Understanding Flood Odds
Recent flooding in the Midwest has been described as a ”500-year flood,” but just what does that mean? According to the U.S. Geological Survey, the occurrence of a 500-year flood doesn’t depend on what happened last year, fifteen years ago (the Great Flood of 1993), or even 100 years ago. Instead, the term “500-year flood” is based on the odds that the degree of flooding will occur. There is only a 0.2 percent chance (one in 500) that a 500-year flood will occur in a particular location in any given year; similarly, a 100-year flood has a one percent chance (one in 100) of occurring in a particular location in any given year.
Experts determine flooding odds by measuring annual peak-flow values with streamgages across the country and then using a model to determine the probability that a flood will occur. Long-term data from streamgages give scientists the information they need to understand floods and define areas that may be flood-prone.
(Sources: USGS. June 20, 2008. “Two 500-Year Floods Within 15 Years – What are the Odds.” http://www.usgs.gov/newsroom/docs/flooding_in_20080620.pdf)
