Overwash

Overwash is the flow of water and sediment over a coastal dune or beach crest during storm events (or other situations with high water). 'Overwash' may refer only to the landward flux of water due to overtopping of a dune system while 'washover' may refer to the sediment deposited by overwash.{{Cite journal|last=Donnelly|first=C..|year=2006|title=State of knowledge on measurement and modeling of coastal overwash|journal=Journal of Coastal Research|volume=22|pages=965–991|doi=10.2112/04-0431.1|author2=Kraus, N.|author3=Larson, M.|number=4|s2cid=131321011}} A common process on barrier islands, Overwash redistributes sediment and facilitates the migration of barrier islands in response to sea level rise.{{Cite journal|last1=Lorenzo-Trueba|first1=Jorge|last2=Ashton|first2=Andrew D.|date=2014-04-01|title=Rollover, drowning, and discontinuous retreat: Distinct modes of barrier response to sea-level rise arising from a simple morphodynamic model|journal=Journal of Geophysical Research: Earth Surface|language=en|volume=119|issue=4|pages=2013JF002941|doi=10.1002/2013JF002941|bibcode=2014JGRF..119..779L|issn=2169-9011|hdl=1912/6714|s2cid=54010731 |hdl-access=free}} Overwash can occur as a result of runup (the maximum height of the swash), or inundation.{{Cite journal|last=Asbury H. Sallenger|first=Jr.|date=2000-01-01|title=Storm Impact Scale for Barrier Islands|jstor=4300099|journal=Journal of Coastal Research|volume=16|issue=3|pages=890–895}}

Deposition from overwash (washover) can be deposited onto the beach berm, the dune, or as far as the back barrier bay, marsh, estuary, or lagoon. Sediment deposits created during overwash exhibit consistent scaling — for example, the distance inland that an overwash deposit extends is proportional to the area that the overwash deposit covers.{{Cite journal|last=Lazarus|first=Eli D.|date=2016-12-16|title=Scaling laws for coastal overwash morphology|journal=Geophysical Research Letters|language=en|volume=43|issue=23|pages=12,113–12,119|doi=10.1002/2016GL071213|bibcode=2016GeoRL..4312113L|issn=1944-8007|doi-access=free}} The distance inland that an overwash deposit extends is also correlated with the volume of sand deposited. The amount of overwash and washover sedimentation also depends on the characteristics of the storm and the setting — coastal development tends to reduce the amount of sand deposited on a barrier island during overwash, with consequences for barrier island evolution.{{Cite journal| last = Rogers | first = L.J..| author2=Moore L. J. | author3=Goldstein E. B. | author4=Hein C. J.| author5=Lorenzo-Trueba J. | author6=Ashton A. D.| year = 2015| title = Anthropogenic controls on overwash deposition: Evidence and consequences| journal = Journal of Geophysical Research: Earth Surface| volume = 120 | issue = 12 | pages = 2609–2624| doi = 10.1002/2015JF003634 | bibcode = 2015JGRF..120.2609R| doi-access = free| hdl = 1912/7946| hdl-access = free}}

The stratigraphic layers of sediment in an overwash deposit can be used to estimate the duration of the storm that caused the deposition.{{Cite journal|last1=Shaw|first1=J.|last2=You|first2=Y.|last3=Mohrig|first3=D.|last4=Kocurek|first4=G.|title=Tracking hurricane-generated storm surge with washover fan stratigraphy|journal=Geology|volume=43|issue=2|pages=127–130|doi=10.1130/g36460.1|year=2015|bibcode=2015Geo....43..127S}} In paleotempestology, overwash deposits are frequently used to reconstruct prehistoric tropical cyclones.{{Cite journal|last1=Bregy|first1=J. C.|last2=Wallace|first2=D. J.|last3=Totten|first3=R. L.|last4=Cruz|first4=V. J.|title=2500-year paleotempestological record of intense storms for the northern Gulf of Mexico, United States|journal=Marine Geology|volume=396|issue=SI: Geological Records of Extreme Wave Events|pages=26–42|doi=10.1016/j.margeo.2017.09.009|year=2018|bibcode=2018MGeol.396...26B|doi-access=free}} Sediment deposited via overwash can also impact the rate of growth of salt marsh plants — a small amount of sand deposited can increase salt marsh plant growth, but too much deposited sand will kill vegetation.{{Cite journal|last1=Walters|first1=David C.|last2=Kirwan|first2=Matthew L.|date=2016-05-01|title=Optimal hurricane overwash thickness for maximizing marsh resilience to sea level rise|journal=Ecology and Evolution|language=en|volume=6|issue=9|pages=2948–2956|doi=10.1002/ece3.2024|pmid=27069590|pmc=4808077}}

During large storms, overwash deposits can self-organize into a periodic, rhythmic pattern, where overwash deposits occur at a regular spacing along the shoreline.{{Cite journal|last1=Lazarus|first1=E. D.|last2=Armstrong|first2=S.|title=Self-organized pattern formation in coastal barrier washover deposits|journal=Geology|volume=43|issue=4|pages=363–366|doi=10.1130/g36329.1|year=2015|bibcode=2015Geo....43..363L|url=https://eprints.soton.ac.uk/400714/1/G36329-Lazarus-accepted-ms-wDR.pdf}}