100 Year Flood

Mar 17, 03:45 PM

This is flood awareness week and the National Weather Service has some nice information on flooding that I would like to share. This first insert is on confusion regarding the frequency of 100 or 500 year floods. As you will read, the odds are very much against it but floods of that magnitude can occur in back to back years!

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.

Cedar Rapids residential area in the 500yr. flood of 2008

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.

Downtown Cedar Rapids submerged June 13th, 2008