It boggles my mind to try and think about what this would mean in Cincinnati alone.
Covington, KY sits behind large dikes and of course the Ohio River is made navigable only by use of dams.
The 1937 Cincinnati flood was major for this part of the world.
The flood plain on which much of Cincy is located has a small river running through the middle of it that the Ohio backs into during floods except for the flood gates. They pump the Mill Creek water over the flood gate in effect to prevent flooding.
It works. I have seen one or two rather large floods here in my life in Cincy but nothing that did much damage to infrastructure. Without all of the dikes, the Cincy flood plain would have to be abandoned.
That would be almost all of the built up urban area.