Editing Levee

{{Short description|Ridge or wall to hold back water}}
{{Other uses}}
{{Use dmy dates|date=March 2020}}
{{Use American English|date=June 2021}}
[[File:River Levee Cross Section Figure.svg|thumb|Components of an artificial levee:
{{ordered list
| 1 = Design high water level (HWL)
| 2 = Low water channel
| 3 = Flood channel
| 4 = Riverside slope
| 5 = Riverside banquette
| 6 = Levee crown
| 7 = Landside slope
| 8 = Landside banquette
| 9 = Berm
| 10 = Low water revetment
| 11 = Riverside land
| 12 = Levee
| 13 = Protected lowland
| 14 = River zone
}}]]
[[File:Sacramento River Levee.jpg|thumb|The side of a levee in [[Sacramento, California|Sacramento]], [[California]]]]

A '''levee''' ({{IPAc-en|ˈ|l|ɛ|v|i}} or {{IPAc-en|ˈ|l|ɛ|v|eɪ}}),{{efn|Sometimes {{gloss|levée}}}}<ref>{{cite OED|levee|39369470}}</ref> '''dike''' ([[American English]]), '''dyke''' ([[British English]]; see [[American and British English spelling differences#Miscellaneous spelling differences|spelling differences]]), '''embankment''', '''floodbank''', or '''stop bank''' is an elevated [[ridge]], natural or artificial, alongside the [[river banks|banks]] of a [[river]], often intended to [[flood control|protect against flooding]] of the area adjoining the river. It is usually [[soil|earthen]] and often runs [[parallel (geometry)|parallel]] to the course of a river in its [[floodplain]] or along low-lying coastlines.<ref>{{Cite journal | title=Levees and Other Raised Ground | author=Henry Petroski  | journal=American Scientist | year=2006 | volume=94 | issue=1 | pages=7–11| doi=10.1511/2006.57.7 }}</ref>

Naturally occurring levees form on river [[floodplain]]s following flooding. Sediment and [[alluvium]] are [[deposition (geology)|deposited]] on the banks and settle, forming a ridge that increases the river channel's capacity. Alternatively, levees can be artificially constructed from [[fill dirt|fill]], designed to regulate water levels. In some circumstances, artificial levees can be [[environmental degradation|environmentally damaging]].<ref>{{Cite web |title=Flood risk reduction with multiple benefits: more space for the river |url=https://www.preventionweb.net/news/flood-risk-reduction-multiple-benefits-more-space-river |access-date=2023-01-22 |website=www.preventionweb.net |date=6 May 2022 |language=en}}</ref>

Ancient civilizations in the [[Indus Valley civilisation|Indus Valley]], ancient Egypt, Mesopotamia and China all built levees. Today, levees can be found around the world, and failures of levees due to erosion or other causes can be major disasters,<ref>{{Cite web |date=11 February 2021 |title=A Look at Preventing Levee Erosion |url=https://www.fema.gov/case-study/look-preventing-levee-erosion |access-date=28 June 2023 |website=[[Federal Emergency Management Agency]]}}</ref> such as the catastrophic [[2005 levee failures in Greater New Orleans]] that occurred as a result of [[Hurricane Katrina]].

== Etymology ==
Speakers of [[American English]] use the word ''levee'', from the French word {{lang|fr|levée}} (from the feminine past participle of the [[French verbs|French verb]] {{lang|fr|lever}}, 'to raise'). It originated in [[New Orleans]] a few years after the city's founding in 1718 and was later adopted by English speakers.<ref>{{OED|levee}} '''1718–1720''': "Dumont Plan, New Orleans" [map label]. Shown in Justin Winsor, (1895) ''The Mississippi basin: The struggle in America between England and France 1697–1763.'' Boston; New York: Houghton, Mifflin and Company {{ISBN|0833747223}}. '''1770''': "The town [New Orleans] is secured from the inundations of the river by a raised bank, generally called the Levée." Philip Pittman, ''The Present State of the European Settlements on the Mississippi; with a geographical description of that river.'' London</ref> The name derives from the trait of the levee's ridges being raised higher than both the channel and the surrounding floodplains.

