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