Among more common dunes are barchans (a Turkish word), i.e., crescent-shaped (in plan) dunes with convex side facing the wind. As the wind blows, it carries the sand up the slope by its force against gravity up to the crest. Leeward of the crest there is relative calm and sand falls along the angle of repose for sand. But the less side is an area of relative calm whence miniature convection and eddy action may develop leading to a slight concavity on the leeward side.
The windward slope is gentler. It may range from 5 to 20 degrees, the more frequent figure about 10 to 20 degree. This face is also known as back-slope. The length of the barchan is the distance from the convex windward tip to the tapering ends of the crescent measured along a straight line, which is parallel to the along-wind axis of the dune. The width is the maximum spread of the dune transverse to the length. It is generally at right angles to the length-wise axis along a line passing through the crest. The maximum length as well as the breadth is known to be 400 meters. The maximum height of barchans is said to be 30 meters.
It is thought that the maximum supply of sand is along the central axis. The supply of sand decreases away from it. This accounts for the barchan and its decreasing dimension towards the horns. But it appears that like most geomorphological features the causes and characteristics of barchans are not fully understood. The winds do not blow along fixed axis and along reserved lines. As such, the above-noted explanation of dune morphology is inadequate and some other explanations are wanted.
When the dunes are active, i.e., advancing and receiving supply of sand from the windward slope the summit and crest coincide; otherwise, the summit is rounded. The slip face consists of two parts. The inner higher part is steeper with gravity slope and slightly concave but the lower parts of the dune towards the horns have a decreasing height and neither their outer nor inner slopes have concavity. These two slopes join in a rounded surface to form a convex slope.
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The horns of the dunes are believed to be fixed by some plants or depressions where the nearness of the water table to the surface may be a fixing factor. But these explanations too are rather inadequate. The barchans may occur in colonies and one wonders if plants or depressions are so regularly distributed as to provide binding sites for the rather uniformly- spaced dunes. A more convincing explanation of the horns becoming static is that they are in the shadow of the main dune mass. Where the barchans are active three cases were postulated by the Swedish traveler of Central Asia, Sven Hedin. If the supply of sand is constant the dune advances regularly. If the supply of sand goes on increasing the advance is marked by increasing height of the barchan but slower (decelerating) forward progress. In a third, case where the supply of sand is decreasing the height goes on falling but the forward march is accelerated.
The lateral parts of the barchans advance more quickly because of their decreasing height. The barchans may occur individually or in colonies or as ‘chainlike swarms’. When in colonies they are generally interlinked and march forward as huge ripples.