Some call it unwatering. After Katrina it took 42 days to get 750 billion gallons of water out of the city. Some drained out naturally the rest was pumped out. The Corps patched the breaches in the levees and created some notches  to improve outflow. During this period most of the city's 174 drainage pumps were not operating so temporary pumps of much smaller capacity were brought in.

STUB

100 billion cubic feet of water.
1 square mile is 28 million square feet
Average depth was 5 feet. Did we flood 400 square miles or was there less water? Maybe I'll buy the 400 square miles idea. But the city is not 20 miles wide by 20 miles tall. Again I believe somebody exaggerated the amount of water in the city. Who cares if the amount was 100 billion or 25 billion cf? Why do authorities exaggerate?