Here is a little history of the levee system in New Orleans to put your image in perspective.
Until the early 20th century, the structures of the city were primarily built on higher ground. Flooding of the swampy areas was a frequent problem.
In 1910, the city began to pump water, with pumps designed by specifically for the purpose, into canals which drain into Lake Pontchartrain. The pump/canal system made a lot more land available for development.
Unfortunately, the removal of ground water combined with erosion has lowered the elevation of the city by about 9 feet in 300 years. The city is completely dependent upon the pump system to remove rainwater, and the city is experiencing multiple pump failures.
Thanks, that's interesting.