Effect of Grain Sorting on the Formation of Bedforms

Grain sorting is a process observed in fluvial, coastal, and estuarian environments whereby the selective transport of different fractions of a sediment mixture gives rise to a non-uniform spatial distribution of the grain size probability density function. The formation of some fluvial bedforms (bedloadsheets and sand ridges) is shown to be generated or dominantly controlled by grain sorting. However, most bedforms (river dunes, free and forced fluvial bars, coastal ripples) are moderately affected by the sorting mechanism which is invariably found to lead to a damping effect on bedform growth. Recent investigations of the above phenomena are reviewed and new methodologic aspects arising in treating the instability of flow and bed topography in the presence of mixtures are pointed out.