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:''This article is about a type of dam. See also Levee (event).
A 'levee', 'levée' (from the feminine past participle of the French verb ''lever'', "to raise"), 'floodbank' or 'stopbank' is a natural or artificial slope or wall, usually earthen and often parallels the course of a river. ^[1] Linguists agree that the term "levee" came into English use in New Orleans circa 1672.

Contents

Artificial levees

Natural levees

Levees in tidal waters

Levee failures

Incidents of Levee Failures and Breaches

Levees in popular culture

References

See also

External links

Artificial levees

The main purpose of an artificial levee is to prevent flooding of the adjoining countryside; however, they also confine the flow of the river resulting in higher and faster water flow.
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.
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 with willows, weighted matting or concrete revetments. Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.
The first levees were 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 600 miles (966 km), stretching from modern Aswan to the Nile Delta on the shores of the Mediterranean. The Mesopotamian civilizations and ancient China also built large levee systems. 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.
In modern times, prominent levee systems exist along the Mississippi River and Sacramento Rivers in the United States, and the Po, Rhine, Meuse River, Loire, Vistula, and Danube in Europe.
The Mississippi River levee system represents one of the largest such systems found anywhere in the world. They comprise over 3,500 miles (5,600 km) of levees extending some 1,000 miles (1,600 km) along the Mississippi, stretching from Cape Girardeau, Missouri to the Mississippi Delta. They were begun by French settlers in Louisiana in the 18th century to protect the city of New Orleans. The first Louisianian levees were about 3 feet (0.9 m) high and covered a distance of about 50 miles (80 km) along the riverside. By the mid-1980s, they had reached their present extent and averaged 24 feet (7 m) in height; some Mississippi levees are as much as 50 feet (15 m) high. The Mississippi levees also include some of the longest continuous individual levees in the world. One such levee extends southwards from Pine Bluff, Arkansas for a distance of some 380 miles (611 km).

Natural levees

The ability of a river to carry sediments varies very strongly with its speed. When a river floods over its banks, the water spreads out, slows down, and deposits its load of sediment. Over time, the river's banks are built up above the level of the rest of the floodplain. The resulting ridges are called natural levees.
When the river is not in flood state it may deposit material within its channel, raising its level. The combination can raise not just the surface, but even the bottom of the river above the surrounding country. Natural levees are especially noted on the Yellow River in China near the sea where oceangoing ships appear to sail high above the plain on the elevated river. Natural levees are a common feature of all meandering rivers in the world.

Levees in tidal waters

The basic process occurs in tidal creeks when the incoming tide carries mineral material of all grades up to the limit imposed by the energy of the flow. As the tide overflows the sides of the creek towards high water, the flow rate at the brink slows and larger sediment is deposited, forming the levee. At the height of the tide, the water stands on the salt-marsh or flats and the finer particles slowly settle, forming clay. In the early ebb, the water level in the creek falls leaving the broad expanse of water standing on the marsh at a higher level.
The area of water on the marsh is much greater than the water surface of the creek so that in the latter, the flow rate is much greater. It is this rush of water, perhaps an hour after high water, which keeps the creek channel open. The cross-sectional area of the water body in the creek is small compared with that initially over the levee which at this stage is acting as a weir. The deposited sediment (coarse on the levee and on the mud flats or salt-marsh) therefore tends to stay put so that, tide by tide, the marsh and levee grow higher until they are of such a height that few tides overflow them. In an active system, the levee is always higher than the marsh. That is how it came to be called "''une rive levée''" or raised shore!

