EM 1110-2-1100 (Part V)
31 Jul 2003
(4) Shoreline change modeling to estimate advance fill and renourishment requirements. In detailed
design, one-line numerical models of shoreline evolution such as the GENEralized model for SImulating
Shoreline Change model (Hanson 1987; Hanson and Kraus 1989) are typically used to provide more realistic
estimates of project longevity and renourishment requirements than may be possible using the analytical
approach discussed previously. Numerical shoreline change models provide the designer with an objective
tool for evaluating a variety of potential project design alternatives, which may involve coastal structures in
addition to beach fill. The use of numerical models of shoreline evolution for the design, optimization, and
comparative evaluation of competing project alternatives has many advantages over the use of the analytical
approach. For example, the GENESIS model allows examination of multiple renourishment cycles leading
to a complete project life cycle, the effects of beach-fill stabilization structures, the effects of many possible
future wave conditions (as defined by the Wave Information Study (WIS) wave hindcast) leading to a suite
of possible future shoreline conditions. The numerical approach allows for the evaluation of the project
design and its performance within the context of a realistic sediment budget developed for the project reach.
Employing a shoreline change model like GENESIS allows the designer to examine project performance
under conditions much more representative of the actual project setting than is possible with the analytical
approach, and is therefore the recommended approach for final design of major projects.
(a) Simple model application.
In this section, the GENESIS model is applied in a simplified manner to simulate the evolution of
an idealized beach-fill project, throughout an anticipated 50-year project life, to investigate the design
issues of advance fill and renourishment requirements. The hypothetical project is 6 km (3.7 miles)
long with a design berm width of 20 m (65 ft). Two series of model simulations were performed for
each of four renourishment intervals, 3, 5, 7, and 9 years. The first series of simulations were for a
hypothetical condition where the project shoreline was subjected to no ambient or background
erosion. The second series of simulations were for a project shoreline experiencing a background
erosion rate of 0.5 m/year. All simulations were made for the entire 50-year project life. Note that
the 3-year renourishment cycle project was simulated as a series of fifteen 3-year cycles plus one 5-
year cycle beginning in project year 45. Likewise, the 7-year renourishment cycle project was
simulated as a series of six 7-year cycles plus one 8-year cycle, and the 9-year renourishment cycle
project was simulated as a series of five 9-year cycles plus one 5-year cycle.
In the execution of the simulations, the computed shoreline position at the end of each renourishment
cycle was used to define the initial shoreline position at the beginning of the next renourishment
cycle, except of course in the project area where the required renourishment fill volume was placed
with tapered transitions to the existing shoreline adjacent to the project. The incident wave climate
was specified as an effective wave condition that approached the project normal to the shoreline and
did not change throughout the simulation. The average berm height and depth of closure were
specified to be 1.5 and 6 m, respectively, for a total active depth of 7.5 m. The preproject shoreline
was straight. The initial construction planform included 2-km-long transitions to the preproject
shoreline position adjacent to the project limits, which were added to reduce the rate of end losses
from the project. The model was then run for the first renourishment interval, an appropriate
nourishment volume was added, the model was run for another renourishment interval, and so on,
until the 50-year life was reached. Each prenourishment shoreline within the project limits was
advanced seaward a sufficient distance so that the predicted shoreline position at the end of the
renourishment cycle was 20 m seaward of the preproject shoreline at all locations within the project,
except within 400 m in from both ends of the 6-km-long project. That is, the design width was
allowed to be violated only at the lateral limits of the project, and for an alongshore distance of only
400 m. The results of these model simulations bring to light a number of important considerations
with respect to beach-fill design.
Beach Fill Design
V-4-55
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