next question: How do you find these people? Especially invalids and the elderly who can't climb onto a roof?
I suppose the people living in areas where they were actually able to loot would survive.
That's the heartbreak of all this. They can't do a "house to house" knock on the door to see who's home. The only way to find them is to drop someone from a chopper onto every roof in the city, chop a hole in the roof, and holler "anybody here?" That would take months.
Anybody still inside and not visible from the air won't be found until the waters receed, and then... well.