Vertical Sorting and Preferential Transport in Sheet Flow with Bimodal Size Distributions of Sediment

The vertical sorting of grains that occurs during the process of sheet flow largely has been ignored in parameterized models for sediment transport. Here we performed computer simulations of sheet flow transport with bimodal distributions of coarse grains under forcing from idealized nearshore waves. The results demonstrate that the mobile bed rapidly experiences vertically segregation of grains by size, where the larger grains migrate to the top of the mobile bed, while the smaller grains percolate to the bottom. Consequently, the larger grains transport at a rate disproportionately higher than their mass fraction compared to the smaller grains.