Where this has been found to be a very expensive mistake, you are now seeing communities buy out homeowners, demolish, and leave the areas empty. Other efforts can be very expensive, but worthwhile if they're protecting enough.

For example of the latter, the Winnipeg floodway is a huge ditch around Winnipeg, Manitoba, Canada. Built in the late '50s, it diverts the Red River around the city, reconnecting further north. It was seen as a huge boondoggle when being built, but has saved much of the city repeatedly since.