When The Levees Broke

Tuesday, April 26, 2022 10:57:41 AM

When The Levees Broke

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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. Some of the earliest levees were constructed by the Indus Valley Civilization in Pakistan and North India from circa BC on which the agrarian life of the Harappan peoples depended. The Mesopotamian civilizations and ancient China also built large levee systems. 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, after the Europeans destroyed Tenochtitlan, the levee was also destroyed and flooding became a major problem, which resulted in the majority of The Lake to be 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. During the Chinese Warring States period , the Dujiangyan irrigation system was built by the Qin as a water conservation and flood control project. The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over 5, km 3, mi of levees extending some 1, km mi 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.

Army Corps of Engineers, in conjunction with the Mississippi River Commission, extended the levee system beginning in to cover the riverbanks from Cairo, Illinois to the mouth of the Mississippi delta in Louisiana. 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 km mi. Reinforcement with geocells provides tensile force to the soil to better resist instability. Artificial levees can lead to an elevation of the natural river bed 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 river bed, even up to a point where the river bed is higher than the adjacent ground surface behind the levees, are found for the Yellow River in China and the Mississippi in the USA. 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. The biggest of these are of course 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. 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 , [14] 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 levees commonly form around lowland rivers and creeks without human intervention. 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 amount 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. Both natural and man-made levees can fail in a number of ways.

Factors that cause levee failure include overtopping, erosion, structural failures, and levee saturation. The most frequent and dangerous is a levee breach. Here, a part of the levee actually breaks or is eroded away, leaving a large opening for water to flood land otherwise protected by the levee. A breach can be a sudden or gradual failure, caused either by surface erosion or by subsurface weakness in the levee. A breach can leave a fan-shaped deposit of sediment radiating away from the breach, described as a crevasse splay. In natural levees, once a breach has occurred, the gap in the levee will remain until it is again filled in by levee building processes. This increases the chances of future breaches occurring in the same location. Breaches can be the location of meander cutoffs if the river flow direction is permanently diverted through the gap.

Sometimes levees are said to fail when water overtops the crest of the levee. This will cause flooding on the floodplains, but because it does not damage the levee, it has fewer consequences for future flooding. Among various failure mechanisms that cause levee breaches, soil erosion is found to be one of the most important factors. Predicting soil erosion and scour generation when overtopping happens is important in order to design stable levee and floodwalls. There have been numerous studies to investigate the erodibility of soils. Briaud et al. By analyzing the results from EFA test, an erosion chart to categorize erodibility of the soils was developed. Hughes and Nadal in [18] studied the effect of combination of wave overtopping and storm surge overflow on the erosion and scour generation in levees.

The study included hydraulic parameters and flow characteristics such as flow thickness, wave intervals, surge level above levee crown in analyzing scour development. According to the laboratory tests, empirical correlations related to average overtopping discharge were derived to analyze the resistance of levee against erosion. So people build levees. Levees may be used to increase available land for habitation or divert a body of water so the fertile soil of a river or sea bed may be used for agriculture. They prevent rivers from flooding cities in a storm surge.

But if a levee breaks, the consequences can be disastrous. Levees are usually made of earth. The natural movement of a body of water pushes sediment to the side, creating a natural levee. The banks of a river are often slightly elevated from the river bed. The banks form levees made of sediment, silt , and other materials pushed aside by the flowing water. Levees are usually parallel to the way the river flows, so levees can help direct the flow of the river. Levees can also be artificially created or reinforced. Artificial levees are usually built by piling soil, sand , or rocks on a cleared, level surface. In places where the flow of a river is strong, levees may also be made of blocks of wood, plastic , or metal.

Where the area beside a river or other body of water is in particular danger, levees may even be reinforced by concrete. People have been building and reinforcing levees since the beginning of civilization. Farmers were able to grow crops like cotton and rice. In addition to creating living space and cropland, levees can also provide a measure of protection from invaders. Levees can make a river like a moat , preventing people from easily invading territory on the other side. Destroying levees can also stop invading forces. In , Chinese leaders intentionally broke levees on the Yellow River to prevent the Japanese military from advancing. More than , people, Japanese and Chinese, died in the resulting flood.

Artificial levees need to be protected. They have to stand up to erosion , or wearing away, by the nearby water. Sometimes, trees and plants like Bermuda grass are planted near levees to anchor the soil. Engineers need to maintain levees with structural work to reinforce the boundaries. In emergencies, temporary levees can be made of sandbags. These soak up the water and usually prevent excess water from seeping past the sand. Artificial levees prevent flooding. But they also create a new problem: levees squeeze the flow of the river. Water levels are higher and water flows faster.

This puts more pressure on levees downstream and makes the water more difficult to control. If levees break, it also makes containing the flood more difficult. Since the 18th century, levees have protected Louisiana and other nearby states from flooding by the Mississippi River. When Hurricane Katrina struck New Orleans in , the levees could not withstand the storm surge. The levees broke, and water flooded 80 percent of the city.

Although most levees exist to control rivers, they can also exist on the coast. The country of the Netherlands has an elaborate system of dikes, levees, and dams to hold back the North Sea. Land for farms, industry, and residential use has been created from land that was once the ocean floor. The Bay of Fundy, which borders the provinces of New Brunswick and Nova Scotia, Canada, has one of the highest tidal ranges in the world. The tidal range reaches more than 17 meters 55 feet in some places.

To make the most use of land that would otherwise be underwater during high tide , Canada has constructed levees along parts of the Bay of Fundy. The levee system along the Mississippi River has some of the longest individual levees in the world. One of these levees stretches south along the river from Pine Bluff, Arkansas, for an entire kilometers miles. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Caryl-Sue, National Geographic Society. Dunn, Margery G. For information on user permissions, please read our Terms of Service.

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