The modern word ''dike'' or ''dyke'' most likely derives from the [[Dutch language|Dutch]] word {{lang|nl|dijk}}, with the construction of dikes well attested as early as the 11th century. The {{convert|126|km|mi|adj=mid|sp=us|-long}} [[Westfriese Omringdijk]], completed by 1250, was formed by connecting existing older dikes. The Roman chronicler [[Tacitus]] mentions that the rebellious [[Batavi (Germanic tribe)|Batavi]] pierced dikes to flood their land and to protect their retreat (70&nbsp;[[Common Era|CE]]).<ref>Tacitus ''Histories'' V 19</ref> The word {{lang|nl|dijk}} originally indicated both the [[trench]] and the [[bank (geography)|bank]]. It closely parallels the English verb ''to dig''.<ref>{{cite web|url= http://nl.aup.nl/books/9789053566534-etymologisch-woordenboek-van-het-nederlands-deel-1-a-t-m-e.html|title= Etymologisch woordenboek van het Nederlands, deel 1: A t/m E&nbsp;– Amsterdam University Press|website= Aup.nl|access-date= 12 February 2015|archive-date= 26 March 2017|archive-url= https://web.archive.org/web/20170326092445/http://nl.aup.nl/books/9789053566534-etymologisch-woordenboek-van-het-nederlands-deel-1-a-t-m-e.html|url-status= dead}}</ref>

In [[Old English language|Anglo-Saxon]], the word {{lang|ang|dic}} already existed and was pronounced as ''dick'' in northern England and as ''ditch'' in the south. Similar to Dutch, the English origins of the word lie in digging a trench and forming the upcast soil into a bank alongside it. This practice has meant that the name may be given to either the excavation or to the bank. Thus [[Offa's Dyke]] is a combined structure and [[Car Dyke]] is a trench – though it once had raised banks as well. In the English [[Midlands]] and [[East Anglia]], and in the United States, a dike is what a [[ditch]] is in the south of England, a property-boundary marker or drainage channel. Where it carries a stream, it may be called a running dike as in ''Rippingale Running Dike'', which leads water from the [[catchwater drain]], Car Dyke, to the South Forty Foot Drain in [[Lincolnshire]] (TF1427). The Weir Dike is a [[soak dike]] in [[Bourne, Lincolnshire|Bourne North Fen]], near [[Twenty, Lincolnshire|Twenty]] and alongside the [[River Glen, Lincolnshire|River Glen]], [[Lincolnshire]]. In the [[Norfolk]] and [[Suffolk]] [[The Broads|Broads]], a dyke may be a drainage ditch or a narrow artificial channel off a river or broad for access or mooring, some longer dykes being named, e.g., Candle Dyke.<ref>
{{cite web
|url= http://www.countrysideaccess.norfolk.gov.uk/news-details.aspx?id=119 
|title= Weavers' Way footpath closure&nbsp;– Decoy Road (Hickling) to Potter Heigham 7 January 2011 – 6 April 2012 
|publisher= Countrysideaccess.norfolk.gov.uk 
|access-date= 2013-05-17
}}
</ref>

In parts of [[United Kingdom|Britain]], particularly [[Scotland]] and [[Northern England]], a dyke may be a field wall, generally made with [[dry stone]].

== 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.

== Failures and breaches ==
{{Main|Levee breach}}

Both natural and man-made levees can fail in a number of ways. Factors that cause levee failure include overtopping, erosion, structural failures, and levee saturation. The most frequent (and dangerous) is a [[levee breach]]. Here, a part of the levee actually breaks or is eroded away, leaving a large opening for water to flood land otherwise protected by the levee. A breach can be a sudden or gradual failure, caused either by surface erosion or by subsurface weakness in the levee. A breach can leave a fan-shaped deposit of sediment radiating away from the breach, described as a [[crevasse splay]]. In natural levees, once a breach has occurred, the gap in the levee will remain until it is again filled in by levee building processes. This increases the chances of future breaches occurring in the same location. Breaches can be the location of [[meander cutoff]]s if the river flow direction is permanently diverted through the gap.

Sometimes levees are said to fail when water overtops the crest of the levee. This will cause flooding on the floodplains, but because it does not damage the levee, it has fewer consequences for future flooding.

