Flooding explained: crests, spillways and levees
Townhomes sit in floodwater from the Mississippi River on Monday in Memphis, Tennessee.
May 10th, 2011
09:43 PM ET

Flooding explained: crests, spillways and levees

The swollen Mississippi River on Tuesday was in the process of cresting many feet above flood stage in Memphis, Tennessee, and residents of states to the south are bracing for serious flooding in their communities.

A slow-moving wave of water has been working its way down the river since torrential rains sparked flooding in the Midwest's Ohio and Mississippi river valleys in late April. Hundreds of households were ordered to evacuate in the Memphis area (see pictures), where the river was expected to crest Tuesday around 48 feet - 14 feet above flood stage and less than a foot under the city's record level set in 1937.

The high water is headed for Mississippi and Louisiana, prompting authorities to open one spillway and consider opening another - moves meant to divert some of the water into less populated or unpopulated areas. Up to 5,000 homes will be evacuated in Mississippi, officials there say. In Louisiana, where the river is expected to crest next week, 21 parishes already have issued emergency declarations.

You will hear a lot about crests, spillways and levees over the next couple of weeks. Below you'll find what these mean and what the Army Corps of Engineers and others are doing to mitigate the flooding threat along the Mississippi.

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How the Mississippi River levees could fail
A levee near Tomato, Arkansas, is typical of those that line the sides of the Mississippi River. They can fail in several ways.
May 9th, 2011
09:14 PM ET

How the Mississippi River levees could fail

[cnn-video url="http://www.cnn.com/video/#/video/weather/2011/05/09/tsr.myers.flooding.explainer.cnn"%5D

Pictures can't describe the misery playing out along the Mississippi River for those unprotected by flood levees and walls. Some homes, farms and businesses will be 25 feet underwater for weeks until the water recedes.

The river is still rising from Memphis, Tennessee, to the south. In Memphis, where the river is expected to crest at a near-record 14 feet above flood stage on Tuesday morning, the water was moving at 2 million cubic feet per second on Monday. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers.

Earthen levees should keep most of the larger towns and cities safe as an extraordinarily high volume of water runs down the Mississippi to the Gulf of Mexico. But levees can fail, in part because moving water has tremendous force. This force will try to erode, saturate, undermine and destroy everything in the way.

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Filed under: Arkansas • Louisiana • Mississippi • Tennessee • Weather