I'm not sure that people are "surprised" that it floods, I think it is just a "surprise" when "the big one / 100 year flood / etc." actually happens.
We all have become accustomed to the tornado warning / flash flood warning and then nothing of significance happens, or at least it doesn't impact our little universe. Unfortunately, every once in a while the "worst case" happens. We all know it is possible, but we get lulled to complacency and then hit in the face with a shovel when it happens. The reaction probably looks alot like surprise. Often times something like this will lead to an "overreaction" (some would say appropriate response) to the next threat ... Think the Houston evacuation clusterfuck for Hurricane Rita after Katrina destroyed New Orleans. Then when "the worst" doesn't happen we go back to the ignore / assume things will be ok part of the cycle until Hurricane Harvey hits at which point the vigilance returns. Wash, rinse, repeat. Hopefully at least some progress will be made in each cycle to make the next "worst case" event less deadly / traumatic.
As an aside, I'll bet the Taco Bell in Belton could tell some stories.