Jonathan Rauch has an interesting article on the Cost-Benefit of the New Orleans levees. It would seem that the plan was to just abandon ship. Money Quote
Yet the most striking fact of the New Orleans catastrophe has received less notice than it deserves: The plan for New Orleans in case of a hit from a very powerful hurricane was to lose the city.
In other words, if a severe hurricane struck, the city’s flooding and abandonment was not what would happen if the plan failed. It was the plan.
New Orleans is built between a lake, a river, and the Gulf of Mexico, and it is lower than the surrounding waters. It was kept dry by an extensive system of levees and pumps. That system was itself contributing to the slow subsidence of the city.