How often does a 100-year flood occur?
Large floods are often given designations as a "one-hundred-year flood" but a 100-year flood does not mean that such a flood occurs once every 100 years; instead it means that there is a one in one-hundred (or 1%) chance of such a flood occurring in a given year.Two 100-year floods could occur a year apart or even a month apart -- it all depends on how much rain is falling or how quickly the snow melts. A "20-year flood" has a one in twenty (or 5%) chance of occurring in a particular year so it would be a less destructive flood than a 100-year flood while a "500-year flood" has a one in 500 (0.2%) chance of occurring so it would be much more catastrophic than a 100-year flood.
