hundred-year flood

Flood magnitude which has a one chance in one hundred of being exceeded in any future one-year period. The occurrence of floods is assumed to be random in time, or a regularity of occurrence is implied. The exceeding of a one percent chance flood is no guarantee, therefore, that a similar size flood will not occur next week. The risk of epxerienceing a large flood within time periods longer than one year increases in a nonadditive fashion. For example, the risk of exceeding a one percent chance flood (i.e., a one hundred year flood) one or more times during a thirty-year period is 25 percent and during a seventy-year period is 50 percent. (Source: SRHNOA)