Barchans

(Turkic), continental desert dunes; hills of friable sand, blown by the wind and not secured by vegetation. Single and grouped barchans blown together on hard ground (with an insufficient quantity of sand) are generally low (from 0.5 to several m) but can with time achieve heights of more than 100 m. They have characteristic half-moon or sickle-shaped outlines in a plane with a long, gently sloping (5°-14°) windward slope and a short, steep (30°-33°) leeward slope blending into a “horn” that extends according to the wind direction. In areas of total sand cover, simple barchan contours of small and medium sizes (up to 10–20 m high)—as well as compound and complex ones—are formed. Where these forms combine with large forms, the relative heights reach 200–300 m and more. Depending on the wind conditions the accumulation of barchans takes various forms: barchan ridges parallel to the prevailing winds or extended by their resultant force, barchan chains perpendicular to opposing winds, barchan pyramids in places of the convection of swirling currents, and so on. Barchans unsecured by vegetation can be moved by the wind with a speed of tens of centimeters to hundreds of meters a year. Barchan massifs with sparse vegetation often contain large supplies of fresh water. Some types of barchan forms are shown in Table 1.