By the time the dome opened as a "last resort" shelter, it was far too late to bus anyone out.

Not to mention insurance issues, the fact that even 100 busses wouldn't have gotten a quarter of those ultimately headed for the Superdome out.

It's pretty easy to armchair quarterback what could've been done, but the fact remains, as late as the mandatory evac was, the NHC had NO in the cone for quite some time, and anyone living on a coastal state should know better than to not monitor the news when there's a named storm (or even better, to not ignore the news between June and November).

*EDIT

Storm surge moves ashore just forward of the eye at approximately the forward speed of the storm itself. Due to pressure gradients close to the eye, there tends to be a sudden build up of the most intense levels, but it's still more or less being "pushed" by the storm itself.