A study of 26 urbanized areas in the United States indicates that the amount of urban area in flood plains ranges from 2.4 percent for Spokane, Wash., to 81 percent for Monroe, La. The median value is 10.5 percent, and the weighted average is 16.2 percent.
The amount of development on these flood plains also varies widely, from 11.3 percent for Lorain-Elyria, Ohio, to 97 percent for Great Falls, Mont. The median value is 57 percent, and the weighted average is 52.8 percent. Attempts to correlate either the extent of urban area in flood plain or amount of development with three readily available indices: depth of flooding, precipitation, and physiography-showed no strong relationships. Some correlation was found between the extent of urban area in flood plain and index of depth of flooding.