changes in beach profile with storm wave predictions (e.g., Dick and DaIrymple, 1984; Brampton and Bevan, 1987~. The site- or project-specific category of field investigation in- cludes studies designed to (1) obtain empirical information on local processes, (2) test or use hypotheses to explain local beach changes, or (3) investigate gross cause-and-effect relationships between fluid motions and sediment response at a location of interest. Excellent examples of site-specific studies relate to human intervention with the nearshore, e.g., the shoreline effects of reduced flooding in the Nile River (Inman and Jenkins, 1985), studies of massive sediment in- jections at San Onofre, California (Grove et al., 1987), and sediment inkling at dredged channels, as is observed in many harbors. The process-oriented category of field investigation relates to fundamental sediment-transport research. Specific theoretical rela- tionships are investigated at the sediment-particle scale of inquiry,

51 and the goals may have longer-term implications related to sediment- transport theory and future applications. Studies of this nature may or may not have immediate application to coastal engineering prom lems but represent potential advances to the field. Historically, ex- perunental research has been carried out in wave tanks or flumes; however, field experiments are beginning to be used to test specific hypotheses, to support improved numerical modeling efforts (e.g., Mason et al., 1987), and to carry out fundamental fluid/sedunent research in the nearshore zone (e.g., Seymour, 1987; Hanes and HuntIey, 1986; Beach and Sternberg, 1988~. Instrumentation requirements are substantially different for these three categories of study. The first category, regional-scale investiga- tion, requires less precise quantitative information covering broader geographical areas. In contrast, process-oriented sediment-transport research requires a wide variety and large number of instruments deployed in precise arrays to document detailed relationships. Study areas are selected on the basis of research interest and place great demand on collection of field data. Site- or project-specific studies fad between the other two categories in requirements for instruments and data collection and may rely, for example, on selected point measurements or Tong-term measurements of bathymetry. Measurement Requirements and Capabilities The measurements required to fulfill coastal engineering needs are extremely varied. This variety can be illustrated) by reviewing the major engineering problems defined earlier in Figure 2-! and Table 3-1 and considering the primary measurements needed to address fluid/sediment interactions (summarized in Table 3-4~. The mea- surement requirements listed under backshore protection (in Table 3-1) emphasize spatial-scale (wide area) monitoring rather than local sensing. Inlet stabilization requires measurements to be made within entrance channels and includes estimates of sediment transport and flux, especially ~ relation to infixing channel entrances. Transport of cohesive sediment (clays and muds) in nearshore regions was not considered in this report, and readers are directed to the report on Sedimentation Control to Reduce Maintenance Dredging of Naviga- tional Facilities in Estuaries (NRC, 1987) for a review of cohesive sediment transport. Harbor protection refers to measurements de- signed to investigate sedimentation associated with major protective structures and the adjacent seabed.

52 The various measurement requirements tabulates] in Table 3-1 have been grouped by category in Table 3-4, which emphasizes the variety of measurements used to address coastal engineering sedi- mentation problems. Included with each measurement requirement is a suggester} measurement performance (resolution or range), a commonly used measurement technique, a designation of present instrument capability, the related development need, and selected references for the sediment and seabed morphology categories. From Table 3-4 it is seen that, generally, methods for measur- ing waves and currents are operational and are capable of measur- ing over the full range of environmental conditions (capability I, Il). In contrast, methods for measuring various aspects of sediment characteristics and coastal morphology are in various developmen- tal stages and their full potential has not been realized (capability Ill, IV). In some cases, measurements are only possible under low- wave conditions (e.g., diver-operated samplers), while in other cases, measurement techniques are lacking (e.g., fast response near-bed sediment-transport sensor). As a result, sediment transport in the nearshore zone is inferred from other measurements (e.g., currents, waves), rather than measured directly. Measurement Needs The measurement capability ratings shown in Table 3-4 indicate that many of the sediment and bed-morphology-related measure- ment techniques and some flow and wave measurements related to fluid/sediment interaction are limited in their present capabilities. The present major measurement deficiencies in fluid/sediment inter- action studies include the following. Near-Bed Sediment Transport While recent advances have been made on rugged solid-state optical sensors capable of measuring suspended sediment concentra- tions In the surf zone (e.g., Downing et al., 1981; Huntley, 1982), a technology does not presently exist to measure sediment concen- "rations or change occurring in the near-bed region (within several centimeters of the seabed). Sediment concentration increases to- ward the seabed; measurements in this area are fundamental to our understanding of total sediment transport. Although nearshore sediment-transport studies using sand tracers and traps have been

