Wikipedia

Levee

For other uses, seeLevee (disambiguation).

Alevee(/ˈlɛvi/or/ˈlɛveɪ/),[a][1]dike(American English),dyke(British English;seespelling differences),embankment,floodbank,orstop bankis an elevatedridge,natural or artificial, alongside thebanksof ariver,often intended toprotect against floodingof the area adjoining the river. It is usuallyearthenand often runsparallelto the course of a river in itsfloodplainor along low-lying coastlines.[2]

Components of an artificial levee:
  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
The side of a levee inSacramento,California

Naturally occuring levees form on riverfloodplainsfollowing flooding, where sediment andalluviumisdepositedand settles, forming a ridge and increasing the river channel's capacity. Alternatively, levees can be artificially constructed fromfill,designed to regulate water levels. In some circumstances, artificial levees can beenvironmentally damaging.[3]

Ancient civilizations in theIndus 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,[4]such as the catastrophic2005 levee failures in Greater New Orleansthat occurred as a result ofHurricane Katrina.

Etymology

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Speakers ofAmerican Englishuse the wordlevee,from the French wordlevée(from the feminine past participle of theFrench verblever,'to raise'). It originated inNew Orleansa few years after the city's founding in 1718 and was later adopted by English speakers.[5]The name derives from the trait of the levee's ridges being raised higher than both the channel and the surrounding floodplains.

The modern worddikeordykemost likely derives from theDutchworddijk,with the construction of dikes well attested as early as the 11th century. The 126-kilometer-long (78 mi)Westfriese Omringdijk,completed by 1250, was formed by connecting existing older dikes. The Roman chroniclerTacitusmentions that the rebelliousBatavipierced dikes to flood their land and to protect their retreat (70CE).[6]The worddijkoriginally indicated both thetrenchand thebank.It closely parallels the English verbto dig.[7]

InAnglo-Saxon,the worddicalready existed and was pronounced asdickin northern England and asditchin 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. ThusOffa's Dykeis a combined structure andCar Dykeis a trench – though it once had raised banks as well. In the EnglishMidlandsandEast Anglia,and in the United States, a dike is what aditchis 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 inRippingale Running Dike,which leads water from thecatchwater drain,Car Dyke, to the South Forty Foot Drain inLincolnshire(TF1427). The Weir Dike is asoak dikeinBourne North Fen,nearTwentyand alongside theRiver Glen,Lincolnshire.In theNorfolkandSuffolkBroads,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.[8]

In parts ofBritain,particularlyScotlandandNorthern England,a dyke may be a field wall, generally made withdry stone.

Uses

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A reinforced embankment

The main purpose of artificial levees is to prevent flooding of the adjoiningcountrysideand 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 alongpolders.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 militarydefences.More on this type of levee can be found in the article ondry-stone walls.

Levees can be permanentearthworksor emergency constructions (often ofsandbags) built hastily in a flood emergency.

Some of the earliest levees were constructed by theIndus Valley civilization(inPakistanandNorth Indiafromc. 2600 BCE) on which the agrarian life of the Harappan peoples depended.[9]Levees were also constructed over 3,000 years ago inancient Egypt,where a system of levees was built along the left bank of theRiver Nilefor more than 1,000 kilometers (600 miles), stretching from modernAswanto theNile Deltaon the shores of theMediterranean.TheMesopotamiancivilizations andancient Chinaalso built large levee systems.[10]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 beforeKing ScorpioninPredynastic 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 techniqueChināmitls) 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 bothriver banks,and becausesiltdeposits raise the level ofriverbeds,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 aspetchel.

Artificial levees require substantial engineering. Their surface must be protected from erosion, so they are planted with vegetation such asBermuda grassin order to bind the earth together. On the land side of high levees, a low terrace of earth known as abanquetteis 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 concreterevetments.Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.

River flood prevention

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Broken levee on the Sacramento River
A levee keeps high water on the Mississippi River from floodingGretna,Louisiana,in March 2005.

Prominent levee systems have been built along theMississippi RiverandSacramento Riverin theUnited States,and thePo,Rhine,Meuse River,Rhône,Loire,Vistula,the delta formed by the Rhine, Maas/Meuse andScheldtin theNetherlandsand theDanubeinEurope.During the ChineseWarring States period,theDujiangyan irrigation systemwas built by theQinas awater conservationand flood control project. The system's infrastructure is located on theMin River,which is the longest tributary of theYangtze River,inSichuan,China.

