Such contingency plans are quite common, are they not? The water has to go somewhere, so better redirect it to the place where it causes the least amount of damage.
watch any video of rescue efforts in modern built areas. Some multi-story buildings with the right structure are fine, people with two story houses sit on their roofs, meanwhile, most cars go for a ride in the direction of the flow.. etc.
The Sacramento river, if flooded by say the dam failure of Oroville, could reach Stockton. If that happens the city would be a foot or two underwater.
Little does anyone know there are contingency plans to literally blow the levees along the river and flood the nearby (mostly) farming land.