Levee & Dyke
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. It is known in Europe as a dike or dyke. The modern word dike is most probably derived from the Netherlands "dijk", where the construction of dikes is well attested since the 12th century. In Anglo-Saxon, the word dic already existed and was pronounced with a hard c in northern England and as ditch in the south.
Levee Failures and BreachesMan-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 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. Properly built levees are armored or reinforced with rocks or concrete to prevent erosion and failure. In New OrleansThe 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 eighty percent (80%) 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 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. Other breachesThe Great Mississippi Flood occurred in 1927 when the Mississippi River breached levees and flooded 27,000 square miles (70,000 km²), 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. On January 5, 2008, a levee in Fernley, Nevada bursts, flooding portions of the town and forcing the evacuations of 3,500 residents. 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, the river delta in the Netherlands 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 leveesLevees 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 watersThe 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. References
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A dike (or dyke) is an artificial earthen wall, constructed as a defense or as a boundary. It is also known in American English (notably in the Midwest) as a levee, from the French word levée (elevated). The best known form of dike is a construction built along the edge of a body of water, to prevent it from flooding onto an adjacent lowland. Dikes 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, dikes 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 dikes. Dikes have also been built as field boundaries and as military defences. More on this type of dike can be found in the article on dry-stone walls. Dikes 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 dike it is known as a cradge. Dikes were first constructed in the Indus Valley Civilization (in Pakistan and North India from circa 2600 BC) on which the agrarian life of the Harappan peoples depended. [1] The modern word dike is most probably derived from the Netherlands "dijk", where the construction of dikes is well attested since the 12th century. 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, 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. Dikes 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 dikes in the area, created for the purpose of farming the fertile tidal flatlands. These dikes are referred to as "aboiteau". A dike made from stones laid in horizontal rows with a bed of thin turf between each of them is known as a spetchel. Dike can also mean a pond in the same way as Australians use the word dam. However, this is more likely in the several other languages which use obviously related words. Frisian is one of them. The Frisians who settled in England with the Angles and Saxons form a linguistic link with Dutch dating from well before the 12th century. See the stories of Saints Boniface and Wulfram. In April 2006, South Korea completed the Saemangeum Seawall, displacing the Afsluitdijk as the longest man-made dike in the world. External links and references
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This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
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