FLOOD FREQUENCY

A statistical expression or measure of how often a hydrologic event of a given size or magnitude should, on an average, be equaled or exceeded. For example, a 50-year frequency flood (2 percent change of occurrence) should be equaled or exceeded, on the average, once in 50 years. Also see Hundred-Year Flood, X-Year Flood, and X-Year Flood, Y-Duration Rain.