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 impoldering, 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. Some of the earliest levees were constructed by the
Indus Valley civilization (in
Pakistan and
North India from ) on which the agrarian life of the Harappan peoples depended. Levees were also 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. Another example of a historical levee that protected the growing city-state of Mēxihco-Tenōchtitlan and the neighboring city of Tlatelōlco, was constructed during the early 1400s, under the supervision of the tlahtoani of the altepetl Texcoco, Nezahualcoyotl. Its function was to separate the brackish waters of Lake Texcoco (ideal for the agricultural technique
Chināmitls) from the fresh potable water supplied to the settlements. However, after the Europeans destroyed Tenochtitlan, the levee was also destroyed and flooding became a major problem, which resulted in the majority of The Lake being drained in the 17th century. 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 ,
Louisiana, in March 2005. Prominent levee systems have been built along the
Mississippi River and
Sacramento River in the
United States, and the
Po,
Rhine,
Meuse River,
Rhône,
Loire,
Vistula, the delta formed by the Rhine, Maas/Meuse and
Scheldt in the
Netherlands and the
Danube in
Europe. During the Chinese
Warring States period, the
Dujiangyan irrigation system was built by the
Qin as a
water conservation and flood control project. The system's infrastructure is located on the
Min River, which is the longest tributary of the
Yangtze River, in
Sichuan,
China. 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 Louisiana levees were about high and covered a distance of about along the riverside. The United States Army Corps of Engineers (USACE) recommends and supports
cellular confinement technology (geocells) as a best management practice. Particular attention is given to the matter of surface erosion,
overtopping prevention and protection of levee crest and downstream slope. Reinforcement with geocells provides tensile force to the soil to better resist instability. Artificial levees can lead to an elevation of the natural riverbed 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 riverbed, even up to a point where the riverbed is higher than the adjacent ground surface behind the levees, are found for the
Yellow River in China and the
Mississippi in the United States.
Coastal flood prevention Levees are very common on the marshlands bordering the
Bay of Fundy in
New Brunswick and
Nova Scotia,
Canada. The
Acadians who settled the area can be credited with the original construction of many of the levees in the area, created for the purpose of farming the fertile tidal marshlands. These levees are referred to as dykes. They are constructed with hinged sluice gates that open on the falling tide to drain freshwater from the agricultural marshlands and close on the rising tide to prevent seawater from entering behind the dyke. These sluice gates are called "
aboiteaux". In the
Lower Mainland around the city of
Vancouver,
British Columbia, there are levees (known locally as dikes, and also referred to as "the sea wall") to protect low-lying land in the
Fraser River delta, particularly the city of
Richmond on
Lulu Island. There are also dikes to protect other locations which have flooded in the past, such as the Pitt Polder, land adjacent to the
Pitt River, and other tributary rivers. 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 the huge levees in the
Netherlands, which have gone beyond just defending against floods, as they have aggressively taken back land that is below mean sea level.
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 examples Natural levees commonly form around lowland rivers and creeks without human intervention. They are elongated 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 number 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 River in
China near the sea, where oceangoing ships appear to sail high above the plain on the elevated river. 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 ==