EM 1110-2-1100 (Part V)
31 Jul 2003
through both simplified and detailed applications of GENESIS for a given project to quantify the
range of uncertainty between the design methods. Coastal engineering judgement, local coastal
experience, and the performance of nearby similarly-situated projects, if any, are important and
should be factored into the overall design and specifically the estimates of project volume
requirements. The results of a detailed GENESIS model application should always be interpreted
in the context of the model calibration and verification. For instance, if the historical rate of erosion
was underpredicted in some specific model region of the simulated shoreline in the calibration and
verification then it should be expected that the model will continue to underpredict erosion in this
area in the simulation of the various design alternatives. Estimates of volume requirements should
be adjusted to account for deficiencies in the model calibration and verification.
One advantage of taking the detailed GENESIS modeling approach in the investigation and
prediction of beach-fill project performance is the opportunity to anticipate possible secondary effects
that have troubled some past projects. For instance, the excavation of a nearshore borrow area or
some preexisting bathymetric feature may significantly alter local breaking wave conditions and
result in the development of a localized zone of accelerated erosion along the project shoreline (a hot
spot). A detailed application of GENESIS would examine the effects of local bathymetry and project
changes in the bathymetry, through the application of nearshore wave transformation models and
inclusion of nearshore wave information in the shoreline change calculations. GENESIS model
predictions based on this type of data can reveal the potential for the development of shoreline
anomalies within the project. Simplified applications of GENESIS or an analysis based on the
analytical approach have no chance of revealing possible hot spot development because the processes
responsible for their development are ignored or are smoothed over in the analysis.
If a good calibration of the GENESIS model cannot be achieved because of complex boundary
conditions (unstructured inlet, randomly fluctuating shoreline), or insufficient or poor data quality,
then a more simplified application of the model is recommended. An alternative to calibrating and
verifying the GENESIS model to historical shoreline position data, is to calibrate GENESIS to an
accepted existing-condition sediment budget developed through an analysis of the available physical
data. In this type of application, a wave time series that reproduces the accepted sediment budget
within the model is developed either from hindcast wave information such as WIS, measured wave
information, or simply synthesized from observations or general wave and wind information
available in a statistical form.
(5) Anticipating hot spots. Hot spots are localized areas within a beach nourishment project that
experience a reduction in beach width (corresponding to a loss of sand volume) significantly greater than
losses that were predicted and/or are observed throughout the rest of the project. Hot spots are problematic
in the sense that the desired level of protection may be compromised locally, whereas the remainder of the
project is functioning well. To achieve the desired level of protection, renourishment in the hot spot area may
be required on a more frequent basis than the rest of the project. Or, some structural solution may be required
to provide the desired level of protection or improve sand retention so that renourishment can be performed
on the same schedule as is planned for the rest of the project. Hot spot mitigation measures will probably
result in additional unanticipated and undesired costs. The public perception of hot spots that develop shortly
after construction can be one of project failure. Therefore, the potential for the occurrence of hot spots should
be investigated during the design process. This can be accomplished through, for example, the method used
by Smith and Ebersole (1997). The GENESIS model together with detailed nearshore wave conditions can
also be used for this purpose (Bodge, Creed, and Raichle 1996).
(a) Hot spots can arise due to a locally strong gradient in longshore sand transport rate, which creates
a divergent zone where, in a net sense, more sand is leaving an area than is entering it. Strong longshore
transport gradients can develop due to a noticeable change in shoreline/bathymetric contour orientation.
Beach Fill Design
V-4-61
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