Posted by: Michael Ritter PhD | June 19, 2008

2008 Midwest Floods of “epic proportions”

The upper Midwest of the United States has been devastated by flooding over the past few weeks due to a series of storm systems that deluged an already wet region with precipitation 400% above normal in some places. As of this writing, the flood waters have started to recede from parts of Iowa and are making their way down the Mississippi. Some are comparing this to “The Great Flood of 1993”, though many differences exist. In many cases the media has been referring to “100- or 500-year floods” occurring in particular communities. But what exactly is a “100-year flood”?

Some hydrologists cringe at the use of “100-year flood” as it may be misinterpreted. Hydrologists would rather describe an event with respect to its recurrence interval.  The recurrence interval  “is based on the probability that the given event will be equaled or exceeded in any given year. (USGS)” It is an estimate of the average time span between past occurrences of random events.

The equation for calculating the Recurrence Interval(RI) is:

RI = (n+1)/m

where n = number of years in the record, m = magnitude ranking

For example; a flood event that had a peak stream discharge of 89,800 cfs ranked as the 4th largest discharge over a 50 year record period.

Therefore the RI is:

RI = (50+1)/4 = 12.75 years meaning the likelihood of a discharge of that magnitude recurring within 12.75 years is fairly high.

100-year floods have a 1 percent chance of occurrence in any given year. So even though the chance is low, it is possible that a 100-year flood could occur in two consecutive years. Unfortunately, that’s what’s happening (as of this writing) to the residents of the small Wisconsin town of Gay’s Mills who experienced a 100-year flood both this year and last and are renewing talk of moving their town.

Table Courtesy USGS

Useful Links:

TPE Link: Floods and Flooding

Reference: USGS. Flood: Recurrence intervals and 100-year floods


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