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General Information Product 106

100-Year Flood–It's All About Chance

By Robert R. Holmes, Jr. and Karen Dinicola

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Introduction

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
703-648-5301

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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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