Ed Wenk, a long-time student of how technology can experience catastrophic failures, has an interesting op ed. on what went wrong with Katrina:
Summarizing the Berkeley report on the New Orleans levee failure, to which he contributed, he says
The Berkeley team found that all modes of failure were preventable at modest additional cost, that the safety margin of 1.3 was too low, that the storm intensity assumed in design was too low and driven by political, not engineering, processes. That piecemeal funding led to piecemeal construction. Looking back, the system was a failure waiting to happen.
The Katrina disaster hatches an imperative we dare not ignore. We can learn from its lessons of human failure how to cope more successfully with extremes of weather. Otherwise, nature will find the flaw.
Or, to put it more succinctly: be afraid. Be very afraid.