Levee

A levee, levée, dike (or dyke), embankment, floodbank or stopbank is a natural or artificial slope or wall to regulate water levels. It is usually earthen and often parallel to the course of a river or the coast.

Etymology

Levee

The word levee, from the French word levée (from the feminine past participle of the French verb lever, "to raise"), is used in American English (notably in the Midwest and Deep South); it came into English use in New Orleans circa 1720. The French pronunciation is , English .

Dike

The modern word dike is most probably derived from the Dutch word "dijk", with the construction of dikes in Holland well attested since the 12th century. The long Westfriese Omringdijk, for instance, was completed by 1250, and was formed by connecting existing older dikes. The Roman chronicler Tacitus however mentions the fact that the rebellious Batavi pierced dikes to flood their land and to protect their retreat (AD 70). The Dutch word dijk meant originally both the trench or the bank. The word is closely related to the English verb to dig (EWN).

In Anglo-Saxon, the word dic already existed and was pronounced with a hard c 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 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 Lincolnshire (TF1427). The Weir Dike is a soak dike in Bourne North Fen, near Twenty and alongside the River Glen, Lincolnshire.

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 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 Civilization (in Pakistan and North India from circa 2600 BC) on which the agrarian life of the Harappan peoples depended. Also 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 , 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.

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 with willows, 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 exist along the Mississippi River and Sacramento River in the United States, and the Po, Rhine, Meuse River, Loire, Vistula, the delta formed by the Rhine, Maas/Meuse and Scheldt in the Netherlands and the Danube in Europe.

The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over of levees extending some 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 Louisianan levees were about high and covered a distance of about along the riverside. By the mid-1980s, they had reached their present extent and averaged in height; some Mississippi levees are as much as 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 .

Coastal flood prevention

Levees are very common on the flatlands bordering the Bay of Fundy in New Brunswick and Nova Scotia Canada. The Acadians who settled the area can be credited with construction of most of the levees in the area, created for the purpose of farming the fertile tidal flatlands. These levees are referred to as "aboiteau". In the Lower Mainland around the city of Vancouver, British Columbia, there are levees to protect low-lying land in the Fraser River delta, particularly the city of Richmond on Lulu Island. There are also levees to protect other locations which have flooded in the past, such as land adjacent to the Pitt River and other tributary rivers.

Natural levees

Levees are commonly thought of as man-made, but they can also be natural. 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

Natural levees may be formed along creek banks that are subject to periodic flooding due to oceanic tides. Levee formation occurs as the incoming tide carries suspended sediment of all grades upstream to the limit imposed by the energy of the tidal flow. As the tidal waters overflow the creek banks, the water flow spreads out to cover a wider area than it did when confined to the stream's main channel. As the water spreads into the flood zone, its flow rate at the brink rapidly slows and much of the sediment that had been carried upstream by the tidal current is deposited along the bank. Over time, during the course of repeated tidal flooding, this sedimentation process forms a levee.

At the height of the tide, the water flow in flooded salt-marsh or flats is the most still 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. 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 and breaches

Man-made 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 levee boils, 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. Often levees are armored or reinforced with rocks or concrete to prevent erosion and failure.

New Orleans

The words levee and levee breach were brought heavily into the public consciousness after the levee failures in metro New Orleans on August 29, 2005 when Hurricane Katrina passed east of the city. Levees breached in over 50 different places submerging 80 percent of the city. Most levees failed due to water overtopping them but some failed when water passed underneath the levee foundations causing the levee wall to shift and resulting in catastrophic sudden breaching. The sudden breaching released highly pressured water that moved houses off their foundations and tossed cars into trees. This happened in the Ninth Ward when the Industrial Canal breached and also in the Lakeview neighborhood when the 17th Street Canal breached. Effects of breached levees are discussed further in and 2005 levee failures in Greater New Orleans, which cites a death toll of 1,464. In New Orleans, the United States Army Corps of Engineers is the Federal agency responsible for levee design and construction as defined in the Flood Control Act of 1965 and subject to local participation requirements, some of which were later waived. Fault has been aimed at the Corps of Engineers, their local contractors, and local levee boards.

North Sea

The St. Elizabeth's flood of 1421 was caused by a surge of seawater being forced upriver during a storm, overflowing the river dikes and submerging approximately of land in the Netherlands. Estimates of people having died range from 2,000 to 10,000. Parts of the submerged lands have still not been reclaimed resulting in the Biesbosch wetlands.

During the North Sea flood of 1953, in the night of 31 January – 1 February 1953, many dikes in the provinces of Zeeland, Zuid-Holland and Noord-Brabant in the Netherlands were unable to withstand the combination of spring tide and a northwesterly storm. The resulting flood killed 1,835 people. A further 307 people were killed by dike breaches in the United Kingdom, in the counties of Lincolnshire, Norfolk, Suffolk and Essex. In the Netherlands these flood was a main reason for the construction of the Delta Works, probably the most innovative and extensive levee system in the world .

Other breaches

1927 - The Great Mississippi Flood of 1927 occurred when the Mississippi River breached levees and flooded , killing 246 people in seven states and displacing 700,000 people.

September 1928: Storm surge from the Okeechobee Hurricane breaches levees surrounding Lake Okeechobee, killing an estimated 2500 people.

Dec 24, 1955 - Just after midnight, a levee on the west bank of the Feather River collapsed just south of Yuba City, Ca., resulting in the drowning of 38 residents. Jan 3, 1976 - A dike failed on the Vliet, a tributuary of the Rupel in Belgium. The village of Ruisbroek was flooded to a depth of 3m and over 2000 people had to be evacuated. This disaster prompted the drafting of Belgium's Sigma Plan as a counterpart to the Dutch Delta Plan. Feb 20, 1986 - A levee on the south bank of the Yuba River collapsed at the northern Sacramento Valley community of Linda, California in Yuba County, inundating and destroying 600 homes. Jan 2, 1997 - A levee on the west bank of the Feather River collapsed at the northern Sacramento Valley community of Arboga, California in Yuba County, killing three people. More than 100,000 people in Yuba and Sutter counties were evacuated. 3 June 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. January 5, 2008 - A levee in Fernley, Nevada burst, flooding portions of the town and forcing the evacuations of 3,500 residents. September 14, 2008 - a levee in Munster, Indiana broke on the Little Calumet River resulting in flooding in most of Munster. August 8, 2009 - Levees fail in Southern Taiwan due to Typhoon Morakot causing widespread flooding in many regions.

February 26, 2010 - Levees were submerged by wind and a huge tide in Vendée, in Western France because of the Xynthia storm.

See also

Bunding

Dam

Floodway

Subsidence

Trench

References

External links

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

New Orleans and the Delta

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

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

(Activists Blocked New Orleans Levee Plan)

Category:Geotechnical engineering Category:Fluvial landforms Category:Riparian Category:Inventions of the Indus Valley Civilization