X-YEAR FLOOD — The magnitude of a flood which has a 1-in-X chance of being exceeded in any future one-year period. For example, a 2-year flood would have a 1-in-2 (50 percent) chance of exceedence in any one year; a 10-year flood, a 1-in-10 (10 percent) chance; a 100-year flood, a 1-in-100 (1 percent) chance, etc. These values are statistically derived, using past flood records. They are used for many reasons, but especially for engineering drainage and water supply structures. As the occurrence of floods is random in time, there is no guarantee that there will not be two X-year floods within a given year. There is also no guarantee that there will be an X-year flood in an X-year time period, or even in a 2X period. Finally, an X-Year, Y-Duration Rain will not necessarily produce an X-year flood. Storm duration and intensity, antecedent moisture and other conditions can cause X-year rains to produce more or less than X-year floods. For example, a 100-year, 6-hour rain over a very dry basin may only produce a 2-year flood, whereas a 5-year, 6-hour rain over a saturated or burned basin could cause a 100-year flood.

X-YEAR, Y-DURATION RAIN — The magnitude of rainfall which has a 1-in-X chance of being exceeded in any future one-year time period with a duration of Y [hours or days]. X-year rains must have durations associated with them; e.g., 25-year, 6-hour rain, 50-year, 24-hour rain, 100-year, 10-day rain, etc. These values are statistically derived using past rainfall records.