Answer:
The city is below sea level.
The city cannot quickly remove water.
Explanation:
Water flows from the place with higher elevation to the place with lower elevation. This make the water from the ocean can easily flowed into new Orleans every time they experience a heavy rain and caused the flood.
One way to deal with this is to created a separate container /dams within the city to remove the water. (they need to find away to relocate the water that flowed in to these container).
New Orleans hasn't develop a good enough mechanism to do this. Making the amount of water that flow in due to the difference of elevation come in at a much faster rate that the city able to remove.