The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over 5,600 km (3,500 mi) of levees extending some 1,000 km (620 mi) along the Mississippi, stretching fromCape Girardeau,Missouri,to theMississippi delta.They were begun by French settlers inLouisianain the 18th century to protect the city ofNew Orleans.[11]The first Louisiana levees were about 90 cm (3 ft) high and covered a distance of about 80 km (50 mi) along the riverside.[11]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 fromCairo, Illinoisto the mouth of theMississippi deltain Louisiana.[11]By the mid-1980s, they had reached their present extent and averaged 7.3 m (24 ft) in height; some Mississippi levees are as high as 15 m (50 ft). The Mississippi levees also include some of the longest continuous individual levees in the world. One such levee extends southwards fromPine Bluff,Arkansas,for a distance of some 610 km (380 mi). The scope and scale of the Mississippi levees has often been compared to theGreat Wall of China.[12]

The United States Army Corps of Engineers (USACE) recommends and supportscellular confinementtechnology (geocells) as a best management practice.[13]Particular attention is given to the matter of surface erosion,overtoppingprevention 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 thebed loadof a river.Alluvialrivers 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 theYellow Riverin China and theMississippiin the United States.

Coastal flood prevention

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Levees are very common on the marshlands bordering theBay of FundyinNew BrunswickandNova Scotia,Canada.TheAcadianswho 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 "aboiteaux".In theLower Mainlandaround the city ofVancouver,British Columbia,there are levees (known locally as dikes, and also referred to as "the sea wall" ) to protect low-lying land in theFraser Riverdelta, particularly the city ofRichmondonLulu Island.There are also dikes to protect other locations which have flooded in the past, such as the Pitt Polder, land adjacent to thePitt River,and other tributary rivers.

Coastal flood prevention levees are also common along the inland coastline behind theWadden Sea,an area devastated by many historic floods.[14]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 theNetherlands,which have gone beyond just defending against floods, as they have aggressively taken back land that is below mean sea level.[15]

Spur dykes or groynes

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See also:JettyandBreakwater (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 therevetment,[16]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

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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 ofsuspended sedimentin 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 mainthalweg.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.[17]

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 theYellow RiverinChinanear the sea, where oceangoing ships appear to sail high above the plain on the elevated river.[18]

Levees are common in any river with a high suspended sediment fraction and thus are intimately associated withmeanderingchannels, 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. Highspring tideswill cause flooding, and result in the building up of levees.

Failures and breaches

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Main article: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 alevee 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 acrevasse 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 ofmeander cutoffsif 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 erosionis 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 andfloodwalls.There have been numerous studies to investigate the erodibility of soils. Briaud et al. (2008)[19]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[20]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.[21][22]

Another approach applied to prevent levee failures iselectrical 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.[23]

Negative impacts

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

Ecological impact

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Levees interrupt floodplain ecosystems that developed under conditions of seasonal flooding.[24]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

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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, therecurrence intervalfor high-water events in the river increases, often requiring increases in levee height.[25]

Levee breaches produce high-energy flooding

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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.[26]

Prolonged flooding after levee failure

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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

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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 themantle,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 insubsidence(sinking of land surface).[27]In coastal areas, this results in land dipping below sea level, the ocean migrating inland, andsalt-water intrudinginto freshwater aquifers.[28]

Coastal sediment loss

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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 "birds-foot delta"extends far out into the ocean. The results for surrounding land include beach depletion, subsidence, salt-water intrusion, and land loss.[29]

See also

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  • Bridge scour– Erosion of sediment near bridge foundations by water
  • Bunding– Retaining wall around pollution source
  • Coupure– Structural feature
  • Dam– Barrier that stops or restricts the flow of surface or underground streams
  • Earth structure– Building or other structure made largely from soil
  • Embankment (earthworks)– Wall or bank to carry a road or rail over low ground or water's edge
  • Flood control– Methods for reducing detrimental effects of flood waters
  • Flood control in the Netherlands– Manmade control of flooding in the Netherlands
  • Lava channel
  • Nullah– Steep, narrow valley
  • Seawall– Form of coastal defence
  • Sleeper dike– Dike that backs up a front-line dike
  • Subsidence– Downward vertical movement of the Earth's surface
  • Trench– Excavated channel in ground

