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October 13, 2016

An assessment of the flooding that occurred in the Meramec River Basin from December 2015–January 2016 is available in a new report from the U.S. Geological Survey. The report includes peak stages and streamflows, historical comparisons and flood-frequency statistics from the record flood.

Results show there is a 0.52 percent chance that a streamflow of the same magnitude occurring in December 2015 (64,000 cubic feet per second) on the Bourbeuse River at Union could happen at the same location in any given year. For the magnitude of flooding that occurred on the Meramec River near Eureka in December 2015 (162,000 cfs), there is a 1.1 percent chance each year of a repeat flood there. These results convey the probability that the streamflow of a certain magnitude will be equaled or exceeded in any year, also known as the annual exceedance probability. The AEP has recently become the standard in referring to flood-frequency estimates because it clearly conveys the probability of a flood occurring during any given year.

“Understanding the probability of the New Year’s 2016 event is extremely useful for emergency managers and city planners in proactively planning and mitigating for future floods,” said USGS National Flood Coordinator, Robert Holmes. “This flood frequency analysis is possible only because of the USGS long-term monitoring of rivers in the Meramec River Basin, with some sites having more than 100 years of continuous data.”

In the past, flood-frequency estimates were presented in terms of the recurrence interval, which represents the average number of years between occurrences of a streamflow of equal or greater magnitude. This term, however, often results in a misunderstanding that a 100-year flood, once it has occurred, will not occur again for another 99 years, which is not necessarily true. A 100-year flood streamflow, for example, is the same as a streamflow having a 0.01 AEP, or a flood streamflow with a 1 percent chance of occurring in any year. For the Bourbeuse River, the December 2015 flooding at Union had a recurrence interval of 191 years and the Meramec River near Eureka showed a recurrence interval of 91 years.

Record water levels were observed at some streamgages on the lower Meramec and Bourbeuse River basins. No record streamflows were observed, but levels were generally ranked in the top five.

For more information on water resources in Missouri, visit the USGS Missouri Water Science Center website.


Photo of Flooding on Mississippi River in December 2015
USGS scientist Chris Rowden drives a research vessel measuring streamflow alongside another field crew on the Mississippi River at St. Louis.Public domain
Photo of Flooding on Mississippi River in December 2015
USGS crews had two research vessels out measuring streamflow on the Mississippi River at St. Louis on New Year's Eve, 2015. USGS scientists Eric Looper and Jason Carron are one of many USGS field crews out in the floodwaters over the holiday.
Photo of Flooding on Mississippi River in December 2015
USGS scientists Chris Rowden, Larry Buschmann and Bob Holmes were on the Mississippi River at St. Louis taking streamflow measurements on New Year's Eve, 2015. This information is critical to the National Weather Service, the U.S. Army Corps of Engineers and emergency managers in making flood predictions and response.

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