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.
Most levees are piles of dirt lined along edges of a river. In theory, vegetation and its roots will keep the dirt stable as water flows by.
Levees can fail if water runs over the top – this is called overtopping. The force of the moving water erodes the soil below, and this can cause a catastrophic breach. The erosion continues until the hole is so large that floodwaters surge into the now unprotected land.
Levees also can fail from below as the water pressure undermines the piled dirt. The water forces itself below the levee and eventually out from the bottom on the dry side of the pile. This is called a sand boil or a mud boil, because of its bubbling appearance.
Also, too many days with floodwater near the top of a levee can saturate the levee’s soil, causing a washout and breach.
The Army Corps of Engineers will be watching for signs of these potential levee failures and will try to take corrective action if it sees them.
"There's always some seepage to be expected generally in the earthen levee systems, and you manage the process by inspecting it, (looking) for the seepage, and (making) sure wherever you find it, you control it,"
retired Army Corps of Engineers Maj. Gen. Thomas Sands said on Monday.
Record or near-record crests are forecast into next week. Sands said that although this month's levels are remarkably high, the levee system has held back high water before.
"The level of degree of confidence in the levee system is basically borne based on the fact that it’s performed so reliably in the past," Sands said.
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