Notes

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  1. ^Sometimes'levée'

References

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  1. ^"levee".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
  2. ^Henry Petroski (2006). "Levees and Other Raised Ground".American Scientist.94(1): 7–11.doi:10.1511/2006.57.7.
  3. ^"Flood risk reduction with multiple benefits: more space for the river".www.preventionweb.net.6 May 2022.Retrieved22 January2023.
  4. ^"A Look at Preventing Levee Erosion".Federal Emergency Management Agency.11 February 2021.Retrieved28 June2023.
  5. ^"levee".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)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 CompanyISBN0833747223.1770:"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
  6. ^TacitusHistoriesV 19
  7. ^"Etymologisch woordenboek van het Nederlands, deel 1: A t/m E – Amsterdam University Press".Aup.nl.Archived fromthe originalon 26 March 2017.Retrieved12 February2015.
  8. ^ "Weavers' Way footpath closure – Decoy Road (Hickling) to Potter Heigham 7 January 2011 – 6 April 2012".Countrysideaccess.norfolk.gov.uk.Retrieved17 May2013.
  9. ^"Indus River Valley Civilizations".History-world.org.Archived from the original on 10 June 2012.Retrieved12 September2008.{{cite web}}:CS1 maint: unfit URL (link)
  10. ^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.
  11. ^abcKemp, Katherine.The Mississippi Levee System and the Old River Control StructureThe Louisiana Environment.Tulane.edu
  12. ^McPhee, John (23 February 1987)."The Control of Nature: Atchafalaya".The New Yorker.Archivedfrom the original on 13 May 2011.Retrieved12 May2011.Republished inMcPhee, John (1989).The Control of Nature.Farrar, Straus and Giroux. p. 272.ISBN0-374-12890-1.
  13. ^Edward B. Perry (September 1998)."levee rehabilitation in USACE Technical Report REMR-GT-26, Innovative Methods for Levee Rehabilitation"(PDF).Dtic.mil.Archivedfrom the original on 8 April 2013.Retrieved3 April2019.
  14. ^"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"(PDF).Waddensea-secretariat.org.Archived(PDF)from the original on 9 October 2022.Retrieved3 April2019.
  15. ^Matt Rosenberg."Dikes of the Netherlands — Geography".Geography.about.com.Archived fromthe originalon 1 February 2009.Retrieved6 December2014.
  16. ^"Hao Zhang, Hajime Nakagawa, 2008,Scour around Spur Dyke: Recent Advances and Future Researches"(PDF).Dpri.kyoto-u.ac.jp.Archived(PDF)from the original on 9 October 2022.Retrieved17 May2013.
  17. ^Leeder, M. R. (2011).Sedimentology and sedimentary basins: from turbulence to tectonics(2nd ed.). Chichester, West Sussex, UK: Wiley-Blackwell. pp. 265–266.ISBN9781405177832.
  18. ^Leeder 2011,pp. 269–271.
  19. ^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.
  20. ^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
  21. ^Karimpour, Mazdak; Heinzl, Kyle; Stendback, Emaline; Galle, Kevin; Zamiran, Siavash; Osouli, Abdolreza (2015). "Scour Characteristics of Saturated Levees Due to Floodwall Overtopping".IFCEE 2015.pp. 1298–1307.doi:10.1061/9780784479087.117.ISBN9780784479087.
  22. ^"Levee Erosion and Scour Potential Due to Floodwall Overtopping (PDF Download Available)".ResearchGate.
  23. ^Arosio, Diego; Munda, Stefano; Tresoldi, Greta; Papini, Monica; Longoni, Laura; Zanzi, Luigi (13 October 2017)."A customized resistivity system for monitoring saturation and seepage in earthen levees: installation and validation".Open Geosciences.9(1): 457–467.Bibcode:2017OGeo....9...35A.doi:10.1515/geo-2017-0035.hdl:11380/1151894.ISSN2391-5447.
  24. ^Knox, R.L., Wohl, E.E., and Morrison, R.R., 2022, Levees don’t protect, they disconnect: A
  25. ^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.
  26. ^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.
  27. ^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.
  28. ^Schmidt, C.W., 2015, Delta Subsidence: An Imminent Threat to Coastal Populations: Environmental Health Perspectives, v. 123, doi:10.1289/ehp.123-A204.
  29. ^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.
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