News stories have said that Hurricane Katrina was a cat 4 and New Orleans levees were built to withstand a cat 3. However, Katrina made landfall quite a bit east of New Orleans, which meant that NOLA did not get cat 4 winds, rains, or flood surges. It turns out that the surge was 11 feet and the floodwalls were supposed to handle 14 feet.

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.
— Experts Say Faulty Levees Caused Much of Flooding By Michael Grunwald and Susan B. Glasser Washington Post Staff Writers Wednesday, September 21, 2005; Page A01