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barchan

n. An arc-shaped sand ridge comprised of well-sorted sand.

Wikipedia
Barchan

A barchan or barkhan dune, (from Russian бархан , from Kazakh бархан ), is a crescent-shaped dune. The term was introduced in 1881 by Russian naturalist Alexander von Middendorf, for crescent-shaped sand dunes in Turkestan and other inland desert regions. Barchans face the wind, appearing convex and are produced by wind action predominately from one direction. They are a very common landform in sandy deserts all over the world and are arc-shaped, markedly asymmetrical in cross section, with a gentle slope facing toward the wind sand ridge, comprising well-sorted sand. This type of dune possesses two "horns" that face downwind, with the steeper slope known as the slip face, facing away from the wind, downwind, at the angle of repose of sand, approximately 30–35 degrees for medium-fine dry sand. The upwind side is packed by the wind, and stands at about 15 degrees. Barchans may be high and wide at the base measured perpendicular to the wind.

Simple barchan dunes may appear as larger, compound barchan or megabarchan dunes, which can gradually migrate with the wind as a result of erosion on the windward side and deposition on the leeward side, at a rate of migration ranging from about a metre to a hundred metres per year. Barchans usually occur as groups of isolated dunes and may form chains that extend across a plain in the direction of the prevailing wind. Barchans and megabarchans may coalesce into ridges that extend for hundreds of kilometers. Dune collisions and changes in wind direction that spawn new barchans from the horns of the old govern the size distribution in a given field.

As barchan dunes migrate, smaller dunes outpace larger dunes, catching-up the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side. The process appears superficially similar to waves of light, sound, or water that pass directly through each other, but the detailed mechanism is very different. The dunes emulate soliton behavior, but unlike solitons, which flow through a medium leaving it undisturbed (think of waves through water), the sand particles themselves are moved. When the smaller dune catches up the larger dune, the winds begin to deposit sand on the rear dune while blowing sand off the front dune without replenishing it. Eventually, the rear dune has assumed dimensions similar to the former front dune which has now become a smaller, faster moving dune that pulls away with the wind.

Barchan dunes have been observed on Mars, where the thin atmosphere produces winds strong enough to move sand and dust.

Usage examples of "barchan".

Each team member carried weapons, but after the training on Barchan, Chan was sure that for Angel and Shikari it was a total waste of effort.

We first noticed it back on Barchan, and it has just gone on getting better and better.

On this third evening, as the dark sands of Barchan gradually cooled, the Pursuit Team settled down to its first strategy session.

No one took much notice of the Shellbacks, or indeed of the whole of Barchan, until one day it was discovered that Shellback flesh was a true delicacy.

For the purpose of his grand project he was quite willing to spend a long stint on Barchan, studying the Dreamsea flora and fauna and shoehorning every misfit species into his scheme.

Sitting near him in the cafeteria was a Pipe-Rilla astronomer, about to leave Barchan en route to the Eta Cass ring system.

He explained his reason for being on Barchan, his notions of taxonomy, and his observations of the Shellbacks.

His immune system had been boosted at the beginning of pursuit team training, making it supposedly robust enough to handle any microorganisms on Barchan or Travancore.

Any kind of dune, from very fine ripples underfoot to truly gargantuan barchan dunes.

By midmorn-ing they have reached the Chinese rover, squat as a black bug in the shadow of the slip face of one of the swarm of barchan dunes blown along the eastern side of the Chasma Boreale.

By midmorning they have reached the Chinese rover, squat as a black bug in the shadow of the slip face of one of the swarm of barchan dunes blown along the eastern side of the Chasma Boreale.

In its place was a narrow, low dune that terminated in a higher barchan of mineral-rich sand.

No one took much notice of the Shellbacks, or indeed of the whole of Barchan, until one day it was discovered that Shellback flesh was a true delicacy.

Looking down at the ice chaos Nadia suddenly remembered what the land had looked like on that first trip, the endless hills and hollows, the funnel-like alases, the great black barchan dunes, the incredible laminated terrain in the last sands before the polar cap .