The curious case of mobility reversal in sediment mixtures

It is common for aggrading deposits of sediment mixtures to show a pattern of downstream fining, whereby characteristic grain size becomes finer in the streamwise direction. Although various factors may contribute to this phenomenon, it can often be explained largely or solely in terms of the variation of grain mobility with grain size in sediment mixtures. That is, finer grains exposed on the surface are typically somewhat more mobile than their coarser neighbors, resulting in preferential transport of finer grains. Experiments reported here, document a reversal in this familiar pattern at slopes in excess of 0.02-0.03. At these higher slopes aggrading and prograding deposits are observed to show downstream coarsening in geometric mean size, indicating a mobility reversal associated with preferential transport of the coarser grains. This rather curious behavior is explored herein.