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- This article is about the type of dam. See also Levee (event) or Levy for fund-raising.
A levee, levée (from the feminine past participle of the French verb lever, "to raise"), floodbank or stopbank is a natural or artificial embankment or dike, usually earthen, which parallels the course of a river. The word levee seems to have come into English through its use in colonial Louisiana.
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 represent 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 behind 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. Though less dramatic as a levee breach, a boil will allow water to flow through or under a levee to resurface on the landside. If left unattended, a boil that is carrying levee material with it can eventually result in a widespread breach.
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 mechicanism for complete breach.
Historical Examples of Levee Failures
The Great Mississippi Flood occurred in 1927 when the Mississippi River breached levees and flooded 27,000 square miles, 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.
During the passage of Hurricane Katrina in August 2005, floodwaters breached levees protecting New Orleans, causing catastrophic flooding and resulting in the total evacuation of the city (effects on levees are discussed further in Effect of Hurricane Katrina on New Orleans).
Floods are not always the cause of levee failures. 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.
Levees in popular culture
The song "I've Been Working on the Railroad" was, according to Carl Sandburg, originally "I've Been Working on the Levee" before the days of railroads.
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.
Don McLean mentions driving his "Chevy to the levee" in his song "American Pie". Canadian band The Tragically Hip penned "New Orleans is Sinking" in 1989.
The song "Row Jimmy", performed and recorded by The Grateful Dead, mentions "Look at Julie down below, the levee doin the do-pas-o". Words by Robert Hunter, music by Jerry Garcia Wake of the Flood " (November 15, 1973) LP: Grateful Dead Records: GD 01.
See also
External links
- New Orleans and the Delta
- DeltaWorks.Org Project of levees, dams and barriers in the Netherlands
- Levees for Greater New Orleans (non-profit)
- Effort to rebuild New Orleans Levees to Category 5 Design (non-profit)
"Levee"