Even in flat country, there is almost always a place either above or away from the worst spots. A couple of small towns have been completely relocated after 2-3 floods. Obviously, many towns were built before data was available, and some chances must be taken.
What is needed is to gradually restrict RE-building after floods in the high-risk areas...and adding serious flood control projects where risks must be weighed against needs.

Yes...this implies similar restrictions about building expensive homes in fire-prone sites or on unstable hillsides.