53 used, the long time-response of the measurement (minutes) does not match the wave-forcing period. Prototype acoustic devices that record grain unpacts in the near-bed region (Downing, 1981; Salkield et al., 1981) have been constructed and deployed. However, an oper- ational solid-state device that can monitor bed-Ioad transport in the near-bed zone is not presently available. Acoustic devices for detecting suspended sediment concentric tion profiles are presently being developed and operated in a variety of laboratories. Although there are some basic problems with the operation of these sensors in the coastal zone (e.g., strong response to bubbles, lirn~ted response to fine sediment), they are perceived to have significant potential. Acoustic backscatter devices are being developed to provide high-resolution profiles of suspended sediment, either looking downward! to the seabed, upward into the lower water column (Lynch et al., 1987), or throughout the water column (see summary in Kraus, 1987~. Acoustic-Doppler devices have been pro- posed for monitoring fluid sediment flows within centimeters of the seabed (Lowe, 1987), thus suggesting a means to estimate bed-Ioad transport. Multifrequency acoustic devices have the potential of re- solving grain size characteristics of suspended load. These devices are evolving rapidly, and they show high potential. Their adaptability to the high sediment concentration and entrained air bubbles common to the nearshore environment, however, is yet to be established. Fluid Turbulence Turbulent velocity fluctuations and the related transfer of mm mentum are the actual mechanisms that maintain sediment suspen- sion within the surf zone. Turbulent fluctuations occur from fluid shear and from breaking waves. Classical sediment-transport the- ory ~ based on boundary shear-generated turbulence. Some present sediment-transport models used in the surf zone consider only forced turbulence from boundary shear, while others consider turbulence from breaking waves to be more important. In all cases, knowledge of turbulent characteristics in the surf zone is almost totally lacking, and existing means to quantify fluid turbulence are inadequate. Methods for measuring turbulence in other fluid environments (including areas of the continental shelf outside the surf zone) have been developed. These include hot film anemometers (e.g., Gust and Weatherly, 1985), acoustic travel-time current sensors (e.g., Williams and Tochko, 1977), and laser-Doppler velocimeters (e.g., Agrawal et

54 al., 1988~. Thus, some forms of technology exist that have potential application. Preliminary measurements of turbulence In the surf zone have been made using hot film probes (George and Flick, 1987~. Bathymetry Requirements for measuring bathymetry exist on several scales. On a large scale, the ability to monitor beach profile changes quickly with I~15 cm accuracy over the range of fair weather and storm conditions is necessary to understand a wide variety of nearshore problems. These include problems related to beach nourishment, long- and short-term beach erosion/accretion, longshore trapping efficiencies, channel sedimentation rates, and total sediment budgets. On a smaller scale, methods are required to monitor changes in seabed elevation caused by scour and erosion at a given point on m~rneter-to-centimeter scales. Measurements of this accuracy are necessary for sediment-transport research directed toward predicting seabed stability and determining the relationships between sedunent transport and changes in beach morphology. Present methods for measuring beach profiles include wading survey techniques (Aubrey and Seymour, 1987), tractor- and sled- mounted transducers, and the motor-driven CRAB device operated by the Corps of Engineers at the Duck, North Carolina, Field Re- search Facility. These techniques provide approximately similar resm lution (1~15 cm) but are limited in operation to low-wave conditions, such as wading survey techniques, or are large devices that cannot be easily moved from site to site, such as CRAB. Instruments for mea- suring small-scale bathymetric changes at a point are presently un- der development (Sallenger, 1989; personal communication). These instruments use miniature, high-frequency echo sounders that are mounted within the water column and "look" downward toward the seabed. The signals are noisy because of bubbles and high sedi- ment concentrations, but these difficulties are being considered and development is proceeding. Directional-Wave Characteristics The partitioning of tote] sediment flux into cross-shore and long- shore components is dependent on the direction of wave propaga- tion in shallow water. Since waves generally break at low angles to the beach, a difference of 1° to 2° in the estimated wave direction

