You bring up a point I can't figure out. From what I understand once the city floods, the water doesn't get out anytime soon (the guy from LSU said 90 days). Once the water level rises above the entry points of the Superdome how is anyone or thing going to get in or out? I would like to think somebody has thought of this, but you never know.
However Moose pointed out that the snakes also head for the highland too, along with rats, fireants, etc.
Boats.
Good and disturbing insight (as to how the heck to get in and out of dome if water seals entrances) . . .
Same thing applies in any building--even a private home: one can picture people inside their houses retreating to the upper floors as the water rises, until they are in their attics under their roofs.
Should the water continue to rise, they will need to break through their roofs to climb outside.
I have heard the authorities have been cautioning peoople to have a hammer or axe on hand for this reason.