WYSO

Commentary: Extreme Precipitation Events And The Miami Valley

Nov 13, 2017

Since the recent hurricanes in the gulf region and the Caribbean, University of Dayton professor Bob Brecha has turned his attention to how the Miami Valley might cope with severe flooding  and takes a look at research done by one of his University of Dayton colleagues for clues.

Extreme weather events were in the news quite a bit this summer, from brutal heat waves in southern Europe, to forest fires in the northwestern US, and maybe most noticeably, the incredible rainfalls brought about by Hurricane Harvey in Houston and the Gulf Coast region.

Harvey was a fairly powerful hurricane that was notable because it hovered over Houston for such a long period of time, and even circled around for a second hit. Some were calling Harvey’s flooding a “500-year event” or even a “1000-year event.”  We have to be careful about the language here. A thousand-year event only has about a tenth of a percent chance of happening in any given year – it doesn’t mean that this was the first time in a thousand years such flooding occurred.  We can’t know that for a young city like Houston.    The strange thing is that Houston has now had so-called 500-year floods for each of the past three years. That begins to sound like estimates for Houston are off somehow.

What about the Miami Valley?   How are we prepared for extreme rainfalls?

Those familiar with Dayton history will know that the great flood of 1913 was a defining event for the city.  However, it was not by any means the first time Dayton flooded – severe floods happened about every twenty years or so prior to 1913.  Political and civic leaders decided to find someone to apply the best science and engineering knowledge possible to prevent future disasters in the city.  Arthur Morgan spent a lot of time, including a trip to Europe, where weather records were much longer, to analyze how often big floods occur.  The result was the establishment of the Miami Conservancy District, and the series of five dry dams on rivers in the watershed of the Great Miami River. The dams are designed to protect against floods up to 40% larger than the one in 1913.  It’s fairly likely that careful engineering design by Morgan means that the full capacity of the retarding areas behind the dams will never be fully used.

One reason that rare floods seem to be coming more often in a place like Houston is that any reliance on historical data means that we are assuming that all of the background conditions remain the same.  But we know that climate change is altering ocean and air temperatures, and therefore increasing the amount of water held in the atmosphere.  If we don’t take those changes  into account, we can’t expect to get the frequency of extreme events right. My colleague Shuang-Ye Wu and other scientists have looked at the expected changes in extreme precipitation events under climate change, and find that the strongest storms will increase the most.  Shuang-Ye has also looked specifically at the Miami Valley and impacts from extreme precipitation. She’s found that there will likely be more extreme events, such as some of those we experienced this past summer and in other recent years, but the century-old flood control system designed by Arthur Morgan should handle even these changes relatively easily.  Of course, as she’s also pointed out, if we were ever to receive 40” of rain at once, all bets are off.  The rainfall leading to the 1913 flood was about 10”, falling on frozen ground.

There are other man-made factors that play into the vulnerability of cities, such as the lack or poor enforcement of zoning regulations, the removal of mangroves and wetlands and other natural buffering mechanisms, and increases in hard surfaces that cause runoff to be more rapid.   We’re lucky here in the Miami Valley that over a century ago, leaders had enough foresight to plan well for the future, and to respect the power of nature while doing so.

Bob Brecha is a professor of Physics and Renewable and Clean Energy at the University of Dayton, and Research Director at UD's Hanley Sustainability Institute.  Follow him on Twitter: @BobBrecha