thumb|Diagram of a barchan

thumb|Typical shape

thumb|Barchan in the [[Namib Desert]]

A barchan or barkhan dune (from Kazakh бархан ) is a crescent-shaped dune.

Russian naturalist Alexander von Middendorf introduced the term in 1881, working from the occurrence of barchans in Turkestan and in other inland desert regions. Barchans "face" the wind, appearing convex, and are produced by wind action predominantly 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 the sand in question, 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 one meter to 100 meters 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 mega-barchans may coalesce into ridges that extend for hundreds of kilometers. Dune collisions and changes in wind direction spawn new barchans from the horns of the old ones and govern the size distribution of a given field.

See also

  • , about snowforms of similar mechanism

References

  • Dune types—Great Sand Dunes National Park