View source for Barchan

{{short description|Crescent-shaped dune}}
{{for multi|the village|Barchan, Iran|the military operation|Barkhane}}
[[File:Barchan.jpg|thumb]]
[[File:Mesquite Sand Dunes.JPG|thumb|Typical shape]]
[[File:Barkhane.jpg|thumb|Barchan in the [[Namib Desert]]]]

A '''barchan''' or '''barkhan dune''' (from [[Kazakh language|Kazakh]] бархан {{IPA|kk|bɑɾˈχɑn|}}) is a [[crescent]]-shaped [[dune]].

Russian naturalist [[Alexander von Middendorf]] introduced the term in 1881,<ref>{{cite web|url=http://www.britannica.com/EBchecked/topic/53068/barchan|title=Barchan - sand dune|quote=The Russian naturalist Alexander von Middendorf is credited with introducing the word into scientific literature in 1881.}}</ref> 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 desert]]s 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&ndash;35 degrees for medium-fine dry sand.<ref name=Pye2008 /><ref>{{Cite journal|last1=Alvarez|first1=Carlos A.|last2=Franklin|first2=Erick M.|date=2017-12-18|title=Birth of a subaqueous barchan dune|journal=Physical Review E|volume=96|issue=6|article-number=062906|doi=10.1103/PhysRevE.96.062906|pmid=29347350|arxiv=1712.07162|bibcode=2017PhRvE..96f2906A|s2cid=25558699}}</ref><ref>{{Cite journal|last1=Alvarez|first1=Carlos A.|last2=Franklin|first2=Erick M.|date=2018-10-19|title=Role of Transverse Displacements in the Formation of Subaqueous Barchan Dunes|journal=Physical Review Letters|volume=121|issue=16|article-number=164503|doi=10.1103/PhysRevLett.121.164503|pmid=30387641|arxiv=1810.11074|bibcode=2018PhRvL.121p4503A|s2cid=53231618}}</ref> The upwind side is packed by the wind, and stands at about 15 degrees. Barchans may be {{convert|9|–|30|m|ft|abbr=on}} high and {{convert|370|m|ft|abbr=on}} 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<ref>{{Cite journal|last1=Hersen|first1=Pascal|last2=Douady|first2=Stéphane|date=2005|title=Collision of barchan dunes as a mechanism of size regulation|journal=Geophysical Research Letters|language=en|volume=32|issue=21|pages=L21403|doi=10.1029/2005GL024179|bibcode=2005GeoRL..3221403H|issn=0094-8276|doi-access=free}}</ref><ref>{{Cite journal|last1=Assis|first1=W. R.|last2=Franklin|first2=E. M.|date=2020-09-28|title=A Comprehensive Picture for Binary Interactions of Subaqueous Barchans|url=https://onlinelibrary.wiley.com/doi/10.1029/2020GL089464|journal=Geophysical Research Letters|language=en|volume=47|issue=18|doi=10.1029/2020GL089464|issn=0094-8276|arxiv=2009.12671|bibcode=2020GeoRL..4789464A|s2cid=221970569}}</ref> and changes in wind direction spawn new barchans from the horns of the old ones and govern the size distribution of a given field.<ref name=Elbelrhiti2005 />

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 (similar to waves passing 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.<ref name=Schwammle2003 />

Barchan dunes have been observed on [[Mars]], where the thin atmosphere produces winds strong enough to move sand and dust.<ref name=mars /> ''Marine barchan dunes'' appear underwater as sands shift their way across the seabed.<ref>{{cite journal|last1=Couldrey|first1=Amelia J.|last2=Benson|first2=Thomas|last3=Knaapen|first3=Michiel A.F.|last4=Marten|first4=Kerry V.|last5=Whitehouse|first5=Richard J.S.|title=Morphological evolution of a barchan dune migrating past an offshore wind farm foundation|journal=Earth Surface Processes and Landforms|date=30 September 2020|volume=45|issue=12|pages=2884–2896|doi=10.1002/esp.4937|bibcode=2020ESPL...45.2884C|s2cid=225400184|url=https://eprints.hrwallingford.com/1434/1/HRPP-21-013.pdf}}</ref><ref>{{cite web|last1=Snieckus|first1=Darius|title=Shifting sands: R&D gurus report in on scour of 'marching' seabed dunes|url=https://www.rechargenews.com/technology/shifting-sands-r-d-gurus-report-in-on-scour-of-marching-seabed-dunes/2-1-899674|website=Recharge|archive-url= https://web.archive.org/web/20201101031539/https://www.rechargenews.com/technology/shifting-sands-r-d-gurus-report-in-on-scour-of-marching-seabed-dunes/2-1-899674|archive-date=1 November 2020|language=en|date=23 October 2020|url-status=live}}</ref>

==See also==
*{{annotated link|Great Sand Dunes National Park and Preserve}}
*{{annotated link|Sastrugi|aka=Zastruga}}, about snowforms of similar mechanism

==References==
{{reflist|30em|refs=
*<ref name=Pye2008>{{cite book|last=Pye|first=Kenneth|title=Aeolian Sand and Sand Dunes|url=https://archive.org/details/aeoliansandsandd00pyek|url-access=limited|year=2008|publisher=Springer|isbn=978-3-540-85910-9|pages=[https://archive.org/details/aeoliansandsandd00pyek/page/n154 138]|author2=Haim Tsoar}}</ref>
*<!--ref name="m-w">[http://unabridged.merriam-webster.com/cgi-bin/unabridged?va=barkhan Merriam-Webster Unabridged - Barkhan]</ref-->
*<ref name=Schwammle2003>{{cite journal|author1=Schwämmle, V.|author2=H.J. Herrmann|name-list-style=amp|title=Solitary wave behaviour of sand dunes|journal=Nature|volume=426|issue=December 11|year=2003|pages=619–620 [https://dx.doi.org/10.1038/426619a Abstract]|doi=10.1038/426619a|pmid=14668849|s2cid=688445}}</ref>
*<ref name=Elbelrhiti2005>{{cite journal|author1=H. Elbelrhiti|author2=P. Claudin|author3=B. Andreotti|name-list-style=amp|title=Field evidence for surface-wave-induced instability of sand dunes|journal=Nature|volume=437|issue=September 29|year=2005|pages=720–723 [https://dx.doi.org/10.1038/nature04058 Abstract]|doi=10.1038/nature04058|pmid=16193049|bibcode=2005Natur.437..720E|s2cid=4411463}}</ref>
<ref name=mars>See for instance: 
*{{Cite APOD|title=Sand Dunes Thawing on Mars|date=March 3, 2008|access-date=}}
*{{Cite APOD|title=Flowing Barchan Sand Dunes on Mars|date=April 20, 2009|access-date=}}</ref>}}

==External links==
{{Commons category|Barchan}}
{{Wiktionary}}

*[https://home.nps.gov/grsa/learn/nature/dune-types.htm Dune types]—Great Sand Dunes National Park
*{{URL|https://data.mendeley.com/datasets/675gwk5jp7/1|The Bibliography of Aeolian Research}}

[[Category:Dunes]]