It's called contributory negligence.
Oy, vey. No, dear, it's not.
For one thing, in Louisiana, they don't have contributory negligence, they have comparative negligence.
Which means, even if you're only partially at fault, you still have to pay.
Further, in Louisiana, the law isn't caveat emptor, because it's not a common law state. There is strict liability for these type of cases. Maybe even absolute liability, I'd have to do more research.
But no matter what, contributory negligence wouldn't be a bar to recovery for an improperly constructed floodwall, even in a common law state, because the people who were flooded out weren't negligent. Imprudent, maybe, but not negligent.