River Dunes and Tidal Sand Waves: Are They Generated by the Same Physical Mechanism?

Many subaerial and subaqueous morphological patterns are repetitive both in space and time, and a characteristic wavelength L and a characteristic period T can be associated with the bottom forms, along with a migration speed cr=L/T. In this contribution we focus our attention on river dunes, which form under steady flows, and on tidal sand waves, which form under oscillatory flows. Now, it is widely accepted that these bedforms arise because of the instability of the plane interface between the riverbed/seabed, made up of cohesionless sand, and the water flowing over it. However, the physical mechanisms commonly suggested to explain the growth of these two bottom forms differ. In the present contribution, the theories of formation of river dunes and sand waves, based on modal stability analysis, are revisited, and it is shown that the different physical mechanisms proposed to explain the formation of river dunes and sand waves exhibit more similarity than was believed until today.