Wednesday, March 21, 2012

Levee breach spared city from brunt of flood

Debra Levey Larson in When faced with a choice between a deluge or a controlled deluge in May 2011 that would protect the city of Cairo, Illinois, the U.S. Army Corps of Engineers chose the latter. But was ordering an intentional breach of the Mississippi River levee at Bird’s Point the right decision?

“The decision was a difficult and complex engineering problem with significant social and political trade-offs between loss of human lives and loss of properties in urban and rural areas,” says University of Illinois researcher Ken Olson. “But it was a calculated risk built on a growing body of river science and prior flooding experiences.”

The city of Cairo occupies a small sliver of land about 2 square miles or 1,300 acres in size, which is only 1 percent of the size of the New Madrid Floodway (210 square miles or 133,000 acres).

“Due to the size of Cairo and lack of an outlet, even if the sea wall or the earthen levee system which protects Cairo had failed, the filling of the city with floodwater would have done little to drop the record Cairo gauge peak of 61.72 feet on May 3, 2011, and threats to downstream levees would have continued to be a high concern.”

According to Olson’s study, published in the Journal of Soil and Water Conservation, the greatest danger to levee failure is constant water pressure against the levee. The weight of the river pushes water underneath the levee, creating boils and undermining the strength of the levee and its capacity to hold water. Consequently, allowing the levee to break on its own would have created much more damage.

...In Olson’s opinion, the failure of the seawall and levee system would have covered the city of Cairo with 22 feet of floodwater for days and could have resulted in significant loss of life and severe damage to more than 600 buildings in the cities of Cairo, Future City, and Urbandale, Illinois...

This NASA shot of Cairo, Illinois, at the confluence of the Ohio and Mississippi Rivers shows how vulnerable it is to flooding

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