Old and new concepts based on coherent structures for understanding sediment transport

Coherent structures develop due to an instability process on smooth beds under turbulent flow conditions and they are relevant for the incipient motion of fine sand sediment. Similar flow structures were found to be relevant for the sediment transport under rough wall conditions, but an instability process could not explain them. In this article we show that rough walls produce a new class of Λ-vortices, these vortices develop from separations on the grains, and are new relevant flow structures.