Measuring and Understanding Coastal Processes (1989) / Chapter Skim
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4. Modeling Coastal Systems
4. Modeling Coastal Systems
Pages 66-86
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From page 66... ...
, while models used in estimating future changes are usually some form of predictive model. Engineering requirements dictate what one wishes to predict, whether it be changes in beach bathymetry or in hydrodynamic forces on a structure under given design storm conditions.
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From page 67... ...
In contrast, laboratory physical models are scaled-down versions of a prototype system in which one employs some scaling considerations. Both field measurements and laboratory measurements are essential means of verifying, based on physical principles, a mathematical mode} or of providing the essential parameters that are not known a · — prlorl.
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From page 68... ...
Such subgrid scale processes must be represented in some ad hoc manner in the model. The classical example is the effect of those turbulent processes that are too small in scale for the adopted average grid scale of the model.
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From page 69... ...
For example, not only does the model provide a rational interpolator, it also provides the means to compute quantities usually not measured directly, like stresses, transport rates, energy fluxes, and energy density. PHYSICAL MODELS Physical models, including hydraulic models, are generally used when flow conditions of a prototype system are not amenable to mathematical analysis.
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From page 70... ...
Calibration and verification of similarity between mode} and prototype requires~detailed field measurements of the spatial and tempm ral distribution and variability of velocity fields. Present technology such as acoustic-Doppler current meters may be able to acquire the necessary prototype data at a point, but large numbers of these sensors are required to verify mode!
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From page 71... ...
Models of storm surges and of slowly evolving circulation on the continental shelf or in estuaries demand adequate information on the spatial distribution of wind stress and on barometric pressure versus time in order to drive them. The modeling of hurricane induced storm surges generally employs an auxiliary storm model for the inclusion of the wind stress and pressure fields associated with such storms (NRC, 1983~.
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From page 72... ...
are heavily contaminated (distorted) by local resonant effects that render inverse methods virtually meaningless because the available models cannot be that site specific.
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From page 73... ...
Numeric 3-D 2, 6, 7 Leendertse and Liu, 1975; Liu and Leendertse, 1987. LEGEND: Need: 1 Major development needed 2 Improve information detail 3 Improve physics 4 Improve efficiency 5 Improve tuning 6 7 8 ~ — v Special data needed Verification needed None
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From page 74... ...
Extensive field measurement of wave growth under carefully selected uniform fetch-limited wave conditions became available in the late 1960s and early 1970s (Mitsuyasu, 1969; Hasselman et al., 1973~. The analysis of these data showed the importance of the nonlinear wave/wave interaction to feed energy into the low-frequency end of the spectrum, rather than direct wind forcing.
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From page 75... ...
resulted that integrates the basic transport equation describing the evolution of the tw - dimensional ocean wave spectrum without additional ad hoc assumptions regarding the spectral shape. The source functions describing the-wind input, nonlinear transfer, and white-capping dissipation are prescribed explicitly.
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From page 76... ...
The determination of am propriate grid scale for bathymetry in refraction models is unsolved and can be critical to the results. For simple bathymetries, the wave refraction models have been verified using detailed laboratory data (WhaTin, 1972; Tsay and Liu, 1982~.
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From page 77... ...
As waves refract and shoal and eventually break, there is a change in wave-induced momentum that must be balanced by a slope in the mean water level or an increase in the bottom shear stress and concomitant nearshore currents. The wave-induced momentum is commonly referred to as radiation stress (Longuet-Higgins and Stewart, 1962~; recent advances in
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From page 78... ...
have established that the driving force for the longshore current is a component of the radiation stress tensor, Slays On the other hand, rip currents can be produced by longshore variation In wave setup (owing to variable wave height and resulting normal radiation stress terms)
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From page 79... ...
The interaction of sea-swell waves with infragravity waves in the nearshore needs to be investigated. Although some generation mechanisms have been proposed and utilized in initial models, the level of understanding is not well developed and field measurements are required for mode!
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From page 80... ...
In summary, the prediction of sediment transport in shallow water is complicated by interaction of the sediment with the driving forces, the mobility of the bed (it is not stationary) , the nonlinear character of waves in shallow water, lack of understanding of turbulent momentum balances in shallow water, and lack of incorporation of cohesion or biological binding effects.
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From page 81... ...
The latter investigators all assumed sediment of uniform grain size, although some discussion of mixtures of grain sizes has been included in the literature (e.g., Kamphuis, 1975; Madsen and Grant, 1975, 1976~. Little comprehensive treatment of nonuniform grain sizes has been available for oscillatory flows.
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From page 82... ...
. A third type of approach to sediment transport has been to relate various sediment parameters to the driving forces, using empirical relationships.
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From page 83... ...
to examune suspended sediments in the surf zone. More complicated models involving turbulence explicitly have been pros posed by a large number of investigators (Beach and Sternberg, 1988; Grant and Madsen, 1979; Glenn, 1983; Smith, 1977; Souisby, 1988~.
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From page 84... ...
that relates theoretical interference patterns of surface gravity and infragravity waves to possible shoreline configurations. Energetics models of beach configuration have been developed based on the initial work of Bagnold and coworkers (Bagnold, 1963; Inman and Bagnold, 1963~.
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From page 85... ...
Bed forms are the response of a deformable sand bed to hydrodynamic shear stresses applied at the surface of that bed. Rarely is a bed perfectly flat; instead it wiB have some scale of structure super~rnposed on it.
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From page 86... ...
86 and stochastic spectral methodologies exist for estunation of wave loading. The newer technology addresses nonlinear coupled models of wave/structure interaction for compliant structures in deep water and Boating tethered structures (e.g., Crandall, 1985; Jeffreys and Patel, 1982; Basu, 1983; Nie~zwecki and Sandt, 1986~.
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