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What is a 100 year flood?


Photo of 1938 flood on South Boulder Creek showing the former dance hall at the Eldorado Springs Resort.
Copyright by the Denver Public Library. See BASIN Gallery for more information

The terms "10 year", "50 year", "100 year" and "500 year" floods are used to describe the estimated probability of a flood event happening in any given year. Their primary use is for determining flood insurance rates in flood hazard areas. Using historic weather and hydrograph data , experts derive the estimated rate of flow or discharge of a river or creek. A 10 year flood has a 10 percent probability of occurring in any given year, a 50 year event a 2% probabaility, a 100 year event a 1% probability, and a 500 year event a .2% probability. While unlikely, it is possible to have two 100 or even 500 year floods within years or months of each other.

Living in or near the foothills of the Rocky Mountains makes flash floods an extremely serious concern. It is not a matter of "if" a major flood will happen but rather when.

Following are links for more information on flash floods in the region.

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Last Page Update - Tuesday December 27, 2005