I looked at the NOAA / USGS Data and at least to this engineer, it seems that the storm surge at NO was probably between 3 and 6 feet. Based on what I've heard about the levee system, I don't think that would be enough to cause the levee failure (actually topping before failure).
My conclusion regarding the storm surge is based on an interpretation of water level data from the USGS gauging stations on the Mississippi River (at Norcross, LA) and tidal gauging at Grand Isle, LA, West Pass, LA and Waveland, MS. The Grand Isle and West Pass gauging stations which were closer to the eye than NOLA, being located southeast thereof, saw water about 6 feet higher than the mean high tide level. Waveland, which is located near Bay of St. Louis / Gulfport, and near to Katrina's 3rd landfall, had water about 8 feet above the mean high tide level before the gauging station stopped recording at about 9 AM last Monday.
More beneath the fold.
The raw data is here
. Other data there for the perusing is historical data on sustained wind speeds, gusts and other meterological stuff.
I haven't seen anyone talking with scientific certainty about the storm surge in NOLA, so I looked around a little bit. It wouldn't surprise me to find out when Katrina is really analyzed that it was the copious precipitation coupled with the back end southerly/ southwesterly winds, not the storm surge, that caused the levee to be overtopped. Once the earthen levee was overtopped it was only a matter of time before scour and erosion would cause a general, larger structural failure.
If this is in deed what happened, the blame for the flooding of NO will fall at the feet of developers / oil companies who destroyed the drainage basin's wetlands in pursuit of the all mighty dollar. And of course there is also blame for the policymakers who let wetlands be destroyed to appease the developers who sweet talked them with a fool's promise of progress.