shoreline positions and waves from 1983 to 1995); and (2) simulating coastal evolution in
connection with the opening of Moriches Inlet and Shinnecock Inlet (simulation period
1931-1983). Objectives of these simulations were to validate that the model could
(1) predict longshore transport rates along a stretch of coastline where trends in the wave
climate and shoreline shape are present at the regional scale, and (2) predict coastal
evolution in terms of shoreline response and changes in the ebb-shoal complex where
regional processes and controls exert a significant influence on local processes.
General Model Setup
The south shore of Long Island is approximately oriented 67.5 deg TN. A coordinate
system was defined that had a similar orientation. In defining the new coordinate system,
baseline points employed by Rosati et al. (1999) were referenced. The lateral boundary
condition of "no shoreline change" was specified based on shoreline measurements
covering 1830 to 1995. Suitable locations for such a boundary condition were identified
approximately 10 km west of Montauk Point and either adjacent to or 15 km east of Fire
Island Inlet, depending on the simulation period. In simulation of inlet openings, the
boundary was placed about 15 km east of Fire Island Inlet to avoid describing spit
movement taking place there. The time step was set at 24 hr and length step at 500 m.
The length of the modeled shoreline in combination with shadowing from surrounding
landmasses made it necessary account for variation in wave climate alongshore. Hindcast
waves from WIS Stations 75, 78, and 81 were input (20-year time series of height, period,
and angle from 1976 to 1995), linearly interpolated between stations. Depth of active
sediment transport was set to 8 m, and the representative median grain size 0.3 mm. The
studied shoreline shows large-scale (i.e., regional) features that persist with time. Without
taking these features into account through yr, diffusion would eliminate them. In Cascade,
yr enters as a source term in the governing transport equation for the local shoreline
evolution y. The shape of yr was determined from spatial filtering of the shoreline
measured 1870 when no inlets existed and using a window length of 7 km.
For the long-term simulation 1931-1983, the equilibrium volumes of the ebb shoals and
bypassing bars were specified as a function of time based on the recorded inlet cross-
sectional areas. In the shorter simulation 1983-1996, the equilibrium volumes were held
constant because the inlets did not change substantially during this time period. Jetty
lengths on each side of the inlets and the time of construction were specified according to
available data. Two sources of sediment were included in the present Cascade simulations
for the FIMP area, input from cliff erosion west of Montauk Point and from beach fills
placed west of Shinnecock Inlet. A total fill volume of about 800,000 m3 was placed west
of Shinnecock Inlet between 1949 and 1983, and another 1,150,000 m3 was put in this area
between 1983 and 1995 (Morang 1999). Smaller beach fills have been placed at other
locations, but were neglected. In the simulations, the beach fill volumes were converted to
sediment sources with constant strength in time and space. Rosati et al. (1999) estimated
the cliff erosion at Montauk Point to yield about 33,000 m3 per year, which input to the
model and introduced as a distributed source with constant strength in time. The
Westhampton groin field was not resolved in the simulations.
Larson, Kraus, and Hanson
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