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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.
Representative terms from entire chapter:
design alternatives
