Sand ripples and dunes

Abstract

Sand or granular patterns may emerge from an erodible bed sheared by a fluid flow in a wide variety of physical environments: in water channels, rivers and coastal areas, in deserts on Earth and under methane or CO2 atmospheres on other planets, in hydraulic engineering and industrial pipe flows. The size of these patterns ranges from the centimeter scale for subaqueous ripples to one hectometer for large river megadunes, from one decameter for the smallest aeolian dunes to one kilometer for the largest ones. Mature, finite-height bedforms are typically asymmetric, with an avalanche slip face on their lee side. More generally, bedforms exhibit different shapes depending on the symmetries of the fluid forcing, or the boundary conditions. The dynamics of these patterns results from the interaction between the fluid flow and the bed topography through particle transport, which takes place in a thin bedload layer in liquids, or a larger saltation layer in gases, or as a suspension. The wide occurence of sand patterns has stimulated a huge number of studies aiming at their understanding. Since the pioneering book of Bagnold (1941), significant progresses have been achieved in the fields of ripples and dunes, coastal forms, and fluvial sedimentary patterns. Some important issues, still debated, are the following. What are the relevant dynamical mechanisms controlling the emergence of bedforms? Do they form by linear instability or non-linear processes like pattern-coarsening? What determines their time and length scales, so different in air and water? What are the similarities and differences between aeolian and subaqueous patterns? What is the influence of the mode of transport: bedload, saltation or suspension? Can bedforms emerge under any hydrodynamical regime, laminar and turbulent? The aim of the presentation is to discuss the above issues and propose, from the recent literature, a unified description of bedform growth and evolution. The following points will be successively addressed: the hydrodynamics above an undulated fixed bottom, the dynamics of non-equilibrium sediment transport, the stability of a flat erodible bed, and finite size effects.