You raise a good point. Such mapping seems a basic necessity to any sort of emergency planning, especially flood evacuation planning. With such maps, and a reasonable estimate of how high the water will get, it should be possible to ID the most flood prone areas and evacuate them to local, higher grounds, instead of trying to evacuate masses of people hundreds of miles.

Where I live, there are detailed maps (by the Feds) of the 100 year flood plane. I think they exist for most of the country on-line. They certainly would be a good place to start the planning.