Editing Levee (section)

== Uses ==
[[File:Versterkt talud. Locatie. Haven van Laaxum 002.JPG|thumb|A reinforced embankment]]

The main purpose of artificial levees is to prevent flooding of the adjoining [[countryside]] and to slow natural course changes in a waterway to provide reliable shipping lanes for maritime commerce over time; they also confine the flow of the river, resulting in higher and faster water flow. Levees can be mainly found along the sea, where dunes are not strong enough, along rivers for protection against high floods, along lakes or along [[polder]]s. Furthermore, levees have been built for the purpose of impoldering, or as a boundary for an inundation area. The latter can be a controlled inundation by the military or a measure to prevent inundation of a larger area surrounded by levees. Levees have also been built as field boundaries and as military [[fortification|defences]]. More on this type of levee can be found in the article on [[dry-stone wall]]s.

Levees can be permanent [[earthworks (engineering)|earthworks]] or emergency constructions (often of [[sandbag]]s) built hastily in a flood emergency.

Some of the earliest levees were constructed by the [[Indus Valley civilization]] (in [[Pakistan]] and [[North India]] from {{circa|2600 BCE}}) on which the agrarian life of the Harappan peoples depended.<ref>{{cite web | title = Indus River Valley Civilizations | website = History-world.org | access-date = 2008-09-12 | url = http://history-world.org/indus_valley.htm | archive-url = https://web.archive.org/web/20120610205538/http://history-world.org/indus_valley.htm | archive-date = 10 June 2012 | url-status = usurped }}</ref> Levees were also constructed over 3,000 years ago in [[ancient Egypt]], where a system of levees was built along the left bank of the [[River Nile]] for more than {{convert|600|mi|km|-2|sp=us|abbr=off|order=flip}}, stretching from modern [[Aswan]] to the [[Nile Delta]] on the shores of the [[Mediterranean]]. The [[Mesopotamia]]n civilizations and [[History of China#Ancient China|ancient China]] also built large levee systems.<ref>Needham, Joseph. (1971). Science and Civilisation in China: Volume 4, Physics and Physical Technology, Part 3, Civil Engineering and Nautics. Cambridge: Cambridge University Press; Brian Lander. "State Management of River Dikes in Early China: New Sources on the Environmental History of the Central Yangzi Region." T’oung Pao 100.4–5 (2014): 325–62.</ref> Because a levee is only as strong as its weakest point, the height and standards of construction have to be consistent along its length. Some authorities have argued that this requires a strong governing authority to guide the work and may have been a catalyst for the development of systems of governance in early civilizations. However, others point to evidence of large-scale water-control earthen works such as canals and/or levees dating from before [[King Scorpion]] in [[Predynastic Egypt]], during which governance was far less centralized.

Another example of a historical levee that protected the growing city-state of Mēxihco-Tenōchtitlan and the neighboring city of Tlatelōlco, was constructed during the early 1400s, under the supervision of the tlahtoani of the altepetl Texcoco, Nezahualcoyotl. Its function was to separate the brackish waters of Lake Texcoco (ideal for the agricultural technique ''Chināmitls'') from the fresh potable water supplied to the settlements. However, after the Europeans destroyed Tenochtitlan, the levee was also destroyed and flooding became a major problem, which resulted in the majority of The Lake being drained in the 17th century.

Levees are usually built by piling earth on a cleared, level surface. Broad at the base, they taper to a level top, where temporary embankments or sandbags can be placed. Because flood discharge intensity increases in levees on both [[Bank (geography)|river bank]]s, and because [[silt]] deposits raise the level of [[Stream bed|riverbeds]], planning and auxiliary measures are vital. Sections are often set back from the river to form a wider channel, and flood valley basins are divided by multiple levees to prevent a single breach from flooding a large area. A levee made from stones laid in horizontal rows with a bed of thin turf between each of them is known as a ''spetchel''.

Artificial levees require substantial engineering. Their surface must be protected from erosion, so they are planted with vegetation such as [[Bermuda grass]] in order to bind the earth together. On the land side of high levees, a low terrace of earth known as a ''banquette'' is usually added as another anti-erosion measure. On the river side, erosion from strong waves or currents presents an even greater threat to the integrity of the levee. The effects of erosion are countered by planting suitable vegetation or installing stones, boulders, weighted matting, or concrete [[revetment]]s. Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.

=== River flood prevention ===
[[File:Sacramento River broken levee.jpg|thumb|Broken levee on the Sacramento River]]
[[File:GretnaLevee.jpg|thumb|A levee keeps high water on the Mississippi River from flooding [[Gretna, Louisiana|Gretna]], [[Louisiana]], in March 2005.]]

