I was wondering that, too. I've been looking at the photos on Fox and can't believe the mess! Just read an article on Fox that said they hope the water will seep out of the city at the rate of one half inch per hour. That's a foot a day for 20 feet of water. Wow! What kind of rot can happen in that time.
That's the rate it can be pumped out. It can't seep out, or reced naturally. Everything flooded is at the natural water level for that part of the planet.
The entire flooded area is inside a levee "ring", and once the levee system is closed, the water can be pumped out of it.