 | The examples and perspective in this article deal primarily with Western culture and do not represent a worldwide view of the subject.(July 2018) |
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.
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 them a symptom of generalized instability of the foundation.[ citation needed ]
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 scoured out by the rushing water. After a breach, a kolk lake can sometimes remain after the water level recedes.
In some cases levees are breached intentionally. This can be done to protect other areas, to drain flooded 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. Again during record-breaking flooding in 2011, the US Army Corps of Engineers blew up a section of a Mississippi River levee with dynamite to open the New Madrid Floodway. The floodway was used for farming and had about 200 residents at the time. The levee at Bird's Point was designed to be removed if necessary so that Mississippi water levels would be lowered, taking pressure off levees for miles upstream in more populated areas such as Cairo, Illinois, and New Madrid, Missouri. [1]
Taking land from the cycle of flooding by putting a dike around it prevents it from being raised by silt left behind after a flood. 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 grows.
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 water at a high velocity that moved houses off their foundations and tossed cars into trees. This happened in the Lower Ninth Ward when the Industrial Canal breached and also in the Lakeview neighborhood when the 17th Street Canal breached. At least 1,464 people perished. 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. [2]
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, South Holland and North 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 this flood was a main reason for the construction of the Delta Works, probably the most innovative and extensive levee system in the world. [3]
 |
A levee, dike, dyke, embankment, floodbank, or stop bank is a structure used to keep the course of rivers from changing and to protect against flooding of the area adjoining the river or coast. It is usually earthen and often runs parallel to the course of a river in its floodplain or along low-lying coastlines.
The Delta Works is a series of construction projects in the southwest of the Netherlands to protect a large area of land around the Rhine–Meuse–Scheldt delta from the sea. Constructed between 1954 and 1997, the works consist of dams, sluices, locks, dykes, levees, and storm surge barriers located in the provinces of South Holland and Zeeland.
Lake Pontchartrain is an estuary located in southeastern Louisiana in the United States. It covers an area of 630 square miles (1,600 km2) with an average depth of 12 to 14 feet. Some shipping channels are kept deeper through dredging. It is roughly oval in shape, about 40 miles (64 km) from west to east and 24 miles (39 km) from south to north.
The 1953 North Sea flood was a major flood caused by a heavy storm surge that struck the Netherlands, north-west Belgium, England and Scotland. Most sea defences facing the surge were overwhelmed, resulting in extensive flooding.
Hurricane preparedness in New Orleans has been an issue since the city's early settlement because of its location.
From 1890 through 2006, the Orleans Levee Board (OLB) was the body of commissioners that oversaw the Orleans Levee District (OLD) which supervised the levee and floodwall system in Orleans Parish, Louisiana The role of the OLB has changed over time. Prior to Hurricane Betsy in 1965, the OLB developed land and sold it to raise money to build and improve flood protection levees. After Betsy, Congress passed the Flood Control Act of 1965 which directed the Army Corps of Engineers to design and build the hurricane flood protection system enveloping New Orleans. Owing to the 1965 legislation, the OLB's duties were now limited to collecting the 30% cost share for project design and construction, and to maintaining and operating completed flood protection structures.
Though Hurricane Katrina did not deal the city of New Orleans a direct hit on August 29, 2005, the associated storm surge precipitated catastrophic failures of the levees and flood walls. The Mississippi River Gulf Outlet ("MR-GO") breached its levees in approximately 15 places. The major levee breaches in the city include the 17th Street Canal levee, the London Avenue Canal, and the wide, navigable Industrial Canal, which left approximately 80% of the city flooded.
On Monday, August 29, 2005, there were over 50 failures of the levees and flood walls protecting New Orleans, Louisiana, and its suburbs following passage of Hurricane Katrina. The failures caused flooding in 80% of New Orleans and all of St. Bernard Parish. In New Orleans alone, 134,000 housing units — 70% of all occupied units — suffered damage from Hurricane Katrina and the subsequent flooding.
