Levee

From Academic Kids

This article is about the type of dam. See also Levee (event)

A levee (from the French for "raised") is a natural or artificial embankment or dyke, usually earthen, which parallels the course of a river. The word seems to have come into English through its use in colonial Louisiana.

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Artificial Levees

Missing image
GretnaLevee.jpg
Levee keeps high water on the Mississippi River from flooding Gretna, Louisiana in March, 2005.

Artificial levees function to prevent flooding of the adjoining countryside; however, it also confines the flow of the river resulting in higher and faster water flow.

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 banks, and because silt deposits raise the level of riverbeds, planning, as well as auxiliary measures are vital.

Prominent levee systems exist along the Mississippi River and Sacramento Rivers in the United States, and the Po, Vistula, and Danube in Europe.

In some countries, levees are known by other names, such as floodbanks or stopbanks.

Levees in Popular Culture

The song "I've Been Working On the Railroad" was, according to Carl Sandburg, originally "I've Been Working On the Levee" before the days of railroads. Other notable mention of levees in music are Led Zeppelin's cover version of a Memphis Minnie track "When the Levee Breaks", and then a Perfect Circles cover of "When the Levee Breaks", also Don McLean's mention of driving his "Chevy to the Levee" in his song "American Pie"


Natural Levees

A natural levee results from the deposit of material by a river during flood stage resulting in the land near a river being raised in elevation. When the river is not in flood state it cuts a channel in the elevated material. Natural levees are especially noted on the Yellow River in China near the sea where ocean going ships appear to sail high above the plain on the elevated river. Natural levees are also present on the Rio Grande River in Colorado's San Luis Valley.

Natural levees are formed as sediment of larger grain size settles out on the banks of channels owing to the drop in flow velocity on the edge of the channel.

Levees in Tidal Waters

The same basic process occurs in tidal creeks when the incomming tide carries mineral material of all grades up to the limit imposed by the energy of the flow. As the tide overflows the sides of the creek towards high water, the flow rate at the brink slows and larger sediment is deposited, forming the levee. At the height of the tide, the water stands on the salt-marsh or flats and the finer particles slowly settle, forming clay. In the early ebb, the water level in the creek falls leaving the broad expanse of water standing on the marsh at a higher level.

The area of water on the marsh is much greater than the water surface of the creek so that in the latter, the flow rate is much greater. It is this rush of water, perhaps an hour after high water, which keeps the creek channel open. The cross-sectional area of the water body in the creek is small compared with that initially over the levee which at this stage is acting as a weir. The deposited sediment (coarse on the levee and fine on the mud flats or salt-marsh) therefore tends to stay put so that, tide by tide, the marsh and levee grow higher until they are of such a hight that few tides overflow them. In an active system, the levee is always higher than the marsh. That is how it came to be called "une rive leve" or raised shore.ja:堤防

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