100-year flood

All you want to know about 100-year flood

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% flood, since it is a flood that has a 1% chance of being equaled or exceeded in any single year. 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.

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 of a certain-size flood occurring during any period can be calculated using PT = 1 – (1-Pf)n where PT is the probability of occurrence for the entire period; Pf is the probability of occurrence in any single year; and n is the number of years. Ten-year floods have a 10% chance of occurring in any given year (Pf =0.10); 500-year have a 0.2% chance of occurring in any given year (Pf =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.

References

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See also

Retrieved from "http://en.wikipedia.org/wiki/100-year_flood"

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