GenCade Basic Assumptions
Because GenCade is a one-line model, it is constrained by the standard
assumptions upon which this type of model is based:
- The beach profile shape remains constant
- The shoreward and seaward depth limits of the profile are constant
- Sand is transported alongshore by the action of breaking waves and longshore currents
- The detailed structure of the nearshore circulation is ignored
- There is a long-term trend in shoreline evolution
The first assumption presumes that the beach profile maintains an average shape that is characteristic of the particular coast, apart from times of extreme change as produced by storms. For example, steep beaches remain steep and gently sloping beaches remain gentle in a comparative sense and in the long term. Although seasonal changes in wave climate cause the position of the shoreline to move shoreward and seaward in a cyclical manner, with corresponding change in the shape and average beach slope, the deviation from average conditions over the total active profile is relatively small.
A second assumption is that sand transport is confined between two well-defined onshore and offshore limiting elevations. The shoreward limit is located at the top of the active berm, and the seaward limit is located where no significant change in bed elevation occurs, the so called depth of closure. Restricting the profile between these two limits provides the simplest way to specify the onshore/offshore boundaries, within which shoreline change can be computed.
The model also requires predictive expressions for the total longshore sand transport rate. For open-coasts, the transport rate is a function of the breaking wave height and direction with respect to the local shoreline orientation. Since the transport rate is parameterized in terms of breaking wave quantities, the detailed structure of the nearshore current pattern does not directly enter into the model formulation. As an extension of the original formulation, GenCade may also represent sediment transport resulting from tide or wind, but only in the presence of breaking waves.
Finally, it is assumed that there is a clear long-term trend in shoreline behavior. This isolates long-term behavior from beach system “noise” produced by storms, seasonal changes in waves, tidal fluctuations, and other cyclical and random events. In essence, the assumption of a clear trend implies that the wave action producing longshore sand transport and boundary conditions are the major factors controlling long-term beach change. This assumption is usually well satisfied at engineering projects involving groins, jetties, and detached breakwaters, which introduce gradients in the transport rate and, in some cases, unambiguous boundary conditions along the neighboring open coast. Furthermore, stable beaches with no long-term trend in shoreline behavior are typically not located where engineering studies are required. However, some beaches dominated by cross-shore transport may exhibit significant variability with no clear trend and yet may experience substantial erosion problems requiring engineering studies.