EM 1110-2-1100 (Part III)
30 Apr 02
smaller than that from oblique wave incidence in an open-coast situation. However, in the vicinity of
structures, where diffraction produces a substantial change in breaking wave height over a considerable length
of beach, inclusion of the second term provides an improved modeling result, accounting for diffraction
effects.
(d) The boundary conditions at the ends of a study area in a shoreline change modeling project must
be specified. There are three common boundary conditions: no sand transport (QR = 0), free sand transport
(dQR/dx = 0), and partial sand transport (QR ... 0). The locations of the study area ends should be selected with
these options in mind. Large headlands or jetties which completely block the longshore transport are good
choices for model boundaries. At these locations QR = 0. Points where the position of the shoreline has not
changed for many years are also good locations for boundaries. At these points, the gradient in longshore
transport is small so that a free transport condition can be specified (dQR/dx = 0). At some locations, the
longshore transport rate is known and can be used as a boundary condition (i.e., artificial sand bypassing at
a jetty). If none of these "good" locations exist, engineering judgment must be used.
(e) In all cases, results should be calibrated and verified using known shoreline positions and wave
conditions for the longest period possible. The modeler also attempts to use wave data applicable to the
period between the dates of the calibration shorelines. The GENESIS technical reference (Hanson and Kraus
1989) discusses in full the operation of shoreline change numerical simulation models.
(2) Input data requirements and model output.
(a) As discussed by Gravens (1991, 1992), preparation and analysis of the input and output data files
occupy a substantial portion, perhaps the majority, of the time spent on a detailed shoreline change modeling
project. Gravens (1991, 1992) stresses that the data gathering organization and analysis process cannot be
overemphasized because (1) it forms the first necessary level in understanding coastal processes at the project
site, and (2) the simulation results must be interpreted within the context of regional and local coastal
processes, and the natural variability of the coastal system. Success in modeling shoreline change depends,
to a large extent, on preparation and analysis of the input data.
(b) General input information required by GENESIS includes the spatial and temporal ranges of the
simulation, structure and beach fill configurations (if any), values of model calibration parameters, and
simulated times when output is desired. Initial and measured (if available) shoreline positions as referenced
to a baseline established for the simulation are also required. Offshore and nearshore (if available) wave
information and associated reference depths are used to calculate longshore sand transport rates. Output
information produced by GENESIS includes intermediate and final calculated shoreline positions, and net
and gross longshore sand transport rates.
(3) Capabilities and limitations.
(a) GENESIS was designed to predict long-term trends of the beach plan shape in its evolution from
one given initial condition. This change is usually caused by a notable perturbation; for example, by beach
fill placement, sand mining, sand discharge from a river, construction of a detached breakwater, or jetties
constructed at a harbor or inlet. In engineering applications and tests of GENESIS, modeled shoreline reaches
have ranged from about 2 to 35 km with a grid resolution of 15 to 90 m, and simulation periods have spanned
from approximately 6 months to 20 years, with wave data typically entered at simulated time intervals in the
range of 30 min to 6 hr (Gravens, Kraus, and Hanson 1991).
(b) Hanson and Kraus (1989) and Gravens, Kraus, and Hanson (1991) discuss the capabilities and
limitations of GENESIS. The model allows an almost arbitrary number and combination of groins, jetties,
Longshore Sediment Transport
III-2-81
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