Sand Seas of the Sahara and Sahel: An Explanation of Their Thickness and Sand Dune Type by the Sand Budget Principle

Abstract

This chapter analyzes the thickness and the type of sand dunes in the sand seas of the Saharas and Sahel using the sand budget principal. The two dynamic aeolian parameters—transportation and deposition—combine to determine the sand budget of the sand seas. The interaction between sand import and sand export within a sand sea determines the thickness of the sand cover and the type of dunes formed. It is found that when the input of sand is greater than the output, the sand budget is positive and consequently the sand sea thickens. Its surface is molded into dunes of deposition coalescing barchans, barchan chains, or transverse chains. This behavior can be seen in the Sahelian sand sea of Niger where the sand cover, molded into large and small sand hills, reaches a thickness of 60 m. The sand budget is positive, even though there are locally conspicuous deflation streaks indicating the beginning of a net loss of sand. It is observed that almost sand-free seas, where the sand dunes are separated by large interdunes, occupy 15 per cent of the Sahara. These indicate a very negative sand budget and the examples are the Great Sand Sea, the Calansho Sand Sea, the Rebiana Sand Sea, the “Erg Chech” and the “Erg Iguidi.”