Flooding threat along Mississippi pits man against nature

A dead fish lies along U.S. 49 W Sunday. The carcass was left behind by the Yazoo River in Silver City, Miss. Floodwaters have swamped Mississippi tributaries such as the Yazoo.

Associated Press

A dead fish lies along U.S. 49 W Sunday. The carcass was left behind by the Yazoo River in Silver City, Miss. Floodwaters have swamped Mississippi tributaries such as the Yazoo.

The snakes are out. And the bears. The gators. The jumbo rodents known as nutria. The feral hogs. They seek higher ground as the floodwaters advance, and that can mean the top of a levee or in someone's back yard.

The humans are scrambling, too. They've filled a million sandbags. The hospitals have stockpiled antivenin in anticipation of a surge in snakebites. Officials the other day shot a 10-foot gator on a levee near New Orleans.

Over the weekend, engineers were tracking roughly 250 seepage points along the levees that line the Mississippi and Atchafalaya rivers. The levees sit on fine sand that lets the river water escape like a convict tunneling out of prison. The water can pop up a mile away. Left to its own devices, one of these sand boils (where water erupts as if from a spring) can undermine a levee and lead to a crevasse, a full levee failure — and disaster.

There's a war under way, fought across the Mississippi's 35,000-square-mile alluvial plain. This is the toughest test since 1973 of the Army Corps of Engineers' flood-control system along the Mississippi, which drains 41 percent of the land in the lower 48, and is now swollen by snowmelt from the Rockies and rainwater from the soggy Ohio River Valley.

For the moment, engineers seem to have the upper hand. They are not claiming victory, however. This is not simply a big flood but also a long flood, one that will last well into June. A flood exists in four dimensions, including time — because a long-duration flood can be more problematic than one that crests and recedes quickly.

"Everything that is part of our toolbox is in use," reported Col. Ed Fleming, New Orleans district commander for the corps. "There's no doubt there is going to be a long crest."

His colleague Mike Stack, chief of emergency management for the corps at New Orleans, said, "The system is under tremendous stress, and it's going to be that way for a while."

Stack added: "It's performing as it's designed."

But as with any complex system of engineering, there are weak points, question marks, vulnerabilities. Powerful forces are being checked with levees made of clay.

"We're not dealing with digital technology. We're dealing with earth," said Joseph Suhayda, a retired Louisiana State University coastal hydrologist. "This goes back to the beginning of civilization. It's available, it's cheap, but it's not very good material."

He went on: "These seepages and sand boils are reflections of the fact that there are some continued deficiencies in the system. This is not a robust system. It's not concrete."

• • •

The Army Corps has long prepared for a hypothetical inundation known as the Design Flood. This flood pretty much fits that template. The flow, measured in cubic feet per second, isn't quite at Design Flood levels, but there are places where the river gauges have measured record-high water, topping the old mark by 3 feet in some spots.

For years, the smart money has bet that, in the protracted wrestling match on the Mississippi between man and nature, nature will ultimately come out on top. The decision, going back to the 19th century, to imprison a naturally meandering river between levees — a feat commonly compared to Great Wall of China — has the inevitable effect of raising the water level downstream.

Levee failures can be killers. National Guard troops now walk the levees day and night. They want to pounce on small problems before they become big problems.

"I'm feeling vigilant. Literally saying prayers every day," said Garret Graves, chairman of the Louisiana Coastal Protection and Restoration Authority. "The potential disaster here is real, and we're doing everything we can to make this go down in history as simply a record-high-water event rather than a record disaster."

• • •

The system is designed to limit the river stages at Baton Rouge and New Orleans during a major flood. The solution is diversion. First, the corps opened the Bonnet Carre spillway north of New Orleans, dumping a portion of the river flow into Lake Pontchartrain. Then the corps opened the Morganza spillway, north of Baton Rouge. That relief valve, completed in 1954, has been opened only once before, during the 1973 flood.

The moves do not come without a cost. Fresh water is surging into normally brackish Lake Pontchartrain, with potentially devastating effects on the marine life there. And to the west, the Atchafalaya River basin is slowly filling with water diverted at Morganza. Residents have evacuated, some unhappy to have their lives disrupted to spare the big cities and industries to the east.

Day by day the water surges Niagara-like through the Morganza spillway and advances through the kingdom of crawfish.

Perhaps the trickiest part is ahead: Once the water hits Morgan City, it will be pinched by levees and floodwalls and will begin to "stack up." As the water rises in the floodplain, it will find ways to spill into adjacent areas in a process known as backflooding.

Graves, the chairman of the Louisiana Coastal Protection and Restoration Authority, said the system may be working now, but it's still not ideal.

"It's a good system. Is it a great system? No. We need more options, ultimately. We need more relief valves. This thing is literally being tested to its rim. It's not a comfortable feeling," he said.

Flooding threat along Mississippi pits man against nature 05/22/11 [Last modified: Monday, May 23, 2011 1:10am]

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