Effect of wave-topography interactions on the formation of sand ridges on the shelf

Abstract

The role of wave-topography interactions in the formation of sand ridges on microtidal inner shelves is investigated with an idealized morphodynamic model. The latter uses the two-dimensional shallow water equations to describe a storm-driven flow on an inner shelf with an erodible bottom and a transverse slope. Both bed load and suspended load sediment transport are included. New are the incorporation of a wave module based on physical principles and a critical shear-stress for erosion. A linear stability analysis is used to study the initial growth of bed forms, by analyzing the initial growth of small perturbations evolving on an alongshore uniform basic state, which describes a storm-driven flow on a microtidal inner shelf. Model simulations show that wave-topography interactions cause the ridges to become more trapped to the coast. Both growth and migration of the ridges are controlled by suspended load transport. The physical mechanism responsible for ridge growth is related to transport by the stormdriven current of sediment that is entrained due to wave orbital motions induced by bed forms. This new mechanism even acts in absence of a transverse bottom slope. The orientation, spacing and shape of the modeled ridges agree well with field observations from different shelves.