Swamped City Faces Reality as Levees Fail
...surge was reported at 17 to 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 Pontchartrain," he adds. "Thats preliminary data. Many of our gauges were washed away."
Another major structure damaged was the Superdome, whose roof membranereplaced in 2000was partially stripped away. It was not structurally compromised by the loss of two roof panels blown off by the winds, says Larry Griffis, long-span expert and president of the Structures Division of Walter P. Moore & Associates, Austin, Texas.
Griffis says 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 roof trusses. He says the extraordinary winds apparently caused some weak welds to fail. "The dome performed very well, near design loads of 100-mile winds and 200-mile gusts," he says.
Also mobilizing to provide engineering assistance to FEMA is joint venture team from DMJM H&N, Los Angeles, and Earth Tech, Long Beach, Calif., says Bill Quade, program director. Quade did similar duty after Ivan last year. "This could be the worst," he says. The team will make a preliminary damage assessment and try to get a ballpark figure for infrastructure damages. "Then well prioritize how to get utilities back up and roads back into shape," Quade says.
A great deal of critical infrastructure must be inspected. Near New Orleans, one key transportation link is the 32-mile causeway and twin span bridge of Interstate 10 across Lake Pontchartrain. Another is an nine-mile connector of LA 1 to Port Fourchon that carries 18% of the nations oil and gas supply. It was believed to be underwater. Ironically, the Louisiana Dept. of Transportation and Development was in the middle of investigating why two bids this July to replace-and elevate-the connector came in $100 million over engineers estimate of $154 million (ENR 7/18, p. 13). The new connector would be 22.5 ft high.
Investigators also were assessing the damage caused by a semi-submersible drilling platform that the storm winds rammed into the Cochran Bridge carrying U.S. Route 98 over the Mobile River in Mobile, Ala. The storm ripped the rig from its dry-dock moorings. As the water receded, the rig wedged itself against the bridges eastern pier, which is somewhat protected by a concrete pier dolphin.
"There is some damage to the bridge," says Tony Harris, special assistant to the director at the Alabama Dept. of Transportation. He says it will remain closed to traffic pending assessment. Dan Miller, structural engineer for the Mississippi Dept. of Transportation, also reported that the I-10 bridge over the Pascagoula River was struck by a barge. Damage has yet to be determined. "Theres a lot of flooding and a lot of damage in general," to highways and roads, says Miller.
The storm also will effect areas that it did not hit directly. Gowen Dischman, HNTB project manager for the $250-million U.S. 82 Greenville Bridge in Mississippi, says it was not damaged, but the hurricane "could pull valuable resources off our project for more dire construction needs."
In Louisiana, engineers say that the states long-term erosion problems have been exacerbated. Dominic Izzo, vice president at DMJM+Harris, says a $35-million plan to augment a barrier island in Terrabonne Bay with 400 acres of additional sands was in 95% design. "We dont know how much of the island is left now," he says. "We were going to introduce new material to provide a barrier that would have mitigated the effects of a storm going into the wetlands."
With three months left in the hurricane season and Katrina still spilling rain into the center of the country, there may be more bad news, 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.
Flowers, speaking hours before the levee failures in New Orleans, pointed out that the river drains about 40% of the continental U.S., a vast area stretching from the Rocky Mountains to the Appalachians. "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," he said. "The longer we have water against the levees from one side and rising floodwaters against them from the other, the more were looking at erosion and possible catastrophic failure." Flowers warning proved prescient.
Naomi says 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 proposal features a 30-mile, 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 were going to spend cleaning up after Katrina," says Naomi.
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