In a way, the city is an island, isolated from the rest of the nation.
If the water rises to the level of the surrounding sea, it's going to be impossible to pump out. Since New Orleans is (or WAS) below sea level, you'll have to pump the water up to get it out -- but first, you'll have to rebuild the dikes below the water level. You also have to have the electircity connected to the pumps, and water levels low enough to accept the water from Lake New Orleans.
Then there's the cleanup. And the toxins now deposited on the city.
New Orleans may no longer exist as a city. It may just exist as a memory in our minds, or a word on a plaque along the lines of "Here was the first American City to literally die, killed by Hurricane Katrina in 2005"