MODEL SIMULATIONS OF BAR EVOLUTION IN A LARGE SCALE LABORATORY BEACH

Abstract

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 hydrodynamics to provide successful predictions of profile erosion and offshore bar migration. Such energetic type sediment transport models (based on Bagnold, Bowen and Bailard equations) were driven with near bottom velocities to predict the offshore migration event observed during the DELILAH 90 experiment (Thornton and Birkemeier, 1996). Further study supports and improves offshore migration prediction based on DUCK 94 experiment measured near bottom velocities (Gallagher et al., 1998). However, wave-averaged transport predictions based on Bagnold or other steady state models (in which fluid acceleration or pressure gradient do not play a significant