Plan for coping with levee break: patch, then pump

Flooding

Katrina's Wake

August 31, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

With a 300-foot section of an earth-and-concrete levee on New Orleans' 17th Street Canal gushing water into Jefferson Parish yesterday, civil engineers and the Army Corps of Engineers faced the daunting task of stopping the leak, then cleaning up the water and the mess left behind.

Plans were to use a giant Chinook helicopter to drop rocks or containers filled with sand as a stopgap measure to close the levee break.

The federal government stood poised to bring in high-volume pumps to help drain the city, thousands of mobile homes to house its residents and front loaders and dump trucks to haul away mountains of debris to be buried or burned.

Even after the levee break is repaired, and the area is slowly pumped dry, property owners will face months of hardship trying to put things right again.

`Really horrible'

"I've seen some flood damage, and it's really horrible," said Steven L. Stockton, deputy director of civil works for the Corps of Engineers.

When the enormity of it all sinks in, the question might become whether enough was done to ensure the integrity of the 17th Street Canal levee, not to mention the 15,000 miles of levees elsewhere around the country, experts said.

"The infrastructure is aging," said Gerald E. Galloway, professor of civil engineering at the University of Maryland, College Park. For seven years he was a member of the Mississippi River Commission, overseeing the Corps of Engineers' work on the lower part of the river.

`Wake-up time'

"I would guess in the next couple of months we're going to see rapidly increasing attention to the levee issue," Galloway said. "New Orleans says it's wake-up time."

But New Orleans was under water yesterday, and water from Lake Pontchartrain was still gushing through a breach in the 17th Street Canal levee about halfway between the historic French Quarter and the lake.

Homes and businesses in Jefferson Parish were inundated. But the Corps Engineers said the levee break does not threaten the entire city.

The city's levees, canals and pumps are divided into sectors that operate somewhat independently, "so if one floods, they don't all flood," said Edward J. Hecker, chief of the Corps of Engineers' homeland security office in Washington.

The break was not the "worst nightmare" many have feared. That would have been a break in the 27-foot levees that hold back the Mississippi, which flows by at a million cubic feet per second.

Even so, most of New Orleans was awash yesterday. Much of the flooding is accumulated rainwater, which could not be pumped out because the city's 22 pumps were overwhelmed by rain, then shut down by inundation or power failures.

Other nearby areas, such as St. Bernard Parish, are flooded by water that poured over the levees along the south shore of Lake Pontchartrain. Once the lake waters receded, that flooding stopped, federal officials said.

Levees fail for a number of reasons, Stockton said. They can be overtopped by rising water, eroded by the swiftly moving water they contain or become waterlogged to the point that the packed earth liquefies and slumps, opening a gap.

Water can also seep through weak spots. Sometimes the breach can be as mundane as a gopher hole. But the moving water picks up silt and clay as it goes, and gradually erodes and undermines the levee.

On the 17th Street Canal levee, a slab of concrete shaped like an upside-down "t" weakened in the storm, said Hecker.

"It's a flood wall embedded in the earthen embankment," he said. "It began to stress [Monday] and ... cantilevered out of the levee." Then the dirt around it washed out and collapsed.

The rushing water tore 4 to 6 feet into the embankment, then cascaded the remaining 10 to 15 feet to the street, said Jeffrey Jensen, flood control program manager for the corps at its Washington headquarters.

Engineers hope the water gushing through the levee will slow or stop once the water in Lake Pontchartrain recedes to pre-storm levels and below the level of the breach.

"It's not subsiding as much as we'd hoped," Jensen said.

The Corps' plan to dump rocks or containers of sand into the gap from the air is a variation on what Stockton described as the standard procedure for mending a levee.

"You typically go in and start dumping large rocks or boulders to slow the flow of water down, which will slow the amount of erosion taking place," he said.

Smaller rocks, gravel and sand can then be poured in to seal the break. Sometimes a temporary ring levee is built outside the breach, often on higher ground where access is easier and the water flow slower.

Access to a levee break can be a problem. Some levees are designed to support truck traffic, but not this one. Barges are better if they can be moved close enough without being drawn into the gap.

"We're working on whether we can get a barge up there," Hecker said. "That's difficult due to the relatively narrow width of the canal."

Once the leak is plugged, the corps will turn to pumping.

Baltimore Sun Articles
|
|
|
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.