Mystery surrounds floodwall breaches

Times Picayune | September 14 2005

One of the central mysteries emerging in the Hurricane Katrina disaster is why concrete floo dwalls 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. (more...)