Among various failure mechanisms that cause levee breaches, [[soil erosion]] is found to be one of the most important factors. Predicting soil erosion and scour generation when overtopping happens is important in order to design stable levee and [[floodwall]]s. There have been numerous studies to investigate the erodibility of soils. Briaud et al. (2008)<ref>Briaud, J., Chen, H., Govindasamy, A., Storesund, R. (2008). Levee erosion by overtopping in New Orleans during the Katrina Hurricane. Journal of Geotechnical and Geoenvironmental Engineering. 134 (5): 618–632.</ref> used Erosion Function Apparatus (EFA) test to measure the erodibility of the soils and afterwards by using Chen 3D software, numerical simulations were performed on the levee to find out the velocity vectors in the overtopping water and the generated scour when the overtopping water impinges the levee. By analyzing the results from EFA test, an erosion chart to categorize erodibility of the soils was developed. Hughes and Nadal in 2009<ref>Hughes, S.A., Nadal, N.C. (2009). Laboratory study of combined wave overtopping and storm surge overflow of a levee. Coastal Engineering.56: 244–259</ref> studied the effect of combination of wave overtopping and storm surge overflow on the erosion and scour generation in levees. The study included hydraulic parameters and flow characteristics such as flow thickness, wave intervals, surge level above levee crown in analyzing scour development. According to the laboratory tests, empirical correlations related to average overtopping discharge were derived to analyze the resistance of levee against erosion. These equations could only fit to the situation, similar to the experimental tests, while they can give a reasonable estimation if applied to other conditions.

Osouli et al. (2014) and Karimpour et al. (2015) conducted lab scale physical modeling of levees to evaluate score characterization of different levees due to floodwall overtopping.<ref>{{Cite book |doi = 10.1061/9780784479087.117|chapter = Scour Characteristics of Saturated Levees Due to Floodwall Overtopping|title = IFCEE 2015|year = 2015|last1 = Karimpour|first1 = Mazdak|last2 = Heinzl|first2 = Kyle|last3 = Stendback|first3 = Emaline|last4 = Galle|first4 = Kevin|last5 = Zamiran|first5 = Siavash|last6 = Osouli|first6 = Abdolreza|pages = 1298–1307|isbn = 9780784479087}}</ref><ref>{{cite web|url=https://www.researchgate.net/publication/275033990|title=Levee Erosion and Scour Potential Due to Floodwall Overtopping (PDF Download Available)|website=ResearchGate}}</ref>

Another approach applied to prevent levee failures is [[electrical resistivity tomography]] (ERT). This non-destructive geophysical method can detect in advance critical saturation areas in embankments. ERT can thus be used in monitoring of seepage phenomena in earth structures and act as an early warning system, e.g., in critical parts of levees or embankments.<ref>{{Cite journal|last1=Arosio|first1=Diego|last2=Munda|first2=Stefano|last3=Tresoldi|first3=Greta|last4=Papini|first4=Monica|last5=Longoni|first5=Laura|last6=Zanzi|first6=Luigi|date=2017-10-13|title=A customized resistivity system for monitoring saturation and seepage in earthen levees: installation and validation|journal=Open Geosciences|volume=9|issue=1|pages=457–467|doi=10.1515/geo-2017-0035|issn=2391-5447|bibcode=2017OGeo....9...35A|doi-access=free|hdl=11380/1151894|hdl-access=free}}</ref>

==Negative impacts==
Large scale structures designed to modify natural processes inevitably have some drawbacks or negative impacts.

; Ecological impact
: Levees interrupt floodplain ecosystems that developed under conditions of seasonal flooding.<ref>Knox, R.L., Wohl, E.E., and Morrison, R.R., 2022, Levees don’t protect, they disconnect: A </ref> In many cases, the impact is two-fold, as reduced recurrence of flooding also facilitates [[land-use change]] from forested floodplain to farms.

; Increased height
: In a natural watershed, floodwaters spread over a landscape and slowly return to the river. Downstream, the delivery of water from the area of flooding is spread out in time. If levees keep the floodwaters inside a narrow channel, the water is delivered downstream over a shorter time period. The same volume of water over a shorter time interval means higher river stage (height). As more levees are built upstream, the [[Return period|recurrence interval]] for high-water events in the river increases, often requiring increases in levee height.<ref>Munoz, S.E., Giosan, L., Therrell, M.D., Remo, J.W.F., Shen, Z., Sullivan, R.M., Wiman, C., O’Donnell, M., and Donnelly, J.P., 2018, Climatic control of Mississippi River flood hazard amplified by river engineering: Nature, v. 556, p. 95–98, doi:10.1038/nature26145.</ref>

