On the Origin of Sediment Ripples

As soon as a granular sediment has been set in motion by the currents of droppable fluids or by wind, sand waves form as smaller ripples or larger dunes. The relevance of this phenomenon lies in the roughness effect against the currents and the influence on sediment loads. Likewise, their physical understanding helps us to estimate past flow conditions by means of fossilized sand waves, as well as those on distant planets with proven ripples and dunes, such as Mars and Titan. In the literature, diagrams exist based on observations for the conditions under which the various forms of sand waves develop. However, the cause of their formation is unclear. Various theories have been discussed regarding the further development of ripples once they have formed, but none of them explains the fundamental mechanism that generates the very first ripples. These occur simultaneously over a large area and almost instantly, with a fairly even distance from crest to crest. This contribution presents a solution for how this is possible based on hydrodynamic instability.