EM 1110-2-1100 (Part V)
31 Jul 2003
volumetric erosion due to a background erosion rate of 0.5 m/year on a 6-km project length with an
average berm height of 1.5 m and a depth of closure of 6 m is 1.13 million cu m over a 50-year
project life. Adding this volume to the calculated project end loss volume (in the absence of
background erosion) of 2.8 million cu m gives a total project volume of 3.93 million cu m which is
only about 60 percent of the 6.52 million cu m estimated in the simulations that included a
background erosion rate of 0.5 m/year. The explanation for this result is that background erosion
outside the project causes the seaward protuberance of the project to become larger each year. At
the end of the 50-year project, the design shoreline is 45 m seaward of the adjacent shorelines which
are unaffected by the project nourishments; whereas in the absence of background erosion, the
protuberance of the design shoreline remains 20 m throughout the 50-year project. The additional
nourishment volume is required to provide a natural transition from the eroded adjacent shorelines
to the design shoreline. Figure V-4-22 shows the calculated shoreline position at the end of the 50-
year nourishment project for each of the project scenarios simulated. It also illustrates the previously
discussed requirement for additional nourishment volume in the presence of background erosion.
Figure V-4-22 also illustrates the beneficial effects a beach-fill project has on the adjacent shorelines
during its lifetime. In the absence of background erosion (Figure V-4-22a) it is seen that the beaches
adjacent to the fill project are advanced seaward approximately half the design width for more than
a project length on both sides of the fill. With a background erosion rate of 0.5 m/year
(Figure V-4-22b) it is seen that the fill project stabilized the shoreline at or seaward of the preproject
shoreline position for a distance of approximately one project length on both sides of the fill.
(b) Recommendations for detailed analysis.
In the previous section, the GENESIS model was applied in a simplified way to estimate volume
requirements for an idealized beach-fill project. That application of the GENESIS model was
referred to as simplified because the model was not calibrated for the specific project reach nor was
a time varying time series of wave information used to represent the environmental forcing.
Furthermore, the simplified method of analysis assumes that the project and adjacent beaches can be
characterized as long straight beaches with uniform and temporally constant background erosion
rates, and that the dominant long-term processes affecting the evolution of the beach-fill project can
be captured by examining the alongshore dispersion of the fill by a persistent effective wave
condition. However, many if not most projects vary significantly from these assumptions and
warrant a more detailed application of the GENESIS model. Gravens, Kraus, and Hanson (1991) and
Gravens (1992) provide detailed information concerning the application of the GENESIS model.
A detailed application involves developing shoreline position and beach profile data sets, analyses
of incident wave conditions including detailed nearshore wave transformation, and selection of a
time-history of representative wave conditions for use in model calibration, verification, and project
forcasting. Typically a detailed wind wave hindcast such as WIS provides the required wave
information and serves as the database defining the local and regional characteristics of the offshore
wave climate in the vicinity of the project. A long, multiyear record of measured wave data may also
suffice.
A critical aspect of the detailed application is the calibration of the model to site specific project
conditions. Calibration involves selection of the domain to be modeled and boundary conditions,
evaluation of model accuracy in reproducing historical shoreline change, and net and gross longshore
sand transport rates. The calibration and evaluation of the GENESIS model serve to demonstrate the
predictive capability of the model at the specific project site. Estimates based on a well-calibrated
and verified GENESIS model are considered superior to those based on a simplified application of
GENESIS. However, it may be informative to examine the difference between estimates generated
Beach Fill Design
V-4-59
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