Friction of wave-induced sheet flow

Abstract For many years it has been assumed that mobile beds obey a rough-boundary friction law. Recently it was shown analytically, and verified by comparison with experimental results, that this assumption is not correct for sheet flow. For this type of flow, where the bed-load particles travel in a layer much thicker than the grain diameter, the equivalent roughness increases in direct proportion to the shear stress, and may become very large. The analysis is now extended from unidirectional to oscillatory motion, evaluating sheet-flow friction and defining the region where this type of flow occurs.