55 can significantly change the computed longshore sediment flux rate. Consequently, improved wave directional resolution ~ required in measurement systems for sedimentation assessment and forecasting. This measurement requirement Is also important for other aspects of coastal engineering, as discussed earlier in this chapter. Spatial-Scale Observations Several nearshore engineering functions, such as beach protection and beach nourishment planning and engineering, require data from simultaneous observations over large sections of the nearshore or coastal zone with methods that may be less accurate or precise than the site-specific requirements discussed in the preceding sections. Remote-sensing techniques have been developed, over the past decade, that may provide significant potential for application in the surf zone. These techniques rely on satellite, airborne, and ground- based sensors that can rapidly scan or observe a variety of param- eters. Examples showing some limited application of these remote- sensing methods Include deterrn~nation of nearshore bathymetry from airborne systems and the use of satellite imagery detecting nearshore turbidity to map coastal circulation patterns. Generally, remote- sensing technology has been developed for use in other fields; how- ever, many of the specific methods have potential for coastal engineer- ing applications. Some examples are briefly summarized in Table 3-5, which also includes some of the in-water acoustic and photographic methods discussed in the foregoing sections. Large-scale spatial av- eraging may limit the usefulness of some of these technologies in the surf zone. Since no agency or institution exists that coordinates or system- atically funds instrument development, the needs of the coastal engi- neer~g community have been addressed historically on an individual basis as dictated by funding for field research. Most of the major co- operative field studies e.g., NSTS, C2S2, Duck '85, SUPERDUCK, etc. (Kraus, 1987) have been responsible for coordinating moderate development efforts carried out by individual participants. This ad hoc approach has been effective in the sense that instrument develop- ment is closely tied to scientific needs. It may not, however, provide adequate or long-term funding and facilities for developing some of the major instrument systems listed in the foregoing discussions.

56 TABLE S-5 Remote-Sensing Instruments, Techniques, and Potential Application in Surf Zone Instruments Potential Application Deployment: SATELLITE/AIRCRAFT Altimeter Scatterometer BAR (Synthetic Aperture Radar) Radiometer (microwave) Color scanner Deployment: AIRCRAFT SCR (Surface Contour Radar) ROWS (Radar Ocean-Ware Spec- trometer AOL (Airborne Oceanographic Lidar) Geostrophic currents, wave and storm surge characteristics Sea state Sea state, wave spectrum, wave direction Suspended sediment distribution Sea surface Directional wave spectrum Surface warren and bottom topography SLAR (Side-Looking Airborne Radar) Mapping ALM (Airborne Electromagnetic Coastal bathymetry Method) Deployment: GROUND-BASED CODAR tOSCR) SLR (Side-Looking Radar) Cameras: time lapse and time exposure; (photographic and video imaging) Coastal circulation Wave runup, breaker conditions, low- frequency wave characteristics, bar morphology Techniques Deployment: IN WATER Acoustic back~catter Laser Doppler Acoustic Doppler Acoustic tomography Suspended sediment concentration, flux, bed-load transport characteristics, local scour and deposition, turbulent and suspended sediment field. F[UID/STRUCTURE INTERACTIONS Structure Apes and Measurement Requirements Coastal structures can be divided into the following categories: Breakwaters Elevated platforms, on piles or cylinders

57 Seawalb and bulkheads Jetties and groins Dredged entrance channels into harbors · Navigation aids . Artificial Elands. Design of each of these types of structures requires knowledge of environmental forces sufficient to establish criteria for location, materials that should be used, and dimensions. These criteria may come from physical models, or, if the physics and mathematics are understood well enough, from numerical models. Breakwaters The purpose of a breakwater may be different in one location than in another location because of different reasons for reducing wave energy. Some of these purposes are to: tion, provide quieter water in an entrance channel for safe naviga- . provide a calm harbor for loading and unloading cargo or passengers from ships, protect the shore from wave damage during storms, and . reduce beach erosion or accumulate sand. Breakwaters can be properly designed only with site-specific data about the wave spectrum—both for significant and maximum waves and wave grouping, information on runup and overtopping, shock pressures exerted by breaking waves, the effect of the breakwa- ter on currents, the intensity of air bubbles during storms, and the engineering characteristics of bottom sediments at the seabed and for some depth below. Breakwaters are of various types. The most common is the rubble mound, faced with large stone or concrete units of di~er- ent shapes. Caisson breakwaters, made of rows of hollow boxes (of concrete, usually) reflect wave energy, since long-period narrow- spectrum waves can cause significant scour. Design of these break- waters requires specific knowledge of the erodibility characteristics of the local seabed. Composite breakwaters, partly caissons and partly rubble, have been built- in many configurations, most notably in Japan. These structures are characterized by a vertical wall near the top so that impact forces are usually a critical design consideration.

