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Corps Scrambling to Plug New Orleans Floodwall Breaches

The day after Hurricane Katrina smashed into the Gulf Coast between New Orleans and Biloxi, leaving a path of death and destruction from New Orleans to Biloxi, the U.S. Army Corps of Engineers was scrambling to plug two floodwall breaches in New Orleans to stop water from Lake Ponchartrain from flowing into the city. Corps personnel from New Orleans, Memphis and St. Louis convened Aug. 30 with hydraulics engineers at Vicksburg District headquarters to confer with state and local officials and contractors on the ground in New Orleans to try to stem the inflow and close a 200-ft-long breach along the 17th St. Canal. "Water was pouring in from broken levees," says Adam Wine, a U.S. Coast Guard spokesman, who flew over the city in a helicopter Monday. "There are some areas where the water was 25-ft deep."

"We think the water overtopped the floodwall last night and scoured out the foundation, leading to a structural failure," says Al Naomi, senior manager and head of the Lake Ponchartrain and hurricane protection plan. The top of the concrete floodwall , which sits atop an earthen levee, was 14-15 ft above sea level at the point of failure, Naomi says. The city is 6 to 10 ft below sea level at that point. Heavy rainfall from Katrina elevated Lake Ponchartrain's flood stage to 5 ft above sea level, Naomi says.

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The New Orleans Sewerage and Water Board operates a vast drainage system designed to collect water during storm events and pump it from the city into the canal and on to the lake. "There's the biggest drainage pumping station in the world at the head of the canal, with many, many 1,000-cfs (cubic feet per second) pumps," Naomi says, "but because of the breach, it's shut down. So water is coming in from the lake to the city. We have to devise a plan to plug the breach and turn the pumps back on." By midday, Tuesday, local accounts were estimating that 80% of the city is flooded.

Downtown New Orleans was not submerged, Wine said, but most of the eastern portion of the Big easy is inundated. In many places, only overpasses break the surface. Elsewhere, he saw burning buildings, out of reach of fire department equipment. The lakefront airport was totally submerged, a ship was aground, and cargo containers were scattered randomly. "I could not believe the devastation," Wine said.

Corps engineers in Vicksburg were in contact with local officials and emergency contractors trying to devise the best plan to plug the breach and reverse the flow. "There are about 30 super-sized sandbags, 3 ft x 3 ft each, near the site,," says Naomi, "with more available nearby. It's very likely that the contractors will fill them and try to put them in place with heavy equipment to plug the hole. Then they would come in and drive sheet piles."

Repairs are also needed at a second breach, at the Inner Harbor Navigation Canal. Naomi says that the floodwall was overtopped there as well. The storm surge was reported at 17-20 ft in St. Bernard Parish, on the Gulf of Mexico south and east of the city, Naomi says. "It was 7 to 8 ft in Lake Ponchartrain," He adds. "That's preliminary data. Many of our gauges were washed away and we have to collate information from the ones still in operation."

Another structure, the Superdome's roof, was not structurally compromised by the loss of two roof panels blown off by the hurricane's winds, says Larry Griffis, president of the Structures Division of Walter P. Moore & Associates, Austin, Texas. Griffis says that wind uplift with pressure of 80 to 100 lb per sq ft appears to have torn off sheets of the single-ply membrane roofing, leaving two 4 x 20-ft holes, but the pressures and the breach did not affect the trusses making up the structure of the dome. He says the extraordinary winds apparently caused the failure of some weak welds. "The dome performed very well, near design loads of 100-mile winds and 200-mile gusts," he says.

Walter P. Moore & Associates was the engineer of record for the Houston Astrodome, whose roof was designed by Roof Structures Inc., the same company that designed the Superdome's roof. Griffis was well acquainted with Roof Structures' founder, Louis Bass, and thus became familiar with the company's patented lamella roof design. The Superdome's roof became a focus of controversy after a freak hailstorm in 1980 apparently caused it to leak. But Griffis says that roof's 25-mil poured-on Hypalon coating was self-healing. The roof was replaced only in 2000 with EPDM, which Griffis describes as an elastomeric roof with single-ply membrane mechanically fastened to the original metal deck.

As Katrina spills rain into the center of the country, the worst may be yet to come, says Robert B. Flowers, CEO of HNTB Federal Services Corp., Washington, D.C.-based engineering security consultant. Flowers, former chief of the Corps, headed the Mississippi River Commission, from 1995-97. The river drains about 40% of the continental U.S., a vast area stretching from the Rocky Mountains to the Appalachians, he points out. "If we get big rainfall up through the Ohio Valley, Kentucky and Tennessee, with water moving downriver for a week or so, we could be looking at a real situation. The longer we have water against the levees from one side and rising floodwaters against them from the other, the more we're looking at erosion and possible catastrophic failure."

Flowers says the loss of Louisiana wetlands over the past 200 years, as the Mississippi River levee system has been put in place, heightened and reinforced, has weakened a natural storm protection buffer zone. "Sediment is pushed out into the Gulf, instead of building up wetlands," he says. "It probably adds to the hypoxic or dead zone, and it certainly doesn't help with hurricane protection." Coast 2050, a wetlands restoration plan, backed by former Louisiana Sen. John Breaux, has helped somewhat, he says, by diverting water and sediment from the river to augment wetland areas. But much more needs to be done.

Naomi says that wetlands "didn't play a particular role in this storm," but he agrees that the storm protection system is overdue for an upgrade. The system is built to withstand a Category 3 hurricane, he says. "We need to build a system that can withstand a Category 5 storm. One preliminary plan would feature a 30-mile flood engineered structural flood control system, similar to those in the Netherlands. "It would cost $2 billion to $2.5 billion, which is a drop in the bucket, compared to what we're going to spend cleaning up after Katrina."

 

Andrew G. Wright and E. Michael Powers with Thomas F. Armistead
Courtesy Engineering News-Record

 

Related Links:

Katrina Floods Gulf Coast, Killing 55
FEMA Head: Katrina Was 'Catastrophic'
Louisiana Officials Fear Major Oil and Gas Corridor May Be Under Water

 

 

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