Beach Nourishment and Protection (1995) / Chapter Skim
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4 Beach Nourishment Project Design and Prediction
4 Beach Nourishment Project Design and Prediction
Pages 82-106
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From page 82... ...
Indeed, the constructed profiles are expected to change significantly during the first several years following construction or renourishment. The constructed profile may not follow the exact design prediction because the coastal processes were different than the available data revealed at the design stage or environmental conditions subsequent to project construction or renourishment varied substantially from the predictions supported by the data.
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From page 83... ...
Projects funded entirely by nonfederal sources may be limited by the amount of available funds, and the objective then becomes placement of material to provide the greatest longevity and maximum dry beach width for the dollars available. SIGNIFICANT PROCESSES IN DESIGN The purposes of a beach nourishment project are to increase the dune and berm dimensions and to advance the shoreline seaward to reduce storm damage
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From page 84... ...
The time scales of these two processes are disparate: profile equilibration occurs in a few years, whereas the alongshore equilibration varies in duration and is related to project length, sediment grain size, and wave environment. For example, a reasonably long project (i.e., alongshore length)
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From page 85... ...
~ \ Sea level b) Elevation view showing original profile, initial placed profile, and adjusted profiles that would result from nourishment project with coarse and fine sands FIGURE 4-1 Sand transport losses and beach profiles associated with a nourished beach.
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From page 86... ...
Under the mobilizing action of waves, the sediment will be transported seaward, gradually approaching an equilibrium profile. The equilibrium profile as generally used by designers of beach nourishment projects is defined as the natural form that the beach would take for a given volume of sand of a particular grain size under the prevailing
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From page 87... ...
If the native sand and the nourishment sand are nearly the same grain size, it is reasonable to assume that the equilibrium profile form will be the same as that of the native beach before nourishment but is simply displaced seaward, and the equilibrium beach width can be calculated using simple equations. However, sand finer or coarser than the native sand will have equilibrium beach profiles that are of flatter or steeper slopes than the native sand, respectively.
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From page 88... ...
Both simple and detailed methods for predicting the rate of alongshore spreading depend primarily on wave height, background erosion rate, and sediment grain size. For projects constructed in the vicinity of engineered structures, a littoral barrier, or a sink, such as an inlet, wave direction also is important.
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From page 89... ...
From an engineering perspective, structures can sometimes benefit beach nourishment projects. Nontraditional devices are more problematic because there is little definitive information on their performance capabilities as well as a history of innovative devices that have failed to live up to their claimed potential.
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From page 90... ...
Prediction of project performance in the presence of terminal structures requires knowledge of both wave height and wave direction, and capabilities are limited for both the preliminary and detailed methods (see Appendix C for further discussion)
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From page 91... ...
FIGURE 4-5 Calculated example of beach nourishment project evolution. 91 lions apply as for terminal structures.
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From page 92... ...
A performance demonstration specification is needed for evaluating the effectiveness of nontraditional shore protection and beach stabilization and restoration devices. The results of such a program would be expected to provide a more complete basis for the probable performance before any interested agency or private buyer commits to their use.
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From page 93... ...
Nevertheless, the depth of closure provides a reasonable boundary condition when the equilibrium dry beach width associated with the addition of a certain quantity of sand of a particular grain size is developed. Sediment size of nourishment material can affect the performance of a project in at least two ways.
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From page 94... ...
and predicted performances of beach nourishment projects:
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From page 95... ...
. ~ FIGURE 4-6 Estimated ability to predict performance relative to various factors that can affect beach nourishment projects.
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From page 96... ...
For preliminary design, estimates of wave height are required, and if the setting involves alongshore sediment transport and structures such as groins or jetties, estimates of wave direction also are required. Usually, at the preliminary level, it is assumed that after equilibration the nourished berm height will be the same as the native berm height, and attention is not directed to other profile characteristics (e.g., dune design)
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From page 97... ...
Fine material also adversely affects project performance. Early projects constructed without regard for grain size performed relatively poorly, and recent developments indicate that nourishment sand that is only slightly smaller than native sand can result in significantly narrower equilibrated dry beach widths compared to sand the same size as (or larger than)
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From page 98... ...
For these reasons, it is essential that project decision makers and designers have a basic understanding of sediment sources, transfer, and placement. The search for suitable material generally involves locating a deposit of sand and gravel of sufficient volume and grain size that could serve as a suitable source.
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From page 99... ...
Typically difficult to obtain permits unless in conjunction with lagoon restoration or navigation projects because of regulations against loss of wetlands. Often low quality because of deposition of fine material.
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From page 100... ...
For example, in Florida numerous projects have nearly depleted economically recoverable sand reserves in state waters. Increasingly, distant sources are being considered for use, including colitic Aragonite sands found in the Bahama Islands (see Appendix F)
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From page 101... ...
At present, the major constraints on the transport and placement of material for beach nourishment from offshore borrow sites are weather-related delays owing to sea state and winds; restrictions on construction activity, methods, and timing relating to environmental concerns; equipment limitations for deepwater dredging; and distances over which sediment must be transported. The construction of beach nourishment projects may involve the use of one or many possible combinations of equipment and techniques, depending on the site, the size of the job, environmental and other constraints, and the level of competition at bidding time.
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From page 102... ...
Based on a general review of documentation for various beach nourishment projects by the committee, application of the best physics to project design has not been uniform among the USACE districts. Modern design profiles in the United States began with development of the "Caldwell Section" for emergency sand dune protection used after the great Ash Wednesday 1962 storm that struct the Mid-Atlantic coast (see Appendix H; Podufaly, 1962~.
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From page 103... ...
These two concepts were developed prior to recent developments in equilibrium beach profiles and in both preliminary and detailed methods for calculating spreading losses. POSTCONSTRUCTION DESIGN REFINEMENT AND CORRECTIVE ACTION As discussed earlier, performance of a beach nourishment project, once constructed, often does not conform to predictions because of limitations in predictive models and supporting data or because the wave climate was different than assumed.
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From page 104... ...
The importance of this fact relative to more traditional beach nourishment, in terms of quantities, is reflected in Tables 14 and 16 in Shoreline Protection and Beach Nourishment Projects of the U.S. Army Corps of Engineers (USAGE, 1994~.
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From page 105... ...
1994. Importance of Beach Profile Surveying and Depth of Closure for Beach Nourishment Project Design.
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From page 106... ...
1994. Shoreline Protection and Beach Nourishment Projects of the U.S.
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