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APPENDIX H U.S. Army Corps of Engineers Design Criteria Historically, the design of coastal defense projects has been based on the specifications of a design storm. For urban coastal areas, protection was designed either for the standard project hurricane as defined by the National Weather Service or some other rare event. In the latter case, this was often the storm (hurricane) of record in the area of interest, with the peak surge elevation trans- ferred to the project site and adjusted to coincide with high tide. Frequency of exceedance determinations, when a tide gauge was not available in or near the area of interest, were innovative, highly variable from locale to locale, and lacked a scientific basis. Early beach fills were frequently designed to protect against erosion and to provide recreation. In such cases, coastal flood protection was not claimed. Most damages prevented were from mitigation of the effects of long-term erosion. Berms were generally 50 to 100 ft wide, as determined by the severity of histori- cal events. Some berm widths were set to optimize the recreation benefits, as long as the historical shoreline was not exceeded, because most benefits were derived from enhanced recreation. The Water Resources Development Act of 1986 re- placed the project purpose of beach erosion control with coastal storm damage reduction and recreation. After the March 1962 northeaster that devastated much of the East coast shorefront areas, Joseph M. Caldwell, a U.S. Army Corps of Engineers (USAGE) engineer, designed a dune and berm cross-section based on results of experiments conducted by using a large wave tank located at the Beach Erosion Board labora- tory (predecessor of the Coastal Engineering Research Center) in Washington, D.C. The "Caldwell section" was used to design the protection of coasts for some 311
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312 BEACH NOURISHMENT AND PROTECTION time thereafter. These were the first uses of berms and artificial dunes as sacrifi- cial shore protection measures. The USACE currently uses a range of approaches for developing a set of storm events to evaluate design features. This change in design approach was part of the USAGE's response to the national economic requirements of the Economic and Environmental Principles for Water and Related Land Resources Implemen- tation Studies that were approved by President Reagan in 1983 (Schmidt, 1994; see Appendix B). The selected approach is based on project scope, availability of data, and level of resources. In the simplest case, hypothetical or historically based surges that represent a limited combination of storm parameters are scaled to define a set of storm events. Recurrence relationships are then obtained from existing elevation frequency curves. A more comprehensive design procedure is normally undertaken for large-scale projects. The prescribed procedure is to use numerical models of physical processes and statistical procedures. Historical storm events are used to define a representative storm training set. The beach's response to each event is determined by numerical models. Statistical procedures are then used to compute frequency relationships and associated error bands for the design parameters of interest and for storm damages. This more rigorous approach can be used to generate continuous frequency-of-occurrence relation- ships for any parameter in the design evaluation process, as well as to provide error-band input for risk-based design. The current prescribed practice of the USACE is to use a set of storm events with a range of return periods to evaluate the cost effectiveness of design alterna- tives. Optimization of the net benefits for an area necessitates evaluation of a range of protection alternatives to determine their costs and the damage reduction benefits that each alternative would produce. The degree of protection, or storm damage reduction, that would be produced by each alternative can be evaluated for a series of storm events with return periods ranging from relatively frequent events (5-year return period or less) to extremely rare occurrences (500-year return period). The amount of damage likely to be produced by this range of storms with a beach nourishment project in place can be compared with the expected damage without the project, with the difference in potential damages representing the expected reduction in damages attributable to the particular de- sign alternative being evaluated. The reduction in potential average annual dam- ages plus the incidental average annual recreational benefits associated with the alternative represents the total annual benefits produced by the alternative. Net benefits for this plan are computed by determining the difference in the annual cost necessary to construct and maintain the alternative and its total annual ben- efits. This same procedure is then used to assess the other design alternatives being considered for a particular beach nourishment project. Protection plans can range from simple beach fills of varying widths to combined fills having both artificial dunes and berms. The singular plan that produces the maximum differ- ence in net benefits is designated as the National Economic Development plan.
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APPENDIX H 313 The cost associated with constructing and maintaining this plan establishes the maximum extent of federal cost sharing available for the project area. The result of this plan formulation process is that the plan does not provide protection for a particular storm. Rather, it is capable in varying degrees of reducing damages associated with essentially all storms Barrett, 1994; see Appendix B). For coastal protection projects, design alternatives can include variations in berm width, berm height, and dune height and the inclusion of fixed structures. Under the USAGE's prescribed procedure, the defined storm events are supposed to be chosen to reflect realistic combinations of various parameters descriptive of historical storm events that have impacted the location of interest. For tropical events, the storms need to define the range of durations, maximum winds, radius of maximum winds, pressure deficits, storm track, and other factors. For extratro- pical events, appropriate descriptors include the range of durations, hydrograph shapes, and maximum winds. Frequency relationships are then assigned to the set of storms. Recurrence relationships are no longer directly assigned to a storm. They are assigned to some measurable characteristic or result of the storm such as maxi- mum surge height. In cases such as beach recession, factors such as stage hydro- graph shape and wave characteristics determine the extent of recession. Because storms are characterized by multiple properties, the set-of-storm-events concept is the preferred approach for analysis and is considered by the USACE to be more useable and realistic than the single design storm method. This approach recog- nizes the beneficial effects a project will have during storm events that have parameters exceeding those that produce zero or minimal damage as well as the probability that such events will occur.