Farouk El-Baz

Document Type

Article

Publication Date

11-28-1980

Abstract

Particulate naterlal in the desert originates froa the disintegration of rock by both machanical and chomical weathering. Thls disintegration exposes the particles to the agenta of erosion, which in turn cause more loosening of particles from the rock. Nind ia the major agent of erosion and deposition in the desert. The wind hurls the finest particles into the atnosphere as duet. The largest particles remain behind and armor tha surface, protecting it from further erosion by the wind. Medium-s1zed particles are winaoved away and deposited downwind as sand shaats and sand dunes This paper summarizes dune shapes and accumulations, their rates of movement and methods of stabilization. Sand accunulations occur in a large number of forms, which makes them hard to classify. However, generally speaking, sand-sized grains accumu- late in the form of sand shects with rippled, undulated surfaces or as free dunes and obstacle-related dunes. Free dunes, which are by far the most abundant, can be isolated simple forms, coalesced conpound groups, or complex accumulatlons. The basic geometric shapes of dunes are linear (longitudinal and seif dunes), crescentic (barchan and parabolic dunes), and domical (including dome, pyramid and star dunes). All types of dunes may be active, sent-fixed or fixed by vegetation. Dune shapes vary considerably in different areas and often in the same locality; little is known about the causes of such variacion. A few dunes may form a bundle, and a larger nunber is commoaly referred to as a dune field, which is here irbitrarily limited to less then 2,000 km². Larger accumulations of sand dunes are known as sand seas because of the repetitive naturc of dunc forma, much likc waves on the sca surface. Detailed knowledge of the texture and morphology of these sand seas was vade possible through the interpretation of photographs and inages taken fron space. For example, data from Neteosat allos the recognition of general sand movement patterns in all of the Sahara. In addition, Landsat images and photographs taken on manned space nissions allot the mapping of dune accunulations such as the Great Sand Sea in thc Western Desert of Egypt or the Takla Makan Desert in nar thwestern China. The motion of dunes is fucled by the wind and depends on ita direction and velocity The rate of advance alsa depends on the siec of the dune. For example, crescentic dunes in the Kharga depression in the central part the Western Desert of EgyPt move at the varying rates of 30 to 100 meters per year; che smallest dunes are the fastesc coving the motion of dunes say heve devastating ef fects on humen sectlenents the desert. Because IOE this, aumerous at tempte have been madc fn various deserts halt the advance of sand dunes. Stabilizetion methods "include the utilization of vegetation by seeding the dunes ard by planting trees on the wind ox lee sides. nother method, which has been employed, in Esypt, India and Chins, uses hay fences to limit the saltstion af sand grains. st11l other methods include the spraying of the sand surface, with crude oil, as has been succesgfully crted fnl Iran and Saudi Arabia, and with chemicals that bind the sand "" grains on che uppermost surfacea of the dunes.

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