A 3-DIMENSIONAL NUMERICAL-MODEL OF SURFACE-WAVES IN THE SURF ZONE ANDLONGSHORE-CURRENT GENERATION OVER A PLANE BEACH

A three-dimensional numerical model is developed for the propagation o f shallow-water short-period surface waves in the surf zone and longsh ore current generation over a plane beach topography. This model, whic h is based on Reynolds-averaged non-linear shallow-water (NSW) equatio ns and, hence, includes implicitly the classical radiation stress conc ept, resolves time- and space-dependence of the sea surface elevation and the velocity fields during one wave cycle (short-wave-resolving). The generation of turbulence by wave breaking and vertical fluid shear above the beach is parameterized by the application of a generalized turbulence energy closure scheme. The instantaneous position of the mo ving shoreline is determined from the model equations during the simul ated propagation process. In the case of a single incoming wave train, the wave amplitude, wave period and angle of incidence are prescribed at an offshore open boundary by application of a forced radiation con dition. For uniform alongshore topographic conditions, when cyclic bou ndary conditions are appropriate at alongshore open boundaries whose p ositions are determined by the alongshore component of wavelength in a n incoming single wave train, the model is used to determine the (mean ) longshore current during one wave cycle. It is shown that the maximu m longshore depth-averaged current occurs at an approximate offshore p osition where the generation of turbulence energy through wave breakin g is a maximum. It is further shown that the cross-shore gradient of t he longshore momentum flux is of predominant importance in generating longshore currents. Experiments are described that determine the depen dence of the computed longshore current on the bottom roughness and th e length scale prescription in that part of the turbulence closure sch eme pertaining to the parameterization of the wave breaking process. T he implications of the model results are discussed in the context of t he longshore bedload transport of sedimentary material. Finally, a com parison is made between the model predictions and observational data o n longshore currents and wave heights. (C) 1998 Academic Press.