Evidence for a new sand transport process from experiments on Saharan dunes

Abstract

Sediment transport functions relate the rate of movement of granular solids per unit width of bed to some characteristics of the shearing fluid^1–3. Many have been applied to sand transport over aeolian dunes^4–6; these were developed for horizontal beds, however, whereas in natural environments there are often appreciable slopes on transport paths⁷. Theoretical analyses have usually been adopted^8–10 to account for the effect of bedslope on the threshold of sediment transport^11,12 and on the transport rate¹³. We report results from 462 experiments in the north-west Saharan sand seas in which we measured the sand transport rate for a range of wind speeds on a variety of up- and down-sloping dune surfaces. The results confirm the theoretical analyses for threshold criteria, but reveal transport rates related to the seventh power of the bedslope, considerably greater than those predicted by existing theory. This suggests that when grains land on the surface, they induce surface gravity flow in a process which has not previously been fully recognized.