Thank you very much.
Following up on Mr. Gerretsen's comments, what usually happens in my area is that it backs up. It doesn't back up into a city; it backs up onto farmland. The farmland isn't seen as being valuable as cities are. As a result, much of the farmland gets flooded to protect the ones downstream.
I'm interested in how flood plain mapping is done. Is it done by a model or by actual events?