I would suggest that you seriously need to go and read in detail the reports by the American Society of Civil Engineers, Interagency Performance Evaluation Task Force (IPET) which was headed by the Army Corp of Engineers (who were obviously accused of lacking credibility), University of California at Berkeley, Louisiana Department of Transportation and others.
There was a total of 50 locations along the levees were problems occurred and of the just under 300 miles of levees, approximately 60% of it ended up being damaged. There was some overtopping but even in those cases, some levees continued to perform except where they were built on dirt or peat that was quick to erode. Like all major disasters, there wasn’t just ‘one thing’ that could be pointed to after the fact… there was a huge number of design flaws with respect to the poor reinforcement of the levees, the fact that much of it was built on substrata of incredibly low and unacceptable shear strength that had been grossly overestimated with regards to its strength, pilings that were not deep enough, levee sections that were not interlocked, lack of maintenance, a fundamental problem with the design of the I-walls (that had been clearly identified in 1984 as being a disaster waiting to happen) and on and on… The real problem was that there were four major levee breaks which occurred where the water level was still below its design elevation i.e. no overtopping required for failure. Some of those were located in neighbourhoods where the residents had been reporting the growing ‘marshes’ in their backyards for several years. It is not possible to summarize fat reports into a couple of concise conclusions but suffice it to say that when the Army Corp of Engineers accepts the responsibility for the mess, what you are obviously dealing with is a massive engineering failure.
I have seen the report. I also toured the catastrophe zones in March 2006. My understanding from the tour and presentation is that the Lower 9th Ward and Lakeview were inundated because the canal floodwalls failed. While there indeed were some levee failures in the network, the catastrophic flooding was due to floodwalls breaking.