ABSTRACT

A three-dimensional, numerical prediction system for storm sedimentation has been constructed to compute a cyclonic wind field, coastal circulation, storm waves generated over the continental shelf, the combined effects of steady currents and waves on the benthic boundary layer, both suspended and bed load transport of sediment, and conservation of the sea floor. It has been used to hindcast the oceanographic and sedimentological responses of the western Gulf of Mexico to four historical tropical cyclones in order to investigate the effects of coastal configuration and storm variability on event bed genesis.

The simulations reveal several common responses to these storms: 1) onshore flow to the fight of the storm track generally transports fine sediment landward; 2) off-shore flow to the left transports coarser sediment seaward; and 3) for an observer facing the coast, right to left along-shelf flow transports finer sediment in deep water and coarser sediment in shallow water. The main source of sandy sediment on the inner shelf is the shoreface which is eroded by currents driven by wind stress, geostrophy, and offshore flows associated with collapse of the coastal setup. Coastal geometry is the dominant factor in determining sedimentation patterns. Along the coast in front of each storm, the volume of sediment transported obliquely onshore or offshore is a function of shelf gradient and oastal configuration. Steeper gradients constrain flow to a more longshore pattern. Concave coastlines promote greater shoreface erosion because of increased coastal setup.

None of the simulations reveals the vertical current structure predicted by the geostrophic model for continental shelf circulation, because the calculated depth of the wind-mixed layer exceeded local water depth everywhere over the shelf. Thus, in these experiments, most of the continental shelf fell within the friction-dominated zone in which the upper and lower boundary layers overlap, resulting in vertically uniform along-shelf flow.