Originally Posted by wikipedia
A one-hundred-year flood is calculated to be the level of flood water expected to be equaled or exceeded every 100 years on average. The 100-year flood is more accurately referred to as the 1% annual exceedance probability flood, since it is a flood that has a 1% chance of being equaled or exceeded in any single year.[1] Similarly, a flood level expected to be equaled or exceeded every 10 years on average is known as a ten-year flood. Based on the expected flood water level, a predicted area of inundation can be mapped out. This floodplain map figures very importantly in building permits, environmental regulations, and flood insurance.
Probability
A 100-year flood has approximately a 63.4% chance of occurring in any 100-year period, not a 100 percent chance of occurring. The probability Pe that a certain-size flood occurring during any period will exceed the 100-yr flood threshold can be calculated using Pe = 1 – [1-(1/T)]n where T is the return period of a given storm threshold (e.g. 100-yr, 50-yr, 25-yr, and so forth), and n is the number of years. The exceedance probability Pe is also described as the natural, inherent, or hydrologic risk of failure.[2][3]
Ten-year floods have a 10% chance of occurring in any given year (Pe =0.10); 500-year have a 0.2% chance of occurring in any given year (Pe =0.002); etc. The percent chance of an X-year flood occurring in a single year can be calculated by dividing 100 by X.
The field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering. This theory is most commonly applied to the maximum or minimum observed stream flows of a given river. In desert areas where there are only ephemeral washes, this method is applied to the maximum observed rainfall over a given period of time (24-hours, 6-hours, or 3-hours). The extreme value analysis only considers the most extreme event observed in a given year. So, between the large spring runoff and a heavy summer rain storm, whichever resulted in more runoff would be considered the extreme event, while the smaller event would be ignored in the analysis (even though both may have been capable of causing terrible flooding in their own right).