The USGS Water Science School

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

by Robert R. Holmes, Jr.

## Can we have two 100-year floods close together?How accurate are estimates of the 1-percent Annual Exceedance Probability (AEP) Flood (also known as the 100-year flood)?

The "100-year flood" is an estimate of the long-term average recurrence interval, which does not mean that we really have 100 years between each flood of greater or equal magnitude. Floods happen irregularly. Consider the following: if we had 1,000 years of streamflow data, we would expect to see about 10 floods of equal or greater magnitude than the "100-year flood." These floods would not occur at 100 year intervals. In one part of the 1,000-year record it could be 15 or fewer years between "100-year floods," whereas in other parts, it could be 150 or more years between "100-year floods."

This graph shows the incidence of the 10-year flood for the Embarras River at Ste. Marie, IL (03345500). The variability in time between "10-year floods" ranges from 4 to as many as 28 years between floods. It shows how irregularly floods have occurred during the past 98 years on the Embarras River near Ste. Marie, IL. The magnitude of the 10-year flood has been determined through statistical analysis to be approximately 31,100 cubic feet per second (ft3/s). You can see from the graph that the actual interval between floods greater than this magnitude ranged from 4 to 28 years, but the average of these intervals is about 10 years.

Admittedly, use of such terms as the "100-year flood" can confuse or unintentionally mislead those unfamiliar with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval terminology. For example, one would discuss the "1-percent AEP flood" as opposed to the "100-year flood."