To address these issues, a new class of numerical model of longshore transport and
coastal change, called Cascade, was developed to represent regional processes extending
hundreds of kilometers and covering several inlets. Time intervals of interest span decades
to centuries. At regional time and space scales, the model includes such phenomena as
inlet creation, ebb- and flood-tidal shoal development, bypassing bars between beaches and
inlets, channel dredging, regional trends in the shape of the coast, relative change in sea
level, wind-blown sand, storms, periodic beach nourishment, and shore-protection
structures such as groins and seawalls. The name Cascade derives from recognition that
processes at different spatial and temporal scales act simultaneously in what can be viewed
as cascading of scales from regional to local. For example, offshore contours of a coastal
region might have a curved trend, upon which local projects are emplaced (and interact)
that may individually appear to have straight trends in shoreline position. The embedding
of local processes requires cascading of information from wide-area to local, from long-
term to immediate, and from project to project site.
In the following, the main components of Cascade are described, including algorithms
for computing breaking wave properties, longshore sediment transport, sediment bypassing
of jetties, and inlet sediment storage and transfer. Validation of Cascade encompassed
general model testing and evaluation for a number of hypothetical cases (not discussed
here), as well as a detailed simulation for the south shore of Long Island, New York. For
the field validation, comparisons between model simulations and measurements were made
for the net annual longshore transport rate and shoreline response after opening of two
inlets (Shinnecock Inlet and Moriches Inlet).
Cascade simulates longshore sediment transport and coastal evolution at the regional
and local scale. Fig. 1 provides an overview of a typical coastal setting to which Cascade
may be applied. The figure shows three barrier islands separated by inlets with and without
jetties, where the sediment is transferred around an inlet through the inlet-shoal complex.
A number of sources (sinks) are shown including cliff erosion and wind-blown sand. The
shoreline of the barrier island chain displays a curved trend at the regional scale with local
variations in between the inlets.
Initial model development focused on developing fast, reliable, and robust algorithms
for calculating waves and sediment transport. Also, an important part was formulation of
realistic boundary conditions, which in some cases constitute complex submodels (e.g.,
jetty bypassing, inlet sediment transfer). Below, a short description is given of the
following model components: (1) governing equations and boundary conditions,
(2) breaking wave properties, (3) longshore sediment transport, (4) bypassing transport, and
(5) inlet sediment storage and transfer. Some of these components required new theory to
be developed, whereas other components were based on previous work. The summary here
mainly covers new developments needed for Cascade.
Larson, Kraus, and Hanson