Katrina never hit New Orleans.
But its storm bands had a most disproportionate effect, as rising waters swamped the substandard levees, which once breached, could not be sealed until water had finally managed to reach its own level. Even then, pumping out the flooded districts was a pretty dicey proposition, as the damaged levees are still far from able to withstand another serious onslaught.
There is still a living core to the city. But it will be a LONG time before there sufficient infrastructure, economic recovery, and adequate housing for a city of 400,000 again.
You're assuming it will or even should be a city of 400,000 again....ever.
Clearly there has to be a city there for industrial and historical purposes. But, clearly that piece of land is not suitable for to support that many people. Not without some SERIOUS terraforming of some kind to get the whole thing above sea level (if that's even reasonable).