NEW ORLEANS, Sept. 20 -- 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.

In the weeks since Katrina drowned this low-lying city, there has been an intense focus on the chaotic government response to the flood. But Ivor van Heerden, the Hurricane Center's deputy director, said the real scandal of Katrina is the "catastrophic structural failure" of barriers that should have handled the hurricane with relative ease.

"We are absolutely convinced that those floodwalls were never overtopped," said van Heerden, who also runs LSU's Center for the Study of Public Health Impacts of Hurricanes.

In an interview Tuesday, Corps spokesman Paul Johnston said the agency still believes that storm surges overtopped the concrete floodwalls near the lake, then undermined the earthen levees on which they were perched, setting the stage for the breaches that emptied the lake into the city.

Johnston said the Corps intends to launch an investigation to make sure it is correct about that scenario. But he emphasized that Katrina was a Category 4 hurricane when it smashed into the Gulf Coast, whereas Congress authorized the Corps to protect New Orleans against a storm only up to Category 3. "The event exceeded the design," Johnston said.

The center's researchers agree that Katrina's initial surge from the southeast overwhelmed floodwalls along the New Orleans Industrial Canal, flooding the city's Lower Ninth Ward as well as St. Bernard Parish. They believe that a little-used Army Corps navigation canal known as the Mississippi River Gulf Outlet helped amplify that surge, although they acknowledge that this surge was larger than the system was designed to control.

But the researchers have strong evidence that Katrina's subsequent surge from the north was several feet shy of the height that would have been necessary to overtop the 17th Street and London Avenue floodwalls. It was the failures of those floodwalls that emptied the lake into the rest of the city, filling most of New Orleans like a soup bowl.

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.

Former representative Bob Livingston (R-La.), who helped lead the charge for Corps projects in Louisiana when he chaired the House Appropriations Committee, noted that the earthen levees along Lake Pontchartrain had all held, while the concrete floodwalls had failed. He was especially concerned about the 17th Street barrier, saying it "shouldn't have broken."

"I don't know if it's bad construction or bad design, but whoever the contractor is has a problem," said Livingston, now a lobbyist on Capitol Hill.

Former senator J. Bennett Johnston (D-La.) said he remembers numerous briefings from Corps officials about the danger of a hurricane overtopping the New Orleans levees. But he said he never envisioned a scenario like this one. "This came as a surprise," he said.

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."

Glasser reported from Washington.

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