Beach Nourishment and Protection (1995) / Chapter Skim
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C Prediction of Beach Nourishment Performance
C Prediction of Beach Nourishment Performance
Pages 167-188
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From page 167... ...
Figure C-1 illustrates the complicated three-dimensional sediment transport patterns associated with various phases of project evolution. Although beach nourishment projects have been carried out actively for several decades, there is still not an adequate methodology to predict their de 167
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From page 168... ...
This is due in part to the complicated alongshore and crossshore transport processes, the near uniqueness of every setting for such projects, and the generally inadequate monitoring of both the forces on and responses of past projects to provide a basis for assessment of available methodologies and guidance for their improvement. This appendix reviews simple analytical and numerical procedures available for prediction of beach nourishment project performance, introduces some lesswell-known behavioral characteristics affecting performance, and provides estimates of predictability under various nourishment scenarios.
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From page 169... ...
The knowledge of alongshore sediment transport is much more advanced than for cross-shore transport, which has been studied actively for only about a decade. Equilibrium Dry Beach Width In cases in which the nourishment material is similar to the native beach material, the additional dry beach width after equilibration, /\yO, can be shown to be approximately V ° h*
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From page 170... ...
In order to apply Equation (C-2) to beach nourishment, a relationship is needed between the sediment-scale parameter, A, and the grain size, D, or equivalent sediment fall velocity, w.
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From page 171... ...
APPENDIX C 171 (a) Intersecting Profile APA;N (b)
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From page 172... ...
for values of hJB of 2 and 4, respectively. Planform Evolution Planform evolution is influenced by the general morphology of the system to be nourished; the simplest case is a long straight beach.
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From page 174... ...
(C 7) in which K is a sediment transport factor usually taken as 0.77 but is probably a function of sediment grain size or other characteristics, H is the wave height, and K iS the ratio of breaking wave height to local water depth (usually taken as 0.781.
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From page 175... ...
2, b = 2E(M- 1) 4 G_ /\YO 7r £ c= (1 _My2 in which l\yO is the initial dry beach width, as defined earlier.
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From page 176... ...
In an idealized fashion the upper portions of the placed profile are "planed off'' by the alongshore transport, leaving the placed material below this level of activity as "residual bathymetry." Although this residual bathymetry is not active in the transport processes, it does influence the wave transformation, in particular wave refraction. The effect of the wave refraction is to cause the quasi-equilibrium planform to remain irregular rather than to be straight, as would be the case if the entire placed planform moved in response to gradients in the alongshore sediment transport.
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From page 177... ...
Numerical Models for Predicting Beach Nourishment Performance Computer Models of Alongshore Shoreline Evolution An important tool in the design and implementation of many beach nourishment projects is the application of computer models that simulate the processes of alongshore sediment transport and the resulting evolution of the shoreline planform. Such models incorporate equations that relate sediment movements to the nearshore waves and currents.
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From page 178... ...
One of the chief contributions of the GENESIS model is that it provides a flexible basis for analyses that can be applied to an arbitrary prototype situation a basis that calculates wave transformations as they shoal and undergo refraction and diffraction, calculates the patterns of alongshore sediment transport, and then determines the resulting shoreline changes. One of the principal modifications of the GENESIS model from earlier models is in the calculation of alongshore sediment transport rates, an approach that includes the transport caused by waves breaking obliquely to the shoreline and alongshore variations in wave breaker heights.
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From page 179... ...
compared GENESIS predictions to the results obtained in physical models, again for a series of detached breakwaters such as those at Lakeview Park but also for the impoundment of sand in a series of groins built across the beach. In all cases, the numerical models closely reproduced the shoreline changes that occurred in the physical models.
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From page 180... ...
Analyses using shoreline models such as GENESIS that include wave refraction have the potential for predicting locations of erosional hot spots and could be used to analyze whether the focus of erosion might be eliminated by dredging the offshore shoals to some determined water depth. Shoreline models such as GENESIS have been discussed here as an aspect of the design of beach nourishment projects and have been used to predict the shoreline evolution of the sand fill.
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From page 181... ...
Although full three-dimensional models that account simultaneously for cross-shore and alongshore sediment transport are available, the commonly applied models such as GENESIS deal only with alongshore evolution of the shoreline, and separate models, such as SBEACH, analyze the cross-shore sediment transport. The models sometimes need recalibration for the specific site of the application or verification during the monitoring phase of a nourishment program because of uncertainties in the transport calculations and because of simplified assumptions that are used in developing the shoreline evolution models.
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From page 182... ...
have described an analytical method, based on observations from numerical models, in which the profile tends to approach the equilibrium in an exponential manner for a constant water level. Kriebel (1990)
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From page 183... ...
Due to the complexity of the interaction of sediment transport processes with coastal structures, prediction of the performance of beach nourishment projects in the presence of coastal structures will usually require the use of numerical models. Knowledge of the interaction of coastal structures with beach systems is on the edge of the state of the art.
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From page 184... ...
FIGURE C-7 Calculated planform and volumetric evolution of an initially rectangular beach nourishment project fronting a seawall. Deepwater waves at 10° to shore normal (from Dean and Yoo, 19941.
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From page 185... ...
In the case of nourishment on a shoreline of compatible sand, owing to the small aspect ratio of the project, the transport patterns can be linearized approximately as the superimposition of alongshore transport on an unperturbed shoreline and normally incident waves acting on the nourishment project. This is the reason that nourishment on a beach of compatible sand will result in little downdrift migration or planform asymmetry.
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From page 186... ...
l991b. Comparison of shoreline change obtained with physical and numerical models.
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From page 187... ...
1989. Beach Profile Evolution in Response to Changes in Water Level and Wave Height.
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From page 188... ...
1974. Offshore Sediment Transport and Equilibrium Beach Profiles.
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