attained, all sediment entering the particular morphologic unit is transferred downstream.
Thus, for each morphologic unit (ebb shoal, bypassing bar, attachment bar), two
equations govern storage and transfer of sediment,
V
Qout = Qin
(15)
Veq
dV
= Qin - Qout
(16)
dt
where Qin (Qout )= transport (from) the morphologic unit, V = sediment volume of the unit,
equilibrium shoal volume based on field data from a large number of United States inlets
approximately corresponding to the sum of the ebb shoal and bypassing bar volumes. To
employ these equations for computing Veq of the different units, some assumptions must be
made concerning the size relationship among them. Here, the assumption is made that the
equilibrium volume ratio between the bypassing bar and the ebb shoal, as well as between
the attachment bar and the ebb shoal, is constant. Presently, these ratios are set to 0.25 and
0.1, respectively. In the general case, transport can be directed both to the left and the
right, and there will be bypassing and attachment bars on both sides of the inlet.
If the cross-sectional area of an inlet changes, it is necessary to allow for a time-varying
Veq. During closure of an inlet, the Veq that the tidal flow can maintain may fall below the
actual volume in the ebb shoal complex, implying that sediment is released to adjacent
beaches. Mathematically, Eqs.15 and 16 can describe this situation, but from a physical
point the release might be too rapid and cause unrealistic local growth of the shoreline. To
remedy this situation, Eq. 15 was changed into a nonlinear relationship according to
Qout=Qin(V/Veq)n, where n is a power. By specifying a value of n<1 for situations where
sediment is released to the beach, the release will be slower than for the linear model. In
simulations for Long Island, n was set to 1.0 for periods when the ebb shoal complex was
growing, whereas n=0.1-0.2 when the shoal experienced reduction in volume.
MODELING EVOLUTION OF THE LONG ISLAND SOUTH SHORE
The south shore of Long Island, New York, was selected as a suitable location for
validating the capability of Cascade to simulate sediment transport and coastal evolution on
regional scale. Several studies (e.g., Kana 1995; Rosati et al. 1999) provide substantial
information for model validation. The stretch includes many coastal features and processes
characterizing evolution on the regional scale. The study area extended from Fire Island
Inlet to Montauk Point (named FIMP in the following) because most available information
originated from this coastal stretch. The area includes two inlets (besides Fire Island Inlet
at the western boundary), namely Shinnecock Inlet and Moriches Inlet. The cross-sectional
areas of the inlets have varied substantially with time, altering the size of the ebb shoal
complexes and sediment removed from the nearshore transport system, providing an
opportunity to model a complex morphologic system.
Two types of simulations were performed with Cascade for the FIMP area:
(1) determining the overall annual net longshore transport pattern along the coast (based on
Larson, Kraus, and Hanson
8
Previous Page