Drainage in New Orleans, Louisiana, has been a major concern since the founding of the city in the early 18th century, remaining an important factor in the history of New Orleans today. The central portion of metropolitan New Orleans is fairly unusual in that it is almost completely surrounded by water: Lake Pontchartrain to the north, Lake Borgne to the east, wetlands to the east and west, and the Mississippi River to the south. Half of the land area between these bodies of water is at or below sea level, and no longer has a natural outlet for flowing surface water. As such, virtually all rainfall occurring within this area must be removed through either evapotranspiration or pumping. Thus, flood threats to metropolitan New Orleans include the Mississippi River, Lake Pontchartrain, canals throughout the city, and natural rainfall. Artificial levees have been built to keep out rising river and lake waters but have had the negative effect of keeping rainfall in, and have failed on numerous occasions.
Flood control is an important issue for the Netherlands, as due to its low elevation, approximately two thirds of its area is vulnerable to flooding, while the country is densely populated. Natural sand dunes and constructed dikes, dams, and floodgates provide defense against storm surges from the sea. River dikes prevent flooding from water flowing into the country by the major rivers Rhine and Meuse, while a complicated system of drainage ditches, canals, and pumping stations keep the low-lying parts dry for habitation and agriculture. Water control boards are the independent local government bodies responsible for maintaining this system.
Floods in the United States are generally caused by excessive rainfall, excessive snowmelt, and dam failure. Below is a list of flood events that were of significant impact to the country during the 20th century, from 1900 through 1999, inclusive.
The June 2008 Midwestern United States floods were flooding events which affected portions of the Midwestern United States. After months of heavy precipitation, a number of rivers overflowed their banks for several weeks at a time and broke through levees at numerous locations. Flooding continued into July. States affected by the flooding included Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri and Wisconsin. The American Red Cross assisted the victims of flooding and tornadoes across seven states and the National Guard was mobilized to assist in disaster relief and evacuation.
In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.
Coastal flooding occurs when dry and low-lying land is submerged (flooded) by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penetrates the inland which is controlled by the topography of the coastal land exposed to flooding. The seawater can flood the land via several different paths: direct flooding, overtopping of a barrier, or breaching of a barrier. Coastal flooding is largely a natural event. Due to the effects of climate change and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected.
The 2011 Souris/Mouse River flood in Canada and the United States occurred in June and was greater than a hundred-year flooding event for the river. The US Army Corps of Engineers estimated the flood to have a recurrence interval of two to five centuries.
The effects of Hurricane Isaac in Louisiana were more severe than anywhere in the storm's path, and included $611.8615 million in damages and five total deaths. Forming from a tropical wave in the central Atlantic, Isaac traversed across many of the Lesser and Greater Antilles, before reaching peak intensity with winds of 80 mph (130 km/h) on August 28, 2012 while in the Gulf of Mexico. Nearing the coast of Louisiana, the Category 1 hurricane slowly moved towards the west, making two landfalls in the state with little change of intensity prior to moving inland for a final time. The hurricane weakened and later dissipated on September 1 while over Missouri. Before landfall, Governor Bobby Jindal declared a state of emergency to the state, as well as ordering the mandatory evacuation of 60,000 residents in low-lying areas of Louisiana along the Tangipahoa River in Tangipahoa Parish.
Coastal erosion in Louisiana is the process of steady depletion of wetlands along the state's coastline in marshes, swamps, and barrier islands, particularly affecting the alluvial basin surrounding the mouth of the Mississippi River. In the last century, Southeast Louisiana has lost a large portion of its wetlands and is expected to lose more in the coming years, with some estimates claiming wetland losses equivalent to up to one football field per hour. One consequence of coastal erosion is an increased vulnerability to hurricane storm surges, which affects the New Orleans metropolitan area and other communities in the region. The state has outlined a comprehensive master plan for coastal restoration and has begun to implement various restoration projects such as fresh water diversions, but certain zones will have to be prioritized and targeted for restoration efforts, as it is unlikely that all depleted wetlands can be rehabilitated.
Climate change in Louisiana encompasses the effects of climate change, attributed to man-made increases in atmospheric carbon dioxide, in the U.S. state of Louisiana.
Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of sea-level rise. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term sustainability of deltas.
{{cite web}}: Missing or empty |title= (help)[ title missing ]