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Intense rainfalls are getting bigger and more frequent, causing local governments, engineers and landowners to rethink whether sewer systems and other drainage features are up to their tasks.
The storm water filtration pond near Cedar Lake in Minneapolis, for instance, needed to be cleaned out six years after it was built, instead of the 25 years designers expected.
"I'm just a guy at the end of the rainfall who has to deal with it," said Brian Wilson, a storm water policy analyst with the Minnesota Pollution Control Agency. "It really got my attention because these big storms seem to just be rolling through more and more frequently."
Researchers point directly at a warming climate as the cause. Warmer air holds more water vapor, and a longer storm season means intense storms can occur more frequently in what once were regarded as the off-seasons, said Jay Lawrimore, a climate scientist with the center. As more northerly latitudes have warmed, the storm-producing clashes between warm and cold air masses have moved northward as well, a key reason why the Upper Midwest has seen a steep increase in extreme precipitation, Lawrimore said.
The Upper Midwest saw a 31 percent increase in "intense" rainfalls -- the statistical 1 percent events -- from 1958 to 2007, over previous decades, according to the National Climactic Data Center. That was the second-highest increase among eight U.S. regions, including Alaska and Hawaii. New England and the Northeast saw a 67 percent increase.
Overall, intense doses of precipitation have become more frequent and more intense in recent decades than at any other time in the historical record and account for a larger percentage of total precipitation, according to a study by the U.S. Climate Change Science Program.
"The atmosphere is laden with more water vapor than it has been historically," University of Minnesota Extension climatologist Mark Seeley said. "I think we've transitioned into conditions where convection [severe storms] is the more dominant form of precipitation. We've seen flash floods in recent years in October, for crying out loud, when we should be seeing mild polar fronts across the state." Any explanation for the increase, other than climate change, would be "a stretch," Seeley said.
How much water in big storms?
Across Minnesota, there were seven 1 percent storms in the 1940s and 17 in the 2000s, though some of that apparent increase might be attributable to an increase in observers across the state, according to DNR climatologist Greg Spoden. In the Twin Cities, where a 1 percent storm brings 6 inches of rain in 24 hours, only two are on record: a 7.28-incher on Aug. 30, 1977, and a 9.15-incher on July 23, 1987.
But what amount of rainfall occurs 1 percent of the time these days? Is it still 6 inches, as it has been since a federal determination in 1961? Or is it 7 inches or 8 inches? It matters, Wilson said, because it's a figure that determines how public infrastructure -- dams, sewer systems, cities -- gets designed.
Wilson has spearheaded an effort to determine more exactly what a 1 percent storm might be these days. With $210,000 from the Environmental Trust Fund and the Minnesota Department of Transportation, Minnesota has joined 10 other Midwestern states in a $3 million study expected to result in new federal figures next year.
Those aren't the only public agencies reassessing rainfall risks. The Minnehaha Creek Watershed District has also launched an effort with local communities to develop new standards not only for floodwater management and flood mitigation, but for how new developments might be designed and located. That might include everything from the size of new sewer pipes to restrictions on impervious pavement. The issues also extend to water quality and habitat, Wilson noted, since heavy rain often carries street pollutants directly into streams and raises their temperature dramatically.
Latham Stack, a consultant on the Minnehaha Creek project, said communities ought to look for a variety of strategies, not just an expensive upgrade of a storm sewer system.
"If a 50- or 100-year storm happens every 10 years, that has some pretty major impacts on the ability of communities to absorb damages," Stack said.
Drought, floods can coexist
Meanwhile, recent weather in Minnesota is hinting that a drought that has persisted for about seven months across much of the state -- longer in the Arrowhead -- might be loosening its grip. But those kinds of variations don't mean the longer-term trends of increases in intense rainfall might also reverse quickly, Seeley said; excessive rain has been overpowering drought trends in recent decades.
At the same time, the classic climate warming scenario dictates that both excess rain and drought are likely simultaneously, as heat evaporates moisture from the land, then dumps it elsewhere in spotty, unpredictable, summer-storm patterns.
"It's all going to be about winners and losers," Seeley said.
Bill McAuliffe • 612-673-7646