58 Another variety of breakwater is that composed of a line or lines of sheeting, sheetpiling, or connected cylinders, forniing a vertical wall. This design also requires detailed information about the foun- dation materials below the seabed and also about scour resistance in front of the structure. Any of the foregoing types of breakwaters may be built with different top elevations, depending on the need to prevent, or to allow, overtopping during storms that are so violent that vessels would not be entering. Still lower top elevations may be built as submerged breakwaters designed to reduce the height c)f waver halt. not to absorb or reflect all wave energy. cat · · ~ . ~ _ _ A special class of structures is floating breakwaters. Their effec- tivenes~ depends on their width as compared with the wave length of incoming waves. Consequently, they are mainly suitable for relatively short wave, such as those usually occur in marinas and other small- boat harbors. They offer advantages, compared to bottom-mounted breakwaters, in sheltered deep water or In regions of unusually high tide ranges. A literature search by the Corps of Engineers Water- ways Experiment Station (WES) shows a wide variety of designs for floating breakwaters, including arrays of tires, boxes, baffles, and diaphragms (COE, 1982~. A working group of PlANC (Permanent International Associa- tion of Navigation Congresses) has been established to analyze 163 breakwaters from many parts of the world in efforts to find common factors among breakwaters that have contributed to their success or failure, consider model tests, evaluate safety factors, and propose ways to respond to engineering and construction problems. This assessment is still underway, but it is clear from such reviews, as well as from experience in the analysis of several failures, that more information is needed about the progress of shock pressure waves through the armor and core materials. Elevated Platforms Elevated platforms supported by piles or cylinders embedded in the sea bottom have been extensively used and researched. Wave forces on the cylinders have been measured, and the body of data is sufficient for design, except for the magnitude of shock pressures for breaking waves. Better knowledge is needed about the effect of groups of piles on the scouring of the seabed. Examples of this lack of understanding are demonstrated by the eject of pilings used in

so construction of several research piers over the past decade (for ex- ample, at the Corps of Engineers facility, Duck, North Carolina, and Scripps Institution of Oceanography) that have caused unexpected deepening near the pier. In addition, predictions of scour depth an a function of wave cInnate and the engineering properties of the beach soils and subsoils are not possible with any great accuracy at this tune. The measurement of the height of storm wave crests ~ a critical requirement for safe design. Many tests and observations of behav- ior of of! industry platforms have shown the need to maintain the elevation of the underside of the deck above the crests of the highest waves, to avoid] excessive impacts. Seawalb The design of seawalIs and bulkheads along the shore requires the same kmds of measurements of wind and wave forces as break- waters, with added emphasis on wave overtopping and the effects of impounded rainwater on the landward side. Jetties ~d Groins Jetties and groins are physically similar to breakwaters but are built perpendicular to the shoreline. In the United States, jetties are structures protecting and stabilizing the entrances to harbors, although the term Jetty is used more broadly elsewhere. Groins, by interfering with the transport of sand along the shore, are used to attempt to stabilize the shoreline position. Design problems for these structures are s~rnilar to the design problems for breakwaters. Entrance Channels Dredged entrance channels into harbors may be considered] coast- al structures with negative elevation. In designing entrance channeh, information is required about the source and movement of sediments that tend to fill up these channels and impede shipping. In addition, entrance channels may cause adverse environmental effects in the form of downdrift erosion when sediment supplies are cut off. The relations between current velocity, wave climate, grain size, distribu- tion of sediments, and turbulence are not clearly understood.

