Well now. It seems the levees broke at less than their claimed design strength. Possible reasons for that are incorrect engineering/fabrication, unforseen forces in the storm surge higher than the design strength, or CORRUPTION. As in cheatin' contractors putting inferior materials and/or workmanship on the job.
Given that we are talking about New Orleans, corruption seems the best bet. There's a contractor somewhere sweating bullets right about now, bet you a dollar.