Prominent levee systems have been built along the [[Mississippi River]] and [[Sacramento River]] in the [[United States]], and the [[Po River|Po]], [[Rhine]], [[Meuse River]], [[Rhône]], [[Loire]], [[Vistula]], the delta formed by the Rhine, Maas/Meuse and [[Scheldt]] in the [[Netherlands]] and the [[Danube]] in [[Europe]]. During the Chinese [[Warring States period]], the [[Dujiangyan irrigation system]] was built by the [[Qin (state)|Qin]] as a [[water conservation]] and flood control project. The system's infrastructure is located on the [[Min River (Sichuan)|Min River]], which is the longest tributary of the [[Chang Jiang|Yangtze River]], in [[Sichuan]], [[China]].

The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over {{convert|3500|mi|km|order=flip|abbr=on}} of levees extending some {{convert|1000|km|mi|abbr=on}} along the Mississippi, stretching from [[Cape Girardeau, Missouri|Cape Girardeau]], [[Missouri]], to the [[Mississippi River Delta|Mississippi delta]]. They were begun by French settlers in [[Louisiana]] in the 18th century to protect the city of [[New Orleans]].<ref name=Kemp>Kemp, Katherine. [http://www.tulane.edu/~bfleury/envirobio/enviroweb/FloodControl.htm ''The Mississippi Levee System and the Old River Control Structure''The Louisiana Environment.] Tulane.edu</ref> The first Louisiana levees were about {{convert|3|ft|cm|-1|order=flip|abbr=on}} high and covered a distance of about {{convert|50|mi|km|order=flip|abbr=on}} along the riverside.<ref name=Kemp/> The U.S. Army Corps of Engineers, in conjunction with the Mississippi River Commission, extended the levee system beginning in 1882 to cover the riverbanks from [[Cairo, Illinois]] to the mouth of the [[Mississippi River Delta|Mississippi delta]] in Louisiana.<ref name=Kemp/> By the mid-1980s, they had reached their present extent and averaged {{convert|24|ft|m|sp=us|order=flip|abbr=on}} in height; some Mississippi levees are as high as {{convert|50|ft|m|sp=us|order=flip|abbr=on}}. The Mississippi levees also include some of the longest continuous individual levees in the world. One such levee extends southwards from [[Pine Bluff, Arkansas|Pine Bluff]], [[Arkansas]], for a distance of some {{convert|380|mi|km|sp=us|order=flip|abbr=on}}. The scope and scale of the Mississippi levees has often been compared to the [[Great Wall of China]].<ref name="NewYorker1987">{{cite magazine|last=McPhee|first=John|title=The Control of Nature: Atchafalaya|magazine=The New Yorker|date=February 23, 1987|url=http://www.newyorker.com/archive/1987/02/23/1987_02_23_039_TNY_CARDS_000347146|access-date=May 12, 2011|archive-url=https://web.archive.org/web/20110513171926/http://www.newyorker.com/archive/1987/02/23/1987_02_23_039_TNY_CARDS_000347146|archive-date=May 13, 2011|url-status=live}}  Republished in {{cite book | author=McPhee, John | title=The Control of Nature | publisher=Farrar, Straus and Giroux | year=1989 | isbn=0-374-12890-1 | page = 272 }}</ref>

The United States Army Corps of Engineers (USACE) recommends and supports [[cellular confinement]] technology (geocells) as a best management practice.<ref>{{cite web|url=https://apps.dtic.mil/sti/pdfs/ADA354949.pdf|archive-url=https://web.archive.org/web/20130408135145/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA354949|url-status=live|archive-date=8 April 2013|title= levee rehabilitation in USACE Technical Report REMR-GT-26, Innovative Methods for Levee Rehabilitation|author=Edward B. Perry|date=September 1998|website=Dtic.mil|access-date=3 April 2019}}</ref> Particular attention is given to the matter of surface erosion, [[wave overtopping|overtopping]] prevention and protection of levee crest and downstream slope. Reinforcement with geocells provides tensile force to the soil to better resist instability.

Artificial levees can lead to an elevation of the natural riverbed over time; whether this happens or not and how fast, depends on different factors, one of them being the amount and type of the [[bed load]] of a river. [[Alluvium|Alluvial]] rivers with intense accumulations of sediment tend to this behavior. Examples of rivers where artificial levees led to an elevation of the riverbed, even up to a point where the riverbed is higher than the adjacent ground surface behind the levees, are found for the [[Yellow River]] in China and the [[Mississippi River|Mississippi]] in the United States.