; Levee breaches produce high-energy flooding
: During natural flooding, water spilling over banks rises slowly. When a levee fails, a wall of water held back by the levee suddenly pours out over the landscape, much like a dam break. Impacted areas far from a breach may experience flooding similar to a natural event, while damage near a breach can be catastrophic, including carving out deep holes and channels in the nearby landscape.<ref>Olson, K., Matthews, J., Morton, L.W., and Sloan, J., 2015, Impact of levee breaches, flooding, and land scouring on soil productivity: Journal of Soil and Water Conservation, v. 70, p. 5A-11A, doi:10.2489/jswc.70.1.5A.</ref> 

;  Prolonged flooding after levee failure
: Under natural conditions, floodwaters return quickly to the river channel as water-levels drop. During a levee breach, water pours out into the floodplain and moves down-slope where it is blocked from return to the river. Flooding is prolonged over such areas, waiting for floodwater to slowly infiltrate and evaporate. 

; Subsidence and seawater intrusion
: Natural flooding adds a layer of sediment to the floodplain. The added weight of such layers over many centuries makes the crust sink deeper into the [[Mantle (geology)|mantle]], much like a floating block of wood is pushed deeper into the water if another board is added on top. The momentum of downward movement does not immediately stop when new sediment layers stop being added, resulting in [[subsidence]] (sinking of land surface).<ref>Simms, A.R., Anderson, J.B., DeWitt, R., Lambeck, K., and Purcell, A., 2013, Quantifying rates of coastal subsidence since the last interglacial and the role of sediment loading: Global and Planetary Change, v. 111, p. 296–308, doi:10.1016/j.gloplacha.2013.10.002.</ref> In coastal areas, this results in land dipping below sea level, the ocean migrating inland, and [[Saltwater intrusion|salt-water intruding]] into freshwater aquifers.<ref>Schmidt, C.W., 2015, Delta Subsidence: An Imminent Threat to Coastal Populations: Environmental Health Perspectives, v. 123, doi:10.1289/ehp.123-A204.</ref>

; Coastal sediment loss
: Where a large river spills out into the ocean, the velocity of the water suddenly slows and its ability to transport sand and silt decreases. Sediments begin to settle out, eventually forming a delta and extending to the coastline seaward. During subsequent flood events, water spilling out of the channel will find a shorter route to the ocean and begin building a new delta. Wave action and ocean currents redistribute some of the sediment to build beaches along the coast. When levees are constructed all the way to the ocean, sediments from flooding events are cut off, the river never migrates, and elevated river velocity delivers sediment to deep water where wave action and ocean currents cannot redistribute. Instead of a natural wedge shaped delta forming, a "[[Bird's-foot delta|birds-foot delta]]" extends far out into the ocean. The results for surrounding land include beach depletion, subsidence, salt-water intrusion, and land loss.<ref>Edmonds, D.A., Toby, S.C., Siverd, C.G., Twilley, R., Bentley, S.J., Hagen, S., and Xu, K., 2023, Land loss due to human-altered sediment budget in the Mississippi River Delta: Nature Sustainability, v. 6, p. 644–651, doi:10.1038/s41893-023-01081-0.</ref>

== See also ==
{{div col|colwidth=23em}}
* {{annotated link|Bridge scour}}
* {{annotated link|Bunding}}
* {{annotated link|Coupure}}
* {{annotated link|Dam}}
* {{annotated link|Earth structure}}
* {{annotated link|Embankment (earthworks)}}
* {{annotated link|Flood control}}
* {{annotated link|Flood control in the Netherlands}}
* [[Lava channel]]
* {{annotated link|Nullah}}
* {{annotated link|Seawall}}
* {{annotated link|Sleeper dike}}
* {{annotated link|Subsidence}}
* {{annotated link|Trench}}
{{div col end}}

== Notes ==
{{notelist}}

== References ==
{{reflist|30em}}

== External links ==
{{commonscat|Dikes}}

* [https://books.google.com/books?id=ayEDAAAAMBAJ&pg=RA1-PA24 "Well Diggers Trick", June 1951, ''Popular Science''] article on how flood control engineers were using an old method to protect flood levees along rivers from seepage undermining the levee
* [http://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-1913.pdf "Design and Construction of Levees" US Army Engineer Manual EM-1110-2-1913]
* [https://www.ciria.org/Resources/Free_publications/I_L_H/ILH_resources.aspx The International Levee Handbook]

{{Coastal management|state=expanded}}
{{Earth's landforms}}
{{Infrastructure}}

{{Authority control}}

[[Category:Dikes| ]]
[[Category:Flood control]]
[[Category:Fluvial landforms]]
[[Category:Riparian zone]]