Ivor van Heerden and others from LSU's Hurricane Center now say that the flooding of New Orleans was caused by either faulty design or fault construction of the floodwalls, rather than overtopping or other explanations: "Experts Say Faulty Levees Caused Much of Flooding". (Note that the WaPo's headline is confused: they aren't talking about the levees).
LSU has constructed computer modeling showing that the surges did not overtop the floodwalls:
...[Van Heerden] 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, [Army Corps of Engineers] 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.
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."