A levee breach or levee failure (the word dike or dyke can also be used instead of levee) is a situation where a levee fails or is intentionally breached, causing the previously contained water to flood the land behind the levee.
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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. 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. Some engineers think that boils lead to a form of internal erosion called piping which undermines the levee. But others consider boils as a symptom of generalized instability of the foundation.
Surface erosion of the surface of a levee is usually caused by the action of wind and water (waves but also normal flow). Erosion can be worsened by pre-existing or new damage to a levee. Areas with no surface protection are more prone to erosion. A levee grazed by certain types of animals, like sheep, can show trails used by the animals where grass does not grow.
Trees in levees are a special risk. A tree can become unstable after the soil of the levee has become saturated with water. When the tree falls the root system will likely take a chunk of the saturated soil out of the levee. This shallow hole can quickly erode and result in a breach. If the tree falls in the water and floats away it can damage the levee further downstream. Floating trees near levees should be quickly removed by the agency responsible for the maintenance of the levee.
Other forms of damage can be caused by ships or other (large) floating objects or from objects in the levee, like traffic signs or fences that are damaged or completely removed by wind or water. Barbed wire fences can collect large amounts of floating plant material, resulting in a large amount of drag from the water. Whole fences can be dragged away by the water.
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 (a storm surge) 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.
After a levee breach a kolk lake can often be seen. This is a crater-like depression just behind the breach where soil and other material has been violently removed by the rushing water. After a breach, a kolk lake will remain when the water level recedes.
In some cases levees are breached intentionally. This can be done to protect other areas or to give back land to nature. In most cases an intentional breach is not without discussion since valuable land is given up. During the Great Mississippi Flood of 1927 a levee was blown up with dynamite to prevent the flooding of New Orleans.
Taking land from the cycle of flooding by putting a dike around it prevents it from being raised by silt left behind after a flooding. At the same time the drained soil consolidates and peat decomposes leading to land subsidence. In this way the difference between the water level on one side and land level on the other side of the dike grew. In some areas reclaimed land is given back to nature by breaching and removing dikes to allow flooding to occur (again). This restores the natural environment in the area. This happened in the Glory River in Iraq
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 the article 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.[1]
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 300 square kilometres (100 sq mi) 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.[2]