If Katrina moves westward even a little from its current track the carnage will be even worse. Should that happen, the storm likely will approach New Orleans a way that its surge would push water into Lake Pontchartrain, which is where the city’s dozens of pumps send overflow water to.
If that happens, and the water in the city has nowhere to go, New Orleans will fill up like a fish bowl.
According to this LSU storm surge map, that may happen anyway (pdf)