Technically, we've had plans for this for over 100 years, since Galveston was destroyed in 1900.
That said, the current, highly detailed set of plans for this scenario is only about 4 years old. However, we know that at least part of the plan works - see the recent Katrina shelter work. By the way, all the big Texas shelters are now almost empty - Reunion Arena was to be empty by today. So we're reset and ready to go there.
TXCN: People unable to get out of Houston who wish to do so should call 311 in the city or 713 837-0311 in outlying areas. Your evacuation will be arranged.
"Technically, we've had plans for this for over 100 years..."
yeah, but all the people who had them that long have all passed away.
Hmmm... I don't know that people in Houston are supposed to be evacuating until the people in the flood plain are out. Of course that doesn't mean they can stop you... I just found out there are no shelters in Houston. It's not considered safe.
But you can't evacuate 4.5 million people. Can't be done.