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09/15/2005:
"Mystery Surrounds Floodwall Breaches"
One of the central mysteries emerging in the Hurricane Katrina disaster is why concrete floodwalls in three canals breached during the storm, causing much of the catastrophic flooding, while earthen hurricane levees surrounding the city remained intact.It probably will take months to investigate and make a conclusive determination about what happened, according to the Army Corps of Engineers. But two Louisiana State University scientists who have examined the breaches suggest that a structural flaw in the floodwalls might be to blame.
"Why did we have no hurricane levee failures but five separate places with floodwall failures?" asked Joseph Suhayda, a retired LSU coastal engineer who examined the breaches last week. "That suggests there may be something about floodwalls that makes them more susceptible to failure. Did (the storm) exceed design conditions? What were the conditions? What about the construction?"
Ivor Van Heerden, who uses computer models to study storm-surge dynamics for the LSU Hurricane Center, has said that fragmentary initial data indicate that Katrina's storm-surge heights in Lake Pontchartrain would not have been high enough to top the canal walls and that a "catastrophic structural failure" occurred in the floodwalls.
Corps project manager Al Naomi said that the Corps' working theory is that the floodwalls were well-constructed, but once topped they gave way after water scoured their interior sides, wearing away their earth-packed bases. But he said some other problem could have caused the breaches.
"They could have been overtopped. There could have been some structural failure. They could have been impacted by some type of debris," Naomi said. "I don't think it's right to make some type of judgment now. It's like presuming the reason for a plane crash without recovering the black box."
Officials long had warned about the danger of levees being topped by high water from a storm surge. Absent topping, floodwalls are supposed to remain intact.
The floodwalls lining New Orleans canals consist of concrete sections attached to steel sheet pile drilled deep into the earth, fortified by a concrete and earthen base. The sections are joined with a flexible, waterproof substance.
Floodwalls were breached in the 17th Street Canal, at two places in the London Avenue Canal, and at two places in the Industrial Canal, Suhayda said. Naomi said last week that one of the Industrial Canal breaches likely was caused by a loose barge that broke through it.
Suhayda said that his inspection of the debris from the 17th Street Canal breach suggests the wall simply gave way. "It looks to have been laterally pushed, not scoured in back with dirt being removed in pieces," he said. "You can see levee material, some distance pushed inside the floodwall area, like a bulldozer pushed it."
He suggested that because the walls failed in a few spots, the flaw may not be in the design but in the construction or materials.
"Those sections in the rest of the wall should have been subjected to the same forces as that section that failed," he said. "Why did one side fail, not the other side?"
Drainage canals typically are lined with floodwalls instead of the wider earthen levees that protect the lakefront because of a lack of space, engineers say.
"It's a right-of-way issue," Naomi said. "Usually, there are homes right up against the canal. You have to relocate five miles of homes (to build a levee), or you can build a floodwall."
Constructing a more expensive earthen levee also would require building farther out into the canal itself, reducing the size of the canal - and the volume of water it could handle.
Naomi said that an earthen levee also could have been breached if the surge had pushed water over the top. "A levee failure might be more gradual than with a floodwall," he said. "It means you may have flooded a little slower."
The central question for engineers investigating the breaches will be whether the floodwalls were topped - and that's still unclear.
The levee system, floodwalls included, is designed to protect against an average storm surge of 11.5 feet above sea level. The Corps adds several more feet of "freeboard" to account for waves and other dynamics.
Naomi said the Industrial Canal floodwalls were topped by water coming in from the east. But scientists don't yet know exactly whether Katrina's Lake Pontchartrain surge was high enough to go over the wall in the two other canals.
Many storm surge gauges stopped functioning during the storm, LSU climatologist Barry Keim, though initial data point to a mi-lake height of eight or nine feet. Heights typically are higher at the Lakefront area because wind pushes water higher against the levees.
Suhayda said the debris line on the lakefront levee adjacent to the canal was "several feet" below the top. The levees are 17 or 18 feet high in that area. The canal levees, however, average only 14 feet. Storm surges have waves and other dynamics that push water still higher than the average height.
"There are big implications for as little as a one-foot change in elevation" of the storm surge, Suhayda said.
If the water did not top the levees, the breaches could prove more mysterious. Typically, the pounding of wave action would be the most likely way to cause a breach, scientists say. But there isn't much wave action in canals.
"Waves constantly breaking on the structure start to erode it and make it become unstable," said LSU coastal geologist Greg Stone, who studies storm-surge dynamics. "But I don't think that was a major factor in the canals. You just don't have the (open area) to allow wave growth to occur."
nola.com
