They could just declare anything less than fifty feet above the river as off limits to all humans. That would make it safe. Maybe...
Look how fast the rivers rose at the start of this video
Timelapse video showing the flooding of the Llano River in Texas on July 4th
https://rumble.com/v6vrq5d-timelapse-video-show-the-flooding-of-the-llano-river-in-texas-on-july-4th.html
The Japanese had ancient stone markers in the hillside that many ignored, and built below them. How many died during the tsunami of 2011 because they forgot the lessons of the past?