Faulty Levees

WaPo reported 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.... 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

And Louisiana's history of corruption at all levels of government is probably the cause of the inadequate construction (too many kickbacks to be able to do a decent job)

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.

The foolishness of building a city with major parts of it 7 to 15 feet below sea level, with major bodies of water on all sides, certainly contributed.