All of them. New Orleans did not, of course, begin as a large city when the French established a trading outpost near the mouth of the Mississippi. I'm ignorant of the period in which New Orleans acquired "large city" status.
Let's begin with that point in time when we first knew for certain that the levees were inadequate to perform the function for which they'd been constructed, and work our way forward from there. Perhaps we will learn enough to do better when, for example, the "Big One" hits the Los Angeles area.
Let's enumerate the lessons (and there are /so/ many) without consideration of the political entities involved. The lessons to be learned won't be made any more clear by allowing their examination to devolve into finger pointing.
There are a lot of lessons (at least hundreds!) to be learned, and one of them is that when major interests are in conflict, /someone's/ ox is going to be gored, and wanting or not wanting that to happen is not necessarily going to affect what happens to the ox.
Another is that as long as we see life as a zero sum game, relying on stakeholders to prevent ox-gorings is only wishful thinking. Some stakeholders delight in seeing others' oxes gored.
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA