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 ?
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.
Even if they disagree, "both papers are providing convincing evidence that the generation of small dunes in a field is a very important ingredient to explain the structure of barchan corridors," says Eric Parteli of the University of Erlangen-Nuremberg in Germany.
It's unclear why barchan dunes exist at all," says Pieter Vermeesch, a geologist at University College London.
The team simulated calving dunes and also re-created real-world barchan patterns.
Scientists have observed both collisions and calving in barchan fields.
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.
The scientists, from Johns Hopkins University and Brigham Young universities in the US, and the University of Sousse in Tunisia, used satellite tracking to chart the progress of the barchan encroaching on Mos Espa, which they claim is moving at a rate of about 15 metres per year and has already reached some buildings on the town's fringe.
In time, they say the town will re-emerge, but is likely to have sustained serious damage from the barchan.