### Flood Awareness Week -- 1 in ??? year Flood Explained

Understanding Flood Recurrence Intervals

The terminology "1 in 100 year flood" is often misinterpreted and can be misleading.  Flood recurrence intervals such as a "100-year" or "500-year" do not represent the number of years between flood occurrences of an associated magnitude. Instead, they are actually a statistical average for recurrence every year.

Image Courtesy of State of Iowa/CAP

A better way to look at it is in terms of the chance that a flood of a particular magnitude could happen each year.  Then the "100-year flood" becomes the "1 in 100 chance flood" each year.  Or, looked at another way, a "1 in 100 chance flood" has a 1 percent chance of occurring in any given year.

 Recurrence intervals and probabilities of occurrences Recurrence interval, in years Probability of occurrence in any given year Percent chance of occurrence in any given year 500 1 in 500 0.2% 100 1 in 100 1% 50 1 in 50 2% 25 1 in 25 4% 10 1 in 10 10% 5 1 in 5 20% 2 1 in 2 50%

http://pubs.usgs.gov/fs/FS-229-96/

http://ga.water.usgs.gov/edu/100yearflood.html

Flood Safety Awareness Week: March 16-20, 2009

Governor Culver Proclaims March Flood Awareness Month in Iowa