60 Navigation Aids Navigation aids to guide ships and boats are a broad class of coastal structures. They may be fixed or floating and consist of buoys, lights, ranges, markers, and communications stations (acoustical, visual, radar, or radioJ. Measurements of environmental forces such as those developed by wind, current, and ice are important to the effective design of navigation aids and their supporting structures. Artificial Islands The creation of land in the ocean for terminals, airports, and other activities has been advanced, particularly by the Japanese, during the past two decades. The fill is typically contained within a conventional breakwater structure so it presents no unique design problems. Measurement Capabilities To measure the dynamics of wave/structure interactions, a vari- ety of instrumentation may be required, including the following: Fast response pressure sensors Strain gauges Tensiometers Accelerometers Anemometers Water-level gauges Current meters Bathymetric and topographic measurement systems Optical motion indicators, remote sensing. Measurement requirements, capabilities, and needs have been summarized in Tables 3-6 and 3-7 for application to structures of two general types: fixed position (rubble mounds, solid blocks, or vertical-walled caissons) and floating breakwaters. These structures may be located on shorelines, harbor entrances, or channels. In situ instruments, imbedded in rubble-mound structures, can present data-Iogging problems because of the susceptibility of cables to dam- age.

61 TABLE 3-6 Assesement of Needs for Measurements of Fluid/Structure Interaction (F~ced-Position Structures) Accuracy Measure- Opera- Measurement Require- Measurement ment Capa- tional Objective meets Technique bility Need Profile movement: 10 cm Sunrey III external 10 cm Survey + strain gauge II 5, 6 internal 5 cm Inertial IV 1 Water level Wave Pressure + charactenstice 10 cm wave riders I 3, 6 Wore direction B° Slope arrays + photos III 2 Wave runup Photos II 5 Wave reflection Photos II 2, 6 Wave transmission Photos II 6 Wave overtopping Photos, Acoustic II ~ Pore pressures Pressure gauge III 1 Currents Toe of structure 10 cm/e Acoustic II 4 Inside structure 10 cm/s Acoustic II 5, 6 Forces Mooring ~t10% Strain gauges I 5, 6 Torsional +10% Strain gauges I 5, 6 Seismic +10% Strain gauges I 5, 6 LEGENDS: Capability: I Very good II Adequate III Not adequate IV None Need: 1 Major development 2 Improve information detail 3 Improve reliability 4 Improve durability 5 Improve installation and use 6 More sensors or lower data cost Measurement Requirements Erosion at the Base of Rubble-Mound Structures If the foundation soils are eroded by storm waves, causing the sediments to move seaward, the undermining will cause raveling of the slope and loss of material. To counteract this effect, the base

62 TABLE 3-7 Assessment of Needs for Measurements of Fluid/Structure Interaction (Floating Breakwatere) Measurement Requirement Measurement Measurement Capability Neede Wave data Currents, instantaneous Time series, in stonne Stochastic Heights Directions Pore pressures, core and foundation in storms Velocity in interstices Rate of overtopping Transmission through reflection, in 3 dimensions Runup Air content of breaking waves Ice forces Storm currents, toe Seismic forces Structure data Cross section, as built Movement of units Internal strain Floating structures Mooring forces Torsional stresses Ice forces Sensors, Adequate; at toe, none Adequate Adequate Adequate; at toe, none Inadequate Sensors, Adequate None None Adequate Partial None None None Sensors, Adequate; Installation, none Adequate Poor, especially for rubble Poor Adequate, outside Adequate Adequate Adequate At toe; installation Improved reliability Improved reliability At toe; installation Better resolution Durable installation Installation, none Development and installation Development Better spatial coverage Development Development Development Development Installation Improvement Development Development Improvement Improvement Improvement . areas are often reinforced with a berm or placed in a previously ex- cavated trench to prevent scour. However, the effectiveness of these eros~on-prevention techniques so far has not been accurately mea- sured. Physical models do not adequately reflect the scale relation- ship between foundation sediments and rubble or armor materiab. Prototype measurements of waves and currents, scour, and transport

