If you look at a cross sectional profile of the city, you will see the it is sort of bowl shaped with a few small ridges through it. Most of the ridges are the result of natural levees formed by the Mississippi River and bayous. Some of the bayous are now gone, both from natural and manmade reasons. The ridges served as early settlements for the Choctaw as well as Europeans. The ridges were also important routes from Lake Pontchartrain to the Mississippi River. Some of the present city was built in filled in areas that used to be in open water.
One of the major concerns with a large hurricane hitting the city would be that if the water tops the current levees, the bowl shape of the city would trap the water. It would take months the pump the water out once they get the pumps working again.