EM 1110-2-1100 (Part V)
31 Jul 2003
(a) Without-project condition.
The existing or without-project condition is included in the optimization process to determine
baseline damages. Morphologic features of the existing beach, such as dune height, berm width, and
offshore profile shape, typically vary along the project study domain. To accurately estimate storm
erosion response for the existing condition, a set of representative morphologic reaches should be
developed to describe variations in profile shape along the project domain. The BMAP software can
be used to define morphologic reaches by analyzing profiles, grouping similar profiles, and
calculating an average representative profile for each reach.
Profile characteristics that should be considered when developing morphologic reaches include dune
height and width, berm width, nearshore and offshore profile slopes, sand grain size, presence of
seawalls or other structures, and proximity to inlets. As part of the economic analysis to evaluate
damages and benefits, the project domain is divided into a series of economic reaches based on value
of property and infrastructure (see Part V-4-1-d). Boundaries of economic domains should also be
considered in morphologic reach delineation to ensure that storm erosion modeling is consistent with
the economic analysis.
(b) Storm selection.
Evaluation of potential storm damages requires selection of a set of storms representative of future
events that may impact the project area. The set of storms should reflect a range of intensities and
frequencies of events consistent with the historical record. In developing the storm set, tropical and
extratropical events should be treated distinctly because of differences in storm characteristics and
frequencies of occurrence.
One approach to storm selection has been to develop a set of storms characterized by peak surge
return period ranging from frequent events (5-year return period or less) to extremely rare events
(100-year return period). Peak surges for selected return periods are obtained from available
stage-frequency information, and representative storm surge hydrographs are developed using
assumed hydrograph shapes and durations. Differences in hydrograph shape between tropical and
extratropical storms should be considered; tropical storms typically have much shorter durations.
The storm surge hydrographs are combined with corresponding wave height and wave period time
histories to fully describe the storms. Using this approach, the frequency of modeled responses are
assumed to correspond to return periods of the input storm surges (e.g., a 50-year storm surge
produces a 50-year erosion response). This assumption simplifies the analysis but is not strictly
accurate because, in addition to peak storm surge, other factors influence the magnitude of erosion
(storm duration, hydrograph shape, and wave characteristics). Inconsistencies may arise with this
approach related to characterizing the storm based on peak surge alone. For example, a 20-year
storm (where frequency is defined solely on maximum surge) may produce more erosion than a
50-year storm, if the 20-year storm has higher waves or a longer duration.
An alternate and preferred approach is to develop a "training set" of storms by selecting events from
historical records and/or hindcasts. A sufficiently long historical period is identified, such as
20 years for extratropical storms or 100 years for tropical storms. All historical events within the
period exceeding a selected threshold are included in the training set. For example, all events which
have a peak surge exceeding 0.3 m may be considered significant from a storm erosion standpoint
and included in the training set. No return periods are assigned to storms in the training set a priori.
Each storm is modeled for the existing condition and each project alternative to calculate
corresponding erosion responses. Using the training set of storms and modeled responses, life cycle
analyses are performed by employing the Empirical Simulation Technique (EST) to generate
Beach Fill Design
V-4-37
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