SEDIMENT FLUX AND AIR-FLOW ON THE STOSS SLOPE OF A BARCHAN DUNE

Measurements of sediment flux on the windward slope of an isolated bar chan using an array of 30 sand traps provide new data that can constra in models of dune dynamics. The data show that at low incident wind sp eeds, flux increases up the dune exponentially, whereas at higher wind speeds the increase with distance approaches linearity. Wind profile measurements, conducted at the same time as the flux measurements, ind icate that, although wind speed at a given height increases by 1.2 tim es from dune toe to brinkline, wind shear velocity derived from the pr ofile data decreases up the dune and is in many cases below transport threshold values. This demonstrates that conventional wind profiles, d erived from anemometry on dunes, do not measure the part of the bounda ry layer that is significant for sediment transport.