The Corps of Engineers issued a report in 1979 that was incorporated into the law. The law says more flooding is OK if flood levels are not increased more than 1 foot. Displacing water in the floodplain forces the water to go elsewhere or can cause water to pool if the runoff route is obstructed. The pools retain flood water longer, causing even more damage. Both cities both want to build in the floodplain because it boosts the tax base and helps the real estate business.
Development means money and money talks.
There has to be compromise between no building in the floodplain and the present unlimited building in the floodplain. Clearly such activity should be severly limited for the health and safety of all. I don't think so. In fact, developers should be held liable for the damages they inflict on others but that will never happen. Health and safety regulations abound, nearly all fully supported by the public.