The formation of tidal sand waves: steady versus unsteady approaches

To investigate the formation of the sand waves generated by tide propagation, the stability of the flat sea bottom forced by tidal currents is often analyzed. Both quasi-steady and unsteady approaches are used. The quasi-steady approaches consider a steady current characterized by a slowly varying strength such that the time t is considered as a parameter in the equations describing the phenomenon. The unsteady approaches take into account the unsteadiness of the forcing flow assuming that the terms related to partial/partial t provide significant contributions to the momentum and sediment balances. In the present paper, the accuracy of the two classes of approach is discussed along with the reliability of their predictions. The use of an unsteady approach appears to be necessary when the suspended sediment transport provides a significant contribution to the total load. A simple diagram is presented to evaluate the need of an unsteady approach.