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Morganza spillway: Flooding farmland to save New Orleans

The US Army of Corps of Engineers has opened the Morganza spillway to prevent the surging Mississippi from endangering New Orleans. But farms and natural habitat will be flooded as a result.

By Staff writer / May 15, 2011

Water diverted from the Mississippi River spills through a bay in the Morganza Spillway. A steel, 10-ton floodgate was slowly raised Saturday for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River, away from heavily populated areas downstream.

Patrick Semansky/AP

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New Orleans

One hour. That’s all it took for the farmland located beside the Morganza floodwall, untouched for 38 years, to turn into a great lake.

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Water from the Mississippi rolled gently but quickly through the natural landscape of trees, grazing land, and swamp late Saturday afternoon, the result of a decision by the US Army Corps of Engineers to open the historic hydraulic system in order to lower river levels cascading towards Baton Rouge and New Orleans.

It will take three days for the water to flow 20 miles south into the Atchafalaya Basin, which drains into the Gulf of Mexico. Along the way, the water’s passage will affect up to 25,000 people living in communities, from small towns populated by just a few hundred to Morgan City, central to the area’s oil and seafood production, that is home to 12,000 people. Sandbagging of roads, interstate highways, and homes in those areas started early last week.

The Morganza is the second floodway opened in Louisiana to mitigate the surging floodwaters driving southward down the Mississippi River over the past three weeks that has displaced hundreds and continues to cover homes and farmland with water.

Mike Strack, chief of emergency management for the Corps’ New Orleans division, says that even though the Corps opened a single bay, there are 124 remaining. The opening will continue to be slow to give wildlife, including black bears, deer and turtles, time to find shelter. However, the flow rate will rise, from 10,000 cubic feet per second (cfs) to about 125,000 cfs, Corps officials said. That represents less than a quarter of the structure’s full capacity of 600,000 cfs.

Last Sunday, the Corps opened all 350 bays at the Bonnet Carre, located closer to New Orleans, releasing water into Lake Pontchartrain and eventually the Gulf of Mexico.

There is still uncertainty about how much water to let through and for what period of time.

Officials say they are aiming for at least three weeks and will involve the opening of at least 15 bays. But as forecasts from the National Weather Service change, it may be longer.

Water is expected to crest at the Morganza in about ten days. How far the river levels rise will determine how much will end up being diverted through the floodway.

The Morganza was built in 1954 and used only one other time before, in 1973. Still there remain uncertainties: Will the Corps be able to close the steel doors of the floodway gates three weeks after they are open? Is using only a quarter of the Morganza’s capacity enough to prevent flooding in New Orleans, where water rests less than three feet from the top of levees?

John Barry, author of “Rising Tide,” considered the definitive history of the 1927 flood, the most disastrous flood in US history to date that displaced over 600,000 people and flooded 26,000 square miles, says the actions taken by the Corps since that incident produced “a very good comprehensive system that includes all sorts of elements including, by a wide margin, the strongest levees in the U.S.”

The agricultural use of the floodplain is understandable because it “is too valuable to just leave fallow,” Mr. Barry says. But opening the Morganza was a calculated risk work taking, he adds. River levels in New Orleans, for example, are expected to crest at 19.5 feet, just inches from the top of the levees there.

Barry says that possibility could lead to a “massive levee breach” where there would be “Niagara Falls or more pouring of the river for an extended period of time.”

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