Hoping to prevent past mistakes during the rebuilding process, civil engineers gathered in New Orleans to determine how Hurricane Katrina breached the city's levees, flooding 80 percent of the Big Easy.

The central issue they grappled with: Did Katrina overwhelm the city's flood defenses with a torrent they weren't designed to contain? Or did faulty construction or maintenance cause them to burst open at water levels well within their capacity?

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Officials from the U.S. Army Corps of Engineers have said the barriers were never intended to withstand a storm as powerful as Katrina. Congress had instructed them to build a network of levees and floodwalls that could withstand a Category 3 storm similar to Hurricane Betsy, which flooded New Orleans in 1965. Katrina was a Category 4 hurricane when it hit, so it would be expected that floodwaters would pour over the levees.

That is evidently what happened on the east side of New Orleans, where an earthen levee was overwhelmed in numerous places by floodwaters surging in from the Gulf of Mexico. But farther west, along the 17th Street and London Avenue canals, engineers have seen few signs that the water ever got high enough to pour over the storm barriers.

Now, experts are asking if the levees failed because the floodwaters rose above them, or if they crumbled when the water was still well below their tops. The issue is critical because engineers do not want to repeat mistakes when they rebuild New Orleans' flood defenses.

Because taller levees need broader bases, the earthen mounds along the 17th Street and London Avenue canals are topped with concrete walls that are designed to increase the barriers' height without taking up land in adjacent neighborhoods. Up to 11 feet high, the walls are anchored to the ground by steel sheets driven into the earthen levee.

Immediately after Hurricane Katrina, Army Corps of Engineers officials hypothesized that parts of the wall had been undermined as the flood poured over them, cascading down the barrier's landward side like a waterfall. The force of the falling water would have scoured out dirt along the wall's foundation and undermined it.

Many of the levee breaches appear to have happened that way, said Raymond Seed, a University of California, Berkeley, civil engineer who headed the National Science Foundation team.

``That was a very common mode, and one lesson there is to prevent the erosion,'' he said. ``I anticipate a number of wall sections will be armored for that in the future.''

But Paul Kemp, a professor at the Louisiana State University School of the Coast and the Environment, and a number of colleagues are convinced that in many locations the water never reached the tops of floodwalls.

They appeal to evidence such as the bathtub ring-like high-water mark that can be seen in many places, indicating that the water never rose more than partially up the walls. They point out that the water never got over the levees along Lake Pontchartrain, where the walls are the same height.

Computer simulations of Katrina performed by researchers at the LSU Hurricane Center also suggest the water never rose high enough to pour over the walls, though in some places it could have gotten close.

A number of engineers suspect a process known as heaving undermined the floodwalls in the London Avenue and 17th Street canals. The pressure exerted by water in the canals would have squeezed soil out from underneath the floodwalls. In some places, Seed said, entire sections of levee embankment appear to have moved as much as 35 feet.

Washington Post:

Louisiana's top hurricane experts have rejected the official explanations for the floodwall collapses that inundated much of New Orleans, concluding that Hurricane Katrina's storm surges were much smaller than authorities have suggested and that the city's flood- protection system should have kept most of the city dry.

The Army Corps of Engineers has said that Katrina was just too massive for a system that was not intended to protect the city from a storm greater than a Category 3 hurricane, and that the floodwall failures near Lake Pontchartrain were caused by extraordinary surges that overtopped the walls.

But with the help of complex computer models and stark visual evidence, scientists and engineers at Louisiana State University's Hurricane Center have concluded that Katrina's surges did not come close to overtopping those barriers. That would make faulty design, inadequate construction or some combination of the two the likely cause of the breaching of the floodwalls along the 17th Street and London Avenue canals -- and the flooding of most of New Orleans.

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On a tour Tuesday, researchers showed numerous indications that Katrina's surge was not as tall as the lakefront's protections. They showed a "debris line" that indicates the top height of Katrina's waves was at least four feet below the crest of Lake Pontchartrain's levees. They also pointed out how the breached floodwalls near the lake showed no signs of overtopping -- no splattering of mud, no drip lines and no erosion at their bases. They contended that the pattern of destruction behind the breaches was consistent with a localized "pressure burst," rather than widespread overtopping.

The center has also completed a computerized "hindcast" of Katrina, which has confirmed the evidence before their eyes. Their model indicates that most of the surge around the lake and its nearby canals was less than 11 feet above sea level, and that none of it should have been greater than 13 feet. The Army Corps's flood-protection system for New Orleans was designed to handle surges of more than 14 feet above sea level.

"This should not have been a big deal for these floodwalls," said oceanographer G. Paul Kemp, a hurricane expert who runs LSU's Natural Systems Modeling Laboratory. "It should have been a modest challenge. There's no way this should have exceeded the capacity."

The center's researchers said it is too early to say whether the breaches were caused by poor design, faulty construction or some combination. But van Heerden said the floodwalls at issue -- massive concrete slabs mounted on steel sheet pilings -- looked more like the sound barriers found on major highways. He also suggested that the slabs should have been interlocked, and that the canals they were supposed to protect should have had floodgates to keep out water from the lake.

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The Corps has not identified the contractors who built the floodgates that failed; Paul Johnston said there will be a full investigation into the breaches.

Congress authorizes flood- control projects -- after receiving recommendations from the Corps -- and the Corps oversees their design and construction.

John M. Barry -- who criticized the Corps in "Rising Tide," a history of the Mississippi River flood of 1927 -- said that if Katrina did not exceed the design capacity of the New Orleans levees, the federal government may bear ultimate responsibility for this disaster.

"If this is true, then the loss of life and the devastation in much of New Orleans is no more a natural disaster than a surgeon killing a patient by failing to suture an artery would be a natural death," Barry said. "And that surgeon would be culpable."

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