A barchan or barkhan dune is an arc-shaped sand ridge, comprising well-sorted sand. This type of dune possesses two "horns" that face downwind, with the slip face (the downwind slope) at the angle of repose of sand, or approximately 35 degrees (Strahler & Archibold, 2008, pg.442). The upwind side is packed by the wind, and stands at about 15 degrees. Simple barchan dunes may stretch from meters to a hundred meters or so between the tips of the horns. The word is of Turkic origin, borrowed into English via Russian.
Simple barchan dunes may appear as larger, compound barchan or megabarchan dunes, which may migrate with the wind. Barchans and megabarchans may coalesce into ridges that extend for hundreds of kilometers.
As barchan dunes migrate, smaller dunes outpace larger dunes, bumping into the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side. The process seems to be similar to waves of light, sound or water that pass directly through each other; the detailed mechanism is, however, very different, being nonlinear. These are known as solitons.
The dunes emulate soliton behavior but unlike solitons, the sand particles do not pass through each other. When the smaller dune rear-ends 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. (Schwämmle & Herrmann, 2003)
Fine examples of barchan dunes are found near La Joya, Arequipa, Peru ( ); a number of dunes are readily visible from the Pan American Highway at the intersection with the Carretera Interoceanica just north of La Joya, where they can be seen passing over cement block buildings. In mid-2010, several smaller barchan dunes are approaching the Carretera from the south. These gray-colored dunes are formed from fine-grained volcanic ash from the Huaynaputina eruption in 1600.
Barchan dunes are also found in the south of the Special Region of Yogyakarta, Indonesia, the largest being at Parangkusumo Beach. These dunes have been built up over thousands of years from the volcanic sands of Mount Merapi.They are also found in the Great Indian Desert in Rajasthan in India.
See also 
- Zastruga, about snowforms of similar mechanism
- Merriam-Webster Unabridged - Barkhan
- See for instance:
- APOD (March 3, 2008). Astronomy Picture of the Day: Sand Dunes Thawing on Mars. NASA
- APOD (April 20, 2009). Astronomy Picture of the Day: Flowing Barchan Sand Dunes on Mars. NASA
- Schwämmle, V., and H.J. Herrmann (2003). "Solitary wave behaviour of sand dunes". Nature 426 (Dec. 11): 619–620 Abstract. doi:10.1038/426619a. PMID 14668849.
- H. Elbelrhiti, P. Claudin, and B. Andreotti (2005). "Field evidence for surface-wave-induced instability of sand dunes". Nature 437 (Sep. 29): 720–723 Abstract. doi:10.1038/nature04058. PMID 16193049.
- Mainguet M. & Dumay F. (2011 Fulltext (PDF)). "Fighting wind erosion. one aspect of the combat against desertification". Les dossiers thématiques du CSFD 3 (Jan. 2011).
Strahler, A. & Archibold, O.W. Physical Geography: Science and Systems of the Human Environment. 4th ed. 2008. John Wiley & Sons, Inc. Hoboken, NJ. pg. 442.
Media related to Barchan at Wikimedia Commons