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General Information Product 106

100-Year Flood–It's All About Chance

By Robert R. Holmes, Jr. and Karen Dinicola

Thumbnail of and link to report PDF (1 MB)


In the 1960's, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the "100-year flood". The term "100-year flood" is part of the national lexicon, but is often a source of confusion by those not familiar with flood science and statistics. This poster is an attempt to explain the concept, probabilistic nature, and inherent uncertainties of the "100-year flood" to the layman.

First posted April 15, 2010

For additional information contact:
Office of Surface Water
415 National Center
Reston, Virginia 20192

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Suggested citation:

Holmes, R.R., Jr., Dinicola, K., 2010, 100-Year flood–it's all about chance: U.S. Geological Survey General Information Product 106, 1 p.

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