Measuring and Understanding Coastal Processes (1989) / Chapter Skim
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3. Processes and Measurement Requirements
3. Processes and Measurement Requirements
Pages 26-65
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From page 26... ...
Eulerian measurements are those collected by moored current meters measuring velocity at a fixed location. Lagrangian measurements are obtained by a tracer (drifter floats, dye, or tagged particles)
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From page 28... ...
After the hydrodynamic forces generated by high- and lowfrequency 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)
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From page 29... ...
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 (~)
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From page 30... ...
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 recording and internal processing capability. Some of the difficulties and problems encountered with Eulerian measurement systems are biological fouling, corrosion, orientation uncertainties, disturbance of the flow fields by the instrument, bubbles, cavitation, extreme storm waves, and disturbance of sensors by fishermen.
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From page 31... ...
FIGURE 3-3 Electromagnetic Current Meter, Model 511. SOURCE: MarshMcBirney, Inc., Gaithersburg, Md.
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From page 32... ...
32 ~ `11 ~ ~ / / / / FIGURE 3-4 75-KHz Self-Contained Acoustic-Doppler Current Profiler. SOURCE: RD Instruments, San Diego, Calif.
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From page 33... ...
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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From page 34... ...
Wave directional buoys are used primarily in deep water to measure the heave, pitch, and roll with accelerometers and an inclinometer. A second type of "orbit following" directional buoy, developed by ENDECO (Brainard and Gardner, 1982)
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From page 35... ...
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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From page 36... ...
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.
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From page 37... ...
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 characteristics in shallow water using three or more bottom-mounted pressure sensors (Higgins et al., 1981~.
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From page 38... ...
7 Radiation stress +10%, 1 Hs
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From page 39... ...
2 Improve information detail 3 Improve reliability 4 Improve durability 5 Improve installation and use 6 More sensors, or lower data cost 7 None
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From page 40... ...
Recent developments in radiation stress (momentum flux) measurements include using differential pressure sensors to make direct measurements of the surface slope (Bodge and Dean, 1984)
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From page 41... ...
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 approximately 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.
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From page 42... ...
Doppler sonars mounted on the floating instrument 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.
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From page 43... ...
5. Improvement is needled in wave directionality measurement and analysis, both in situ and remote.
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From page 44... ...
[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.
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From page 45... ...
It also demands accurate bathymetric data near shore, and land elevations in the backshore if overland flooding is an unportant consideration. Numerical models for storm surges need water-level data during storm events and adequate astronomical tide data to verify the models.
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From page 46... ...
Verification 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.
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From page 47... ...
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 measured by a variety of means including tracking of drifters, in situ current meters, and remote acoustic-Doppler systems.
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From page 48... ...
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 during normal tidal regimes and during storm- or earthquake-induced anomalous events. "Strategic placement" means that which will allow inferences to be drawn in terms of such phenomena as wave setup,
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From page 49... ...
F[UID/SED=ENT INTERACTIONS One of the ultimate goals of coastal engineering research is to understand and to predict shoreline stability and morphological changes in response to the variety of processes that occur in the coastal environment. The engineer must understand the processes to be able to undertake projects that address such concerns as beach nourishment, sedimentation associated with coastal structures, erosionaccretion patterns along exposed coasts or in the vicinity of mIets or navigational channels, and sediment response associated with dredging activities.
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From page 50... ...
Typical examples include use of time-lapse photography to map spatial and temporal changes in longshore bar morphology relative to wave conditions (Holman and Lippman, 1987~; correlation of LandSat 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 includes studies designed to (1)
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From page 51... ...
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.
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From page 52... ...
The present major measurement deficiencies in fluid/sediment interaction 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 concentrations In the surf zone (e.g., Downing et al., 1981; Huntley, 1982)
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From page 53... ...
Fluid Turbulence Turbulent velocity fluctuations and the related transfer of mm mentum are the actual mechanisms that maintain sediment suspension within the surf zone. Turbulent fluctuations occur from fluid shear and from breaking waves.
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From page 54... ...
sediment flux into cross-shore and longshore components is dependent on the direction of wave propagation in shallow water. Since waves generally break at low angles to the beach, a difference of 1° to 2° in the estimated wave direction
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From page 55... ...
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.
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From page 56... ...
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)
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From page 57... ...
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 breakwater 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.
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From page 58... ...
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.
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From page 59... ...
The relations between current velocity, wave climate, grain size, distribution of sediments, and turbulence are not clearly understood.
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From page 60... ...
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 variety 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.
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From page 61... ...
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.
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From page 62... ...
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 .
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From page 63... ...
Storm Surge During the height of a storm, mean sea level may rise considerably 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.
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From page 64... ...
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.
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From page 65... ...
tures, the types and directions of breaking waves attacking structhe 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.
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