Editing Dune (section)

==Sub-aqueous dunes==
{{main|Ripple marks}}
Sub-aqueous ([[underwater]]) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural [[Channel (geography)|channels]] such as rivers and estuaries, and also form in engineered [[canal]]s and pipelines.<ref>{{cite journal |doi=10.1016/j.powtec.2008.04.065 |title=Morphology and displacement of dunes in a closed-conduit flow |journal=Powder Technology |volume=190 |issue=1–2 |pages=247–251 |year=2009 |last1=Franklin |first1=E.M. |last2=Charru |first2=F. |arxiv=1608.07729 |s2cid=93576651 }}</ref><ref>{{cite journal |doi=10.1017/S0022112011000139 |title=Subaqueous barchan dunes in turbulent shear flow. Part 1. Dune motion |journal=Journal of Fluid Mechanics |volume=675 |pages=199–222 |year=2011 |last1=Franklin |first1=E. M. |last2=Charru |first2=F. |bibcode=2011JFM...675..199F |s2cid=51792324 }}</ref><ref>{{cite journal |doi=10.1016/j.expthermflusci.2015.10.017 |title=The formation and migration of sand ripples in closed conduits: Experiments with turbulent water flows |journal=Experimental Thermal and Fluid Science |volume=71 |pages=95–102 |year=2016 |last1=Cardona Florez |first1=Jorge Eduar |last2=Franklin |first2=Erick de Moraes |arxiv=1608.04792 |bibcode=2016ETFS...71...95C |s2cid=119268350 }}</ref> Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical [[bedform]] construction.<ref>Prothero, D. R. and Schwab, F., 1996, ''[[Sedimentary Geology (journal)|Sedimentary Geology]]'', pg. 45–49, {{ISBN|0-7167-2726-9}}</ref> In the case of sub-aqueous barchan dunes, sediment is lost by their extremities, known as horns.<ref>{{Cite journal|last1=Alvarez|first1=Carlos A.|last2=Franklin|first2=Erick M.|date=18 December 2017|title=Birth of a subaqueous barchan dune|journal=Physical Review E|volume=96|issue=6|pages=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=19 October 2018|title=Role of Transverse Displacements in the Formation of Subaqueous Barchan Dunes|journal=Physical Review Letters|volume=121|issue=16|pages=164503|doi=10.1103/PhysRevLett.121.164503|pmid=30387641|arxiv=1810.11074|bibcode=2018PhRvL.121p4503A|s2cid=53231618}}</ref>

These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in [[wavelength]] and height. The shape of a dune gives information about its formation environment.<ref>{{cite web |url=https://www4.uwm.edu/course/geosci697/ripple-indices/Ripple%20Indices.html |title=Ripples |access-date=11 January 2018 |url-status=live |archive-url=https://web.archive.org/web/20180501201547/https://www4.uwm.edu/course/geosci697/ripple-indices/Ripple%20Indices.html |archive-date=1 May 2018  }}</ref> For instance, rivers produce asymmetrical ripples, with the steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments.

Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river [[flooding]].