Desert Sandflow Basins and a Model for the Development of Ergs

Abstract

The movement and deposition of sand in deserts is not a random phenomenon. Sand flows in patterns analagous to drainage sysems that are closely related to the wind systems though not to topography. Sand-flow maps of the Sahara, interpolated from meteorological and bedform data, are presented. At every point a mean potential sandflow resultant can be calculated for each specified combination of grain size, time interval and sampling area, and these may all be different. Given a particular sandflow system and regional sand deflation pattern (as determined largely by the substrate) it is possible to predict where and how sand deposits will develop. The development of ergs is radically affected by the universal occurrence of bedforms and their associated secondary flow systems. One effect of this is that ergs are rarely in equilibrium with their present mean sandflow system. Another, paradoxically, is that deposition may occur in areas where the sandflow is not saturated and where the mean winds are accelerating and diverging. Estimates of natural sand deflation rates are produced and these show how most wind-blown sand comes directly or indirectly from alluvium.