We investigate tidal sandbanks with a nonlinear morphodynamic model. The model consists of shallow water flow (including Coriolis and friction effects), bedload and suspended load transport to describe the sea bed evolution. We look into the finite amplitude behaviour of sandbanks with preferred spacing and orientation from linear theory. In case of a suspended load sediment transport formulation, this leads to equilibrium profiles, which express a balance between drag effects and slope effects on sediment transport. For a bedload transport formulation the sea bed develops into an unrealistic pattern with sharp crests that tend to touch the water surface. The stability properties of the suspended load equilibrium differ from those of a flat bed. We discuss the implications of these results for the understanding of sandbanks as well as the modelling of human intervention at the sea bed, such as large-scale offshore sand extraction.
