Model Forcing
Calculation of morphology change was conducted during a time in which
controlling factors ranged from primarily tidal to storm dominated. The simulation
time interval was specified to coincide with the natural deflection and migration of
the main navigation channel through the ebb shoal. Data collected during two
SHOALS surveys conducted on August 13, 1997 and May 28, 1998 provide
bathymetric change information from which the calculations can be verified. M2D
was forced by both water-surface elevation along the seaward boundary and wave-
driven radiation stress gradients within the interior of the domain. Boundary
conditions for the local model (M2D) were extracted from the regional ADCIRC
model of Long Island and mapped to the seaward boundary, Quogue Canal boundary,
and Shinnecock Canal boundary. This method of boundary specification preserves
temporal and spatial variation of tidal properties at the boundaries. Thus, interior
response to spatial amplitude and phase variation is included in the calculations. In
the nearshore region, the coastal tidal current is represented which enhances the
natural westward migration of the ebb jet.
Incident wave field characteristics (height, period, and direction) were obtained
from NDBC Station 44025 and used to construct spectral energy input files for
STWave. The spectrums were applied along the offshore boundary and propagated
to the shoreline by STWave. During the simulation wave conditions were updated at
3-hr intervals and radiations stress gradients were calculated across the STWave
model domain.
Simulation Results
Results presented in this section represent a 15-week-long time interval that
started on August 13, 1997 and ended on November 30 1997. To reproduce
morphology change the model simulation was condensed to a 4-week-long sequence
during which incident wave fields with wave heights greater than 1 m and periods
larger than 7 sec were modeled. These wave fields were chosen to represent events
that forced peak transport of littoral material. By including August and September,
when wave forcing was relatively weak, tidal control on morphology change could be
evaluated. Sediment transport rates were computed each hour and the inlet
morphology was updated four times each day based on 6-hr averages of the sediment
transport rates. Calculated depth was output every 24 hr, and bathymetric change
(Figs. 11-13) was plotted at the end of each month.
During the months of August and September, Shinnecock Inlet experienced
relatively low wave energy. Therefore, during this time morphology change
responded to tides and relatively weak waves. Wave heights rarely exceeded 1.5 m
and periods ranged between 5 and 9 sec. The longest-period swell (11 to 14 sec) to
reach the inlet between the 1997 and 1998 surveys occurred in mid-September.
Although the swells approached from the south and lasted four days, wave heights
did not exceed 1.75 m. The absence of long-period swells in the wave record during
the simulation owes to a reduction in hurricane activity during El Nino years
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