Levee failures

Levees can fail in a number of ways. The most frequent (and dangerous) form of levee failure is a ''breach''. A ''levee breach'' is when part of the levee actually breaks away, leaving a large opening for water to flood the land protected by the levee. A breach can be a sudden or gradual failure that is caused either by surface erosion or by a subsurface failure of the levee. Levee breaches are often accompanied by boils boil, or ''sand boils''. A sand boil occurs when the upward pressure of water flowing through soil pores under the levee (underseepage) exceeds the downward pressure from the weight of the soil above it. The underseepage resurfaces on the landside, in the form of a volcano-like cone of sand. Boils signal a condition of incipient instability which may lead to erosion of the levee toe or foundation or result in sinking of the levee into the liquefied foundation below. Complete breach of the levee may quickly follow.
Sometimes levees are said to fail when water ''overtops'' the crest of the levee. ''Levee overtopping'' can be caused when flood waters simply exceed the lowest crest of the levee system or if high winds begin to generate significant swells in the ocean or river water to bring waves crashing over the levee. Overtopping can lead to significant landside erosion of the levee or even be the mechanism for complete breach.

Incidents of Levee Failures and Breaches

The Great Mississippi Flood occurred in 1927 when the Mississippi River breached levees and flooded , killing 246 people in seven states and displacing 700,000 people.
In the North Sea flood of 1953, levees and flood defenses collapsed in the United Kingdom and the Netherlands, killing over 2,100 people.
On June 3, 2004, Jones Tract, an inland island that is protected by a series of levees located in the Sacramento-San Joaquin Delta, failed. Though the exact cause of the levee failure is not known, the breach in the levee allowed water from the Middle River to flood the island.
'In New Orleans', levees breached in 53 different places when Hurricane Katrina passed east of the city on August 29. 2005. Eighty percent (80%) of the City was submerged. High pressured water coming through the Industrial Canal breach into the Lower Ninth Ward residential neighborhood tossed cars into trees and moved homes off from their foundations. Effects of breached levees are discussed further in Effect of Hurricane Katrina on New Orleans and 2005 levee failures in Greater New Orleans, which cites a death toll of 1,464. In New Orleans, the US Army Corps of Engineers is, by federal mandate, the sole agency responsible for levee design and construction as defined in the Flood Control Act of 1965.

Levees in popular culture

The song "The Little Things Give You Away" by American band Linkin Park mentions Levees in the lyrics "The levees are breaking." The song itself was written after a trip to floodstricken New Orleans after Hurricane Katrina.
The American folksong "I've Been Working on the Railroad" was, according to Carl Sandburg, originally from an Irish folksong called "I've Been Working on the Levee" before the days of railroads.
The song "Row Jimmy", recorded by The Grateful Dead (1973), mentions "Look at Julie down below, the levee doin the do-pas-o".
Don McLean mentions driving his "Chevy to the levee" in his song "American Pie".
The song "When the Levee Breaks" written and first recorded by Memphis Minnie and Kansas Joe McCoy in 1929, and later covered by Led Zeppelin and others, was about the Great Mississippi Flood.
"The Levee's Gonna Break" is a song written and recorded by Bob Dylan from his album, Modern Times.
Canadian band The Tragically Hip penned "New Orleans is Sinking" in 1989.
The song "The Levee", recorded by Jonny Lang mentions "Take me down, to the levee where the river flows, gonna throw my blues down the levee and let them go".
Behind The Levee is an album by New Orleans band The Subdudes
The song "He Made a Woman out of Me", written by Fred Burch and Donald Hill and appearing in the movie ''Crossroads'', mentions "I was born on a levy/A little bit south of Montgomery".
"When the Levees Broke: ''A Requiem in Four Acts"'' is a 2006 documentary film directed by Spike Lee, about the devastation of New Orleans, Louisiana due to the failure of the levees during Hurricane Katrina in 2005.
"Hooch", a song by the band Everything states "Let's get real, Let's get heavy/ Till the water breaks the levee".

References

1.

See also

★ Dam

★ Dike

★ Floodway

★ Subsidence

External links

★ Levees.Org (activist group in New Orleans to Hold the Corps Accountable)

★ New Orleans and the Delta

★ DeltaWorks.Org Project of levees, dams and barriers in the Netherlands

★ Effort to rebuild New Orleans Levees to Category 5 Design (non-profit)