The movement of desert sand

Abstract

It is well known that on a dry sand beach and, on a much larger scale, on sand-strewn desert country the wind, if above a certain strength, will cause the surface sand grains to rise and to travel down-wind as a low-flying cloud. The mechanism, however, by which ( a ) the grains composing this cloud are raised, ( b ) the rate of mass movement of the sand depends upon the wind velocity, or ( c ) the wind velocity close to the surface is affected by the presence of the sand cloud, does not appear to have been previously investigated experimentally. This mutual interaction of wind and sand grains is of interest both in connexion with the problem of the tendency of sand to heap itself up into dunes even in totally flat uniform plains, and also for the light it may throw on certain aspects of the allied problem of the transport of sediment by liquid currents. Sand found in the desert is usually composed of rounded quartz grains whose sizes range from small pebbles 2 to 3 mm. in diameter down to small particles 0∙01 mm. in diameter, which must be regarded as dust. Mechanical analysis of eolian sand for grain size show, when curves of percentage weight are plotted against grain size, that the peaks of such curves never occur on the small side of 0∙15 mm. diameter. Sand having this smallest peak size is found at the crests of dunes. Here the grains approach uniformity of size, so that the diagrams are sharp-peaked. On the other hand, sand deposits clear of the actual dunes give broad, low diagrams with the peak at a larger diameter. In every case the diagrams show only a few per cent by weight at a size of 0∙03 mm. In fact, it is a peculiarity of all sand accumulations that they are practically free from dust.