From a report/document published on 8/24/06, Civil and Environmental Engineering Department, UC, Berkeley:

In its final report published on July 31, 2006, the team (of 38 engineers and investigators led by Professor Raymond Seed) concluded that the several dozen levee failures in this catastrophic event occurred for a number of reasons, including the choice of materials used in the levee construction, the challenging geology and unstable soils upon which they were built, efforts to achieve economic savings at the expense of reduced margins of safety and engineering lapses associated with failure to anticipate critical failure modes and mechanisms specific to some of the failure sites. Their research indicated that a majority of the levees failed primarily as a result of human error, and not because Hurricane Katrina was an exceptionaly large hurricane.

To read the entire report, go to:
www.ce.berkeley.edu/news/view.php?item=47