Barchans


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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.

B. A. FEDOROVICH

References in periodicals archive ?
In computer simulations of crashing dunes, barchans arranged themselves into patterns found in nature--evidence that collisions control dune field behavior, the team concludes August 7 in Geophysical Research Letters.
Interactions between crescent-shaped, or barchan, dunes stabilize the sand mounds and explain how vast swarms of them can persist over time, two new studies find.
Moving at around 15m a year, the front edge of the barchan appears to have made contact with some of the Mos Espa buildings earlier this year, and is encroaching on Qui-Gon's Alley.
The barchan will likely continue on its journey past the city site, which in due course will re-emerge from the sand, but it is anticipated that it will not remain unscathed.
As well as Mars, barchans have been photographed on Titan, Saturn's largest moon.
The remnants of similar fossil seif dunes, remodelled from barchans, have been described from the Al Dabb'iya Peninsula but there they are capped by Pleistocene marine sediments which accumulated when sea level was about 7-8m higher than that of today (about 125 Ka).
The dunes formed in this way are barchans, nebka, rehbub, and occasionally belts.
Most of the A-la Shan Desert is occupied by an accumulation of mobile barchans.
In the depressions between the barchans and among the rolling dunes, the sand sedge (Carex physodes) forms a dense deep-rooted green carpet.
Wells are also dug farther away from cultivated ground, even in shifting sands and barchans, where there is no water table and takyrs are absent.
The lower mobility and higher water content of the dunes in the fields of barchans mean that they can be colonized by pioneer plants that stabilize the sand even more.
The moss Tortula caninervis, for example, lives in the depressions between fixed barchan dunes in the deserts of middle Asia and Kazakhstan, and, in some places (see p.