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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 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. When such an emergency bank is added on top of an existing levee it is known as a cradge.
Levees 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. Also 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.
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 with willows, weighted matting or concrete revetments. Separate ditches or drainage tiles are constructed to ensure that the foundation does not become waterlogged.
River flood prevention
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 Louisianan 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).
Coastal flood prevention
Levees 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 levees in the area, created for the purpose of farming the fertile tidal flatlands. These levees are referred to as "aboiteau". In the Lower Mainland around the city of Vancouver, British Columbia, there are levees to protect low-lying land in the Fraser River delta, particularly the city of Richmond on Lulu Island. There are also levees to protect other locations which have flooded in the past, such as land adjacent to the Pitt River and other tributary rivers.
Natural levees
Levees 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.
To log this cache, you will have to complete a few requirements. All requirements must be done with in 3 days of logging or it will be deleted.
1) Take a picture of yourself or your geocaching group with the levee in the background.
2) Calculate the flow rate in "Feet Per Second" for Tennessee River at this location. This may be done by measuring a distance along each of the rivers banks, floating an object from your starting point to the ending point, and recording the time it takes the object to float the distance. Once the "Distance" and "Time" are determined, calculate the "Feet Per Second".
Additional Hints
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