If that stuff is ever truly condemned a level of government will pay them fair market rate (1), and there will be finagling to ensure that rate is appropriate. You forget that this is a country that has seen the mississippi flood its banks for three centuries, and rather than not develop these flood prone areas we opt to develop them, allow them to be destroyed, and offer federally subsidized flood insurance to rebuild them to be inevitably destroyed again in the future.
1. https://en.wikipedia.org/wiki/The_Sunken_City