=== Coastal flood prevention ===
Levees are very common on the marshlands bordering the [[Bay of Fundy]] in [[New Brunswick]] and [[Nova Scotia]], [[Canada]]. The [[Acadians]] who settled the area can be credited with the original construction of many of the levees in the area, created for the purpose of farming the fertile tidal marshlands. These levees are referred to as dykes. They are constructed with hinged sluice gates that open on the falling tide to drain freshwater from the agricultural marshlands and close on the rising tide to prevent seawater from entering behind the dyke. These sluice gates are called "[[aboiteau]]x". In the [[Lower Mainland]] around the city of [[Vancouver]], [[British Columbia]], there are levees (known locally as dikes, and also referred to as "the sea wall") to protect low-lying land in the [[Fraser River]] delta, particularly the city of [[Richmond, British Columbia|Richmond]] on [[Lulu Island]]. There are also dikes to protect other locations which have flooded in the past, such as the Pitt Polder, land adjacent to the [[Pitt River]], and other tributary rivers.

Coastal flood prevention levees are also common along the inland coastline behind the [[Wadden Sea]], an area devastated by many historic floods.<ref>{{cite web|url=http://www.waddensea-secretariat.org/sites/default/files/downloads/03.1-coastal-defence-10-05-14.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.waddensea-secretariat.org/sites/default/files/downloads/03.1-coastal-defence-10-05-14.pdf |archive-date=2022-10-09 |url-status=live|title=Trilateral Working Group on Coastal Protection and Sea Level Rise (CPSL), Wadden Sea Ecosystem No. 25 by Jacobus Hofstede, Common Wadden Sea Secretariat (CWSS), Wilhelmshaven, Germany, 2009|website=Waddensea-secretariat.org|access-date=3 April 2019}}</ref> Thus the peoples and governments have erected increasingly large and complex flood protection levee systems to stop the sea even during storm floods. The biggest of these are the huge levees in the [[Netherlands]], which have gone beyond just defending against floods, as they have aggressively taken back land that is below mean sea level.<ref>{{cite web|url=http://geography.about.com/od/specificplacesofinterest/a/dykes.htm|title=Dikes of the Netherlands&nbsp;— Geography|author=Matt Rosenberg|website=Geography.about.com|access-date=6 December 2014|archive-date=1 February 2009|archive-url=https://web.archive.org/web/20090201074700/http://geography.about.com/od/specificplacesofinterest/a/dykes.htm|url-status=dead}}</ref> <!-- {{Citation needed|date=June 2013}} Some source reference may be reused from the Netherlands article -->

=== Spur dykes or groynes ===
{{see also|Jetty|Breakwater (structure)}}
These typically man-made hydraulic structures are situated to protect against erosion. They are typically placed in alluvial rivers perpendicular, or at an angle, to the bank of the channel or the [[revetment]],<ref>{{cite web|url=http://www.dpri.kyoto-u.ac.jp/nenpo/no51/ronbunB/a51b0p64.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.dpri.kyoto-u.ac.jp/nenpo/no51/ronbunB/a51b0p64.pdf |archive-date=2022-10-09 |url-status=live |title=Hao Zhang, Hajime Nakagawa, 2008, ''Scour around Spur Dyke: Recent Advances and Future Researches''|website=Dpri.kyoto-u.ac.jp|access-date=2013-05-17}}</ref> and are used widely along coastlines. There are two common types of spur dyke, permeable and impermeable, depending on the materials used to construct them.

=== Natural examples{{anchor|Natural levee|Natural levees}}===
Natural levees commonly form around lowland rivers and creeks without human intervention. They are elongated ridges of mud and/or silt that form on the river floodplains immediately adjacent to the cut banks. Like artificial levees, they act to reduce the likelihood of floodplain inundation.

Deposition of levees is a natural consequence of the flooding of meandering rivers which carry high proportions of [[suspended sediment]] in the form of fine sands, silts, and muds. Because the carrying capacity of a river depends in part on its depth, the sediment in the water which is over the flooded banks of the channel is no longer capable of keeping the same number of fine sediments in suspension as the main [[thalweg]]. The extra fine sediments thus settle out quickly on the parts of the floodplain nearest to the channel. Over a significant number of floods, this will eventually result in the building up of ridges in these positions and reducing the likelihood of further floods and episodes of levee building.<ref>{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins : from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=9781405177832 |edition=2nd |pages=265–266}}</ref>

If aggradation continues to occur in the main channel, this will make levee overtopping more likely again, and the levees can continue to build up. In some cases, this can result in the channel bed eventually rising above the surrounding floodplains, penned in only by the levees around it; an example is the [[Yellow River]] in [[China]] near the sea, where oceangoing ships appear to sail high above the plain on the elevated river.{{sfn|Leeder|2011|pp=269-271}}

Levees are common in any river with a high suspended sediment fraction and thus are intimately associated with [[meander]]ing channels, which also are more likely to occur where a river carries large fractions of suspended sediment. For similar reasons, they are also common in tidal creeks, where tides bring in large amounts of coastal silts and muds. High [[Spring tide#Range variation: springs and neaps|spring tides]] will cause flooding, and result in the building up of levees.