In THE PERFECT STORM: The Science Behind Subrogating Catastrophic Flood Losses, storm water expert witness Richard Van Bruggen writes:
In contrast to a “floodplain”, “floodways” are determined within the floodplain.
Any encroachment or development on floodplains reduces the flood carrying capacity of a river, increasing flood heights in adjacent areas. In order to limit floodplain development within a central channel area of a river where most of the flood water conveyance occurs, floodways are established. Usually, there is no development allowed in the floodway. Flood damage which occurs in a floodway presents opportunities for subrogation. However, one example to the contrary is in Sonoma County, California, where development in the floodway can exist but must have any net hydraulic effect on the conveyance of the river and homes must be on piers at a minimum level.