63 are required; numerical and physical modeb need to be developed on the basis of these measurements. Shock Pressure Implosion When a wave breaks directly against a rubble slope, or against a concrete wall embedded within it, the shock pressure rapidly pen- etrates the interstices. These shock pressures are suspected of being involved in rearrangement of the structure and breaking of concrete units. Measurements are needed of the pressure gradients that exist at the peak of such ~xnplosions, and of the movement of individual elements of concrete, stone, gravel, and sand as they are jostled by the impacts. Storm Surge During the height of a storm, mean sea level may rise consid- erably as a result of the combined effects of wind setup, reduction in barometric pressure, and tides or Ketches. This brings the zone of magnum wave action nearer to the top of the structure, where the structure is often the most vulnerable. There are few reliable measurements available at this time of the structural ~rnpact or the structural response, so that models are of questionable value. Loss of Fine Particles Uncler the cyclic action of waves, fine particles in the breakwater may work their way through the gaps in the armor (these channels are called piping), or subarmor, and escape. Progressive piping may then cause cavities within the breakwater and loss of support for materiab above, leading to eventual reduction of the crown elevation and allowing detrimental overtopping. At present, there is no tech- nique for detecting and locating internal cavities and piping; such information could affect design and material selection criteria. Overtopping If sufficient "green waters crosses the top of the rubble mound, the crests can attack the structure and flood the interstices at the roadway or crown. Both tests and prototype experience show that flows that overtop are more likely to cause damage. In fact, breakwa- ter ~roundheads," or ends, are known to be most vulnerable and are

64 often designed with heavier units or flatter slopes, or both. Repeated overtopping is thought by some to be a dominant factor in struc- tural failure. Better measurements of wave/structure interaction are needed. Armor Decomposition Wave attack, sun and salt, heat and cold (and perhaps ice), and the unp act of smaller stones and movement of units themselves ad cause gradual deterioration of stone or concrete. The rounded corners of old armor units attest to this effect. As the units become more spherical they become less stable and shift position. Efforts have been made to build scale models with units of softer material, to reproduce this effect, but these efforts have not been wholly satisfactory. A long- term monitoring program of armor decomposition would provide needed information. Multidirectional Waves When waves from different directions approach a breakwater, convergence of crests can produce a local breaking wave far larger and steeper than the average conditions. There is a need for more directional wave measurements in the vicinity of breakwaters. Breakwater Stability Irnp act of concentrated wave energy against any structure at the top of a rubble mound can precipitate a loss of slope stability. A rubble-mound slope has been characterized as an incline close to the failure point. For reasons of economy, slopes are often made as steep as is believed to be practical; there is a Tower factor of safety common to the design criteria for rubble-mound structures than for steel and stone structures. There is a need for more mea- surements of rubble-mound breakwater on prototype and mode! rubble-mounded structures in order to improve understanding of stability coefficients. Summed Instruments are available to measure many parameters related to interaction between water and structures. Some parameters cannot be measured, largely because the sensors and data links have not

65 been successfully deployed in the harsh environment of a stormy sea. Measurements that are needed but cannot yet be measured adequately on fixed structures are . tures, the types and directions of breaking waves attacking struc- the nature of movement of different structural components during storm conditions, pore pressure within the interstices and in the foundation, wave transmission and reflection, torsional stresses in floating structures, scour depths as they progress during storms, the air content of water at contact points, the velocity of wave crests, the amount and nature of overtopping, and the amount of wave runup. The priority research areas are those that will lead to more rational design of coastal structures. This research focuses on mea- surement of the pressures and forces, not only external but also internal, from storm waves at the instant of breaking on structures, so that the behavior observed can be understood and predicted. To undertake much of this research it will be necessary to develop in- stallation methods for mounting instrumentation on or in structures and to recover the data.

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Much of the U.S. coastline is rapidly changing—mostly eroding. That fact places increasing pressure on the planners and managers responsible for coastal development and protection, and could have a direct effect on many of the 125 million Americans living within 50 miles of the coast who rely on its resources and beaches for their livelihood or recreation. Although rapid advances have been made in the measurement systems needed to understand and describe the forces and changes at work in the surf-zone environment, their potential for allowing more accurate and reliable planning and engineering responses has not been fully realized. This book assesses coastal data needs, instrumentation, and analyses, and recommends areas in which more information or better instrumentation is needed.

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