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The USGS Water Science School

The 100-Year Flood—It's All About Chance (USGS)

by Robert R. Holmes, Jr.

 How accurate are 100-year flood estimates?
 Can we have two 100-year floods close together?

Flooding and USGS

Flooding and high water are of major interest not only here at the U.S. Geological Survey (USGS), but to most of the population of the United States. The USGS conducts research on the physical and statistical characteristics of flooding, estimating the probability of flooding at locations around the United States and attempting to understand how the frequency of flooding changes with urbanization, climate variability, and other factors The term "100-year flood" is often used to describe a flood of great magnitude .... but what exactly is a "100-year flood"?

Robert Holmes, USGS's National Flood Program Coordinator, offers up the following explanation of the 100-year flood that we can all understand. You can see Robert's full poster explaining the concept of the 100-year flood at

What is a Flood?

Picture of flooding in St. Louis, 1993. Credit: Srenco Photography, used with permission.A flood is any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. Floods occur for many reasons, such as long-lasting rainfall over a broad area, locally intense thunderstorm- generated rainfall, or rapid melting of a large snow pack with or without accompanying rainfall. Because floods result from many different circumstances, not all floods are equal in magnitude, duration, or effect. Placing floods in context allows society to address such issues as the risk to life and property, and to study and understand the environmental benefits of floods. Trying to place contextual framework around floods is where such terms as "100-year flood" came into being.

So what is a 100-year flood and how is it determined?

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".

Scientists and engineers frequently use statistical probability (chance) to put a context to floods and their occurrence. If the probability of a particular flood magnitude being equaled or exceeded is known, then risk can be assessed. To determine these probabilities all the annual peak streamflow values measured at a streamgage are examined. A streamgage is a location on a river where the height of the water and the quantity of flow (streamflow) are recorded. The U.S. Geological Survey (USGS) operates more than 7,500 streamgages nationwide (see map) that allow for assessment of the probability of floods. Examining all the annual peak streamflow values that occurred at a streamgage with time allows us to estimate the AEP for various flood magnitudes. For example, we can say there is a 1 in 100 chance that next year's flood will equal or exceed the 1-percent AEP flood.

More recently, people talk about larger floods, such as the "500-year flood," as tolerance for risk is reduced and increased protection from flooding is desired. The "500-year flood" corresponds to an AEP of 0.2-percent, which means a flood of that size or greater has a 0.2-percent chance (or 1 in 500 chance) of occurring in a given year.

 Sources and more information

Related topics:

Impervious surfaces and flooding  Rivers and sediment  Rain  100-year floods
Stormflows   Streamflow patterns  Measuring streamflow   Floods Q&A
A water monitoring site   High-water marks

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