Modeling sand bank formation around tidal headlands

Abstract

Sandbanks are often found in the vicinity of coastal headlands around which tidal flows are strong enough to generate significant tidally-forced residual eddies, typically with scales of 2-10 km. One popular hypothesis is that these sandbanks are generated by a 'tidal stirring' mechanism in which the inward-directed pressure gradient associated with these residual eddies produces an inward-directed movement of sand near the seabed. This hypothesis predicts asymmetric sandbank formation when planetary vorticity is significant compared to the relative vorticity of the residual eddies. This mechanism is tested with a numerical sediment transport model, using idealized symmetrical coastline geometry and tidal forcing that represents conditions similar to regions where these tidal headland sandbanks are known to occur. For both suspended and bedload simulations, we find that nearly symmetric sandbanks form, and that the sediment transport patterns that are responsible for building and maintaining the banks are due more the patterns of shear stress and sediment flux that occur over the course of the tidal cycle rather than to the characteristics of the tidally-averaged residual fields. We also find that sediment supply can be an important factor in controlling the nature of the resulting sandbanks.