MODEL SIMULATIONS OF BAR EVOLUTION IN A LARGE SCALE LABORATORY BEACH.pdf

MODEL SIMULATIONS OF BAR EVOLUTION IN A LARGE SCALE LABORATORY BEACH P. Teran Cobo 1 , J. T. Kirby 1 , M. C. Haller 2 , H. T. Ozkan-Haller 3 , J. Magallen 2 , G. Guannel 3 The current paper reports a test of a coupled Boussinesq/bottom boundary layer model for cross-shore transport, applied under accretional wave conditions. Measurements of free surface elevation, fluid velocities and beach profile evolution were obtained during the CROSSTEX experiment carried out in summer 2005. The model calculations are conducted in predictive mode in order to assess model skill in performing short term assessments of beach profile evolution for single storm events. Fifty twenty-minute long simulations are carried out consecutively, spanning the duration of an accretional event. Strong discrepancies such bar height overprediction arise due to various factors, of which the most significant is the wave reflection on the 1/20 beach slope. INTRODUCTION Cross-shore sediment transport processes on simple longshore uniform beaches result from a complex superposition of two and three-dimensional hydrodynamic patterns. Storms produce energetic sea states with waves that break and create turbulence that suspends large amounts of sediment. Under highly energetic storm conditions, breaking waves force near bottom steady flows, also called undertow (Dally and Dean, 1984; Sallenger and Howd, 1989). The equilibrium beach profile will change due to gradients in offshore transport induced by cross-shore undertow, rapidly creating an offshore sandbar. In the presence of a sand bar, wave breaking is enhanced near the bar crest, and reduced near the bar trough (Lippman et al., 1996). Also, bar-intensified undertow has been observed (Sallenger and Howd, 1989; Haines and Sallenger, 1994), with a maximum just shoreward the bar crest (Gallagher et al., 1998). The dominant role of the undertow during erosional events in energetic surf zones allows models based on quasi-steady h