SUSPENDED SEDIMENT TRANSPORT IN THE SURF ZONE - RESPONSE TO INCIDENT WAVE AND LONGSHORE-CURRENT INTERACTION

This paper reports the results of a field experiment carried out in the inner surf zone of a dissipative beach at San Marine, Oregon, in 1984. Observations were made at a location 70 m seaward of the mean high water line (mean depth 1.3 m) during a period when significant offshore wave heights were 3-5 m and the surf zone was 500 m wide. Instruments were deployed in a closely spaced array and measurements included cross-shore and longshore velocity, sea surface fluctuations, and sediment concentration profiles. The results reported in this paper are from a particular 44-min data run characterized by high waves and strong longshore currents. In the first section of this paper the field measurements are summarized to provide insight into the resuspension process and observed sediment concentrations and fluxes on time scales including incident and infragravity bands, and mean values for the entire data set. In the second section a numerical model is presented that incorporates the nonlinear interaction of waves and currents and calculates velocity profiles, boundary shear stress, suspended sediment profiles, and longshore and cross-shore particle flux over 0.5 s intervals throughout the time series. Finally, model calculations of mean suspended load and net longshore and cross-shore flux of sediment are compared to observations. The results show that the model calculates mean suspended load and net longshore and cross-shore flux of sediment within a factor of two of the observations. The model suggests that, for this data set, incident waves and longshore currents contribute equally to the suspended load. For the longshore particle flux, the current contribution is approximately 1.7 times the wave contribution (63% vs 37%), primarily because currents carry suspended sediment higher in the water column where it is advected by stronger longshore currents. Additionally, model results suggest that wave/current interactions enhance suspended sediment load and longshore sand transport by approximately 50-60% over the transport forced by waves alone.