Size of suspended sediment over dunes

Abstract

Samples of suspended sediment were collected at four elevations simultaneously over two-dimensional mobile dunes in two mixtures of 0.5 mm sand in a laboratory flume channel. A constant sampling position relative to the dunes was maintained by adjusting the translation rate of the sampling carriage to be the same as the migration rate of the dunes. The rate of change of concentration of the suspended sediment with distance from the bed for individual grain sizes was found to be related to the part of the dune over which the samples were collected. The suspended sediment concentration over the lowest third of the flow depth was nearly constant over the trough of the bed forms, while concentration varied rapidly with depth over the brink point, with intermediate rates of change, similar to measurements over a flat bed, over the rest of the dune. The concentration of coarser grains was reasonably explained by assuming they were in intermittent suspension from the brink point of one dune to that of the next. Differences between the vertical changes in grain size were reduced when the sizes were plotted relative to the mean crest elevation of the dunes rather than the local bed elevation. The Rouse equation was found to yield accurate predictions of median grain sizes above the crest level using a reference sample, while median grain size was found to be nearly constant below the level of the crest.