N.O. is below Lakefront. Any storm water within the metro area has to be pumped out. Once the surge breached the leaves, it was up to the roof tops and higher in lower areas.
I know how NOLA flooded...where...why. That doesn't change the surges. 12' at Lakefront and 16' (Est.) around Plaq. Parish. The reason it was up to the rooftops is...well...when you build a city in a bathtub and water starts coming over the side...you flood to your rooftops.