Sorting in grain flows at the lee side of dunes

Sediment sorting at the lee side of ripples, dunes and bars has already been recognized long ago. A predictive model of the sorting is necessary but unavailable for implementation in sediment transport models for sediment mixtures. Relevant processes in sedimentological and physical literature are reviewed and compared to the sparse data of sediment sorting. A synthesis is given of the most important variables governing the sorting processes for the benefit of future experimentation and modelling. These variables are the sorting (standard deviation, skewness and bimodality) of the sediment mixture delivered to the brink point, the height of the dune or bar relative to the average grain size of the mixture, the velocity of the flow above the brink point relative to the settling velocity for all grain size fractions, and the frequency of the grain flows. In addition, the initiation mechanism and frequency of the grain flows affect the pattern and effectiveness of sorting.