A TWO-SCALE APPROACH TO NEARSHORE SEDIMENT TRANSPORT MODELING

Beach morphology models are all dependent on empirical devices, to a degree, either through a direct prediction of bottom position changes or by modeling sediment transport and concentration. The dependence on site-specific parameters for calibration limits the predictive capability of these models. The majority of morphology models rely on a solution of the time-averaged balances in the nearshore environment where important wave-related processes such as skewness and breaking generated turbulence are neglected or simplified. Alternative models focus on phase-dependent transport, which may correctly predict intra-wave processes but poorly represent the slow sediment advection due to undertow and longshore current. This effort focuses on mitigating these deficiencies with a one-dimensional nearshore morphology modeling system. The Boussinesq and phase-averaged circulation equations with transport are solved in sequence, and predictions of the wave-related closures provide a rational estimate of momentum and sediment flux in the circulation model. Laboratory measurements indicate the importance of the breaking process in sediment entrainment, and a simple turbulence balance is included in the phase-dependent computations. The predicted sediment concentration levels are shown to be influenced by surface breaking. Early results for application to a laboratory beach in equilibrium show a onshore flux associated with the breaking waves and an offshore transport due to the currents. The balance in transport is in reasonable agreement except in the region of the lower swash.