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".
Photo by Geff Hinds, Tacoma News Tribune Flood designations are based on statistical averages, not on the number of years between big floods. The estimates are only as good as the available data. Flood designations are updated as more data are collected or when the conditions change in a river basin. The "100-Year Flood" BIG FLOODS COULD HAPPEN AGAIN IN WASHINGTON DURING ANY YEAR Rivers across the Nation seem to be rising to record flood levels almost every year. In Washington, more than one 100-year flood has happened on a few rivers in just the past several years. How can 100-year floods happen so often? WHY DON'T THESE FLOODS HAPPEN EVERY 100 YEARS?
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