A LEVEE (/ˈlɛvi/ ), DIKE, DYKE, EMBANKMENT, FLOODBANK or STOPBANK is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels. It is usually earthen and often parallel to the course of a river in its floodplain or along low-lying coastlines.
* 1 Etymology
* 2 Uses
* 3 Natural examples * 4 Failures and breaches * 5 See also * 6 Notes * 7 References * 8 External links
The modern word dike or dyke most likely derives from the Dutch word
dijk, with the construction of dikes in
Frisia (now part of the
In Anglo-Saxon , the word 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 midlands and north of
England, and in the United States, a dike is what a ditch is in the
south, a property-boundary marker or small 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
In parts of Britain , particularly
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 polders . Furthermore, levees have been built for the purpose of empoldering, 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 defences . More on this type of levee can be found in the article on dry-stone walls .
Levees can be permanent earthworks or emergency constructions (often of sandbags ) built hastily in a flood emergency. When such an emergency bank is added on top of an existing levee it is known as a cradge.
Some of the earliest levees were constructed by the Indus Valley
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 river banks , and because silt deposits raise the level of 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 revetments . Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.
RIVER FLOOD PREVENTION
Prominent levee systems have been built along the Mississippi River
The Mississippi levee system represents one of the largest such
systems found anywhere in the world. It comprises over 3,500 miles
(5,600 km) of levees extending some 1,000 kilometres (620 mi) along
the Mississippi, stretching from Cape Girardeau ,
Artificial levees can lead to an elevation of the natural river bed
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. Alluvial rivers with intense accumulations of
sediment tend to this behavior. Examples of rivers where artificial
levees led to an elevation of the river bed, even up to a point where
the river bed is higher than the adjacent ground surface behind the
levees, are found for the Yellow
COASTAL FLOOD PREVENTION
Levees are very common on the marshlands bordering the
Coastal flood prevention levees are also common along the inland
coastline behind the
Wadden Sea , an area devastated by many historic
floods. 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 of course the huge
levees in the
SPUR DYKES OR GROYNES
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 , 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 levees commonly form around lowland rivers and creeks without human intervention. They are elongate 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 amount 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.
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
Levees are common in any river with a high suspended sediment fraction, and thus are intimately associated with meandering 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 tides will cause flooding, and result in the building up of levees.
FAILURES AND BREACHES
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 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 cutoffs 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 floodwalls . There have
been numerous studies to investigate the erodibility of soils. Briaud
et al. (2008) used
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.
* ^ Longman Dictionary of Contemporary English
* ^ Cambridge Advanced Learner\'s Dictionary
* ^ Henry Petroski (2006). "Levees and Other Raised Ground". 94
(1). American Scientist: 7–11.
* ^ "levee".
Oxford English Dictionary (3rd ed.). Oxford University
Press . September 2005. (Subscription or UK public library membership
Tacitus Histories V 19
* ^ "Etymologisch woordenboek van het Nederlands, deel 1: A t/m E
— Amsterdam University Press". aup.nl.
* ^ "Weavers\' Way footpath closure — Decoy
* This article incorporates text from a publication now in the public domain : Chisholm, Hugh, ed. (1911). "Levée". Encyclopædia Britannica (11th ed.). Cambridge University Press.