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John Myers, Forum Communications, Published July 21 2012

NOAA report: Bigger, harder rainfalls the new norm

DULUTH, Minn. – Even back in the late ’90s, Bruce Wilson was seeing the concrete-and-asphalt examples of how a warmer, wetter climate was affecting Minnesota.

Wilson, a research scientist for the Minnesota Pollution Control Agency, and engineers were scratching their heads about why the stormwater retention ponds designed to help keep Minneapolis lakes clean were clogging so fast with street grit.

“We built them to last 25 years between cleanouts, but they were filling up after five years. We couldn’t figure it out until we looked at the rainfall data,’’ Wilson said. “The design was based on 50-year-old rainfall data without realizing we were in a whole new world now when it comes to rainfall.”

Heavier rains happening more often “were just power-washing the city’s streets more than we realized,’’ Wilson said. “We’re not just getting more rain, we’re getting bigger rainstorms.”

Wilson is part of a network of scientists and engineers across an 11-state region trying to explain the new reality.

“You can’t expect infrastructure based on 1960s engineering standards – which were based on rainfall data from 1930 to the 1950s – to hold up to what we’re seeing now,’’ he said. “We have to change the way we design and build things.”

Climate scientists say we are seeing more storm events like the one that caused flash flooding and widespread destruction this summer in and around Duluth, and they say it’s not just by chance.

While there’s been much discussion about a warming climate making heat waves and droughts more intense, like the summer of 2012, there’s been less public talk about how warmer air also holds more water vapor and is more unstable.

Data shows that up to 80 percent of the weather reporting stations across Minnesota are seeing significantly more rain falling than they did 50 years ago, said Mark Seeley, University of Minnesota Extension climatologist and a well-known expert on Minnesota climate issues.

And data shows the rain is coming down harder.

Our warmer, moister atmosphere is bringing fewer widespread gentle rainfalls and more gully-washers, Seeley said. And our gullies – our ditches, storm sewers, culverts, reservoirs, sewerage pumps and even bridges – weren’t designed to handle them.

“We used to average about 60 percent of our total precipitation from big thunderstorm events. Now that’s closing in on 75 percent,’’ Seeley said.

Seeley made another point: These aren’t projections. The change to bigger storms has already happened over the past 50 years. The next 50 years are expected to bring even more change, especially in northern latitudes.

Oddly enough, the increase in heavy rain thunderstorms with fewer region-wide rains is also spurring the opposite effect: More periods of drought in places thunderstorms miss.

“It’s problematic, not just from the amplified rainfall totals, but because with thunderstorms you end up getting more winners and losers,’’ Seeley said. “More heavy rain in one place and then nothing someplace else. Or maybe more heavy rain one month and not much for months after that.’’

That kind of volatility is producing huge swings in weather that weren’t the norm a half-century ago, Seeley said.

Windom, Minn., experienced its wettest May in recorded history this year, with 11 inches falling. That was followed by Windom’s driest June, with just 0.75 inches.

“When you depend on thunderstorms for your precipitation, it’s feast or famine,’’ Seeley said. “It’s like Mother Nature is saying: ‘You think you know how unpredictable I can be? You think you knew my old variability? Let me show you what I can do now.’ ”

Other scientists have described it as Mother Nature on steroids: Sure, she hit home runs before, but not this many.

The problem has been widespread. According to a U.S. Global Change Research Program’s report last year, the amount of rainfall in the heaviest 1 percent of storms – the 100-year rainfall – increased 31 percent on average across the Midwest from 1958 to 2007. But it shows up more in some places. Chicago has seen an 80 percent increase in what used to be the 100-year rainstorm, Wilson said.

Minnesotans have plenty of experience in recent years – Hokah, Roseau, Cannon Falls and, last month, Duluth – more record rainfalls of 8, 10, even 15 inches at a time, more costly damage and more photos of people driving boats down city streets.

Parts of Carlton, Pine and Douglas counties were hit with 7 inches or more rain last August, wiping out roads and bridges and causing mudslides. Some of the same areas got hit with that much rain, and damage, again in June – what used to be 100-year floods, twice in 10 months.

Designing with better data

A new study by the National Oceanic and Atmospheric Administration due in November will show rainfall data for thousands of weather-reporting stations across Minnesota, North Dakota, South Dakota, Wisconsin, Michigan, Iowa, Missouri, Colorado, Nebraska and Kansas.

Science and engineering experts from the PCA and Minnesota Department of Transportation are taking part in the effort that will update precipitation rainfall data to help refigure engineering of storm water systems and roads.

The new data “will show how many times a year should we expect a 1-inch rain or an 8-inch storm, and how do we size our runoff pipes to prevent flooded roads and homes?’’ Wilson said. “The project will recalibrate our expectations up to the 1,000-year storm using data from the past 50 years of rainfall records.”

The numbers already show, for example, that the so-called 1 percent-chance storm – the 100-year rain event for Minneapolis – used to be about 6 inches, Wilson said. Now, it’s 9.3 inches. And the 6-inch rain might be coming every 25 or 10 years.

“We need to get past this debating the cause and figure out how to adapt to the new climate we have,’’ Seeley said.

Jim Benning, Duluth’s director of Public Works, said city engineers will continue to build storm water sewers to easily handle the 10-year storm, “but if MnDOT says the 10-year storm has changed, we’ll be engineering to the new 10-year data.”

Benning said the city can’t afford to rebuild its entire infrastructure to handle 9-inch rainstorms, or even 5-inch storms, even if they are going to happen more often. And even if the city had the money to double the size of culverts, bridges and storm sewers, there isn’t space under the streets alongside all the other utilities, Benning noted.

“We’re looking at places where it makes the most sense to increase capacity, especially along natural streams where it bottlenecks at a bridge,’’ Benning said. “That’s where we had the most problems (in the June flood). If we can give that stream more space, that will help keep the stream in its place even if we get more of these bigger rains. That’s where we’re making changes.”

Diane Desotelle’s job at Minnesota SeaGrant in Duluth is specifically focused on climate change and helping communities adapt to the new realities. So far, she said, the effort is going slowly.

“It is going to take some time to incorporate adaptation into policy and procedures,’’ she said. The June flood and other extreme events may help nudge people toward change. But “the dollars are mostly being spent on just putting things back, because that’s what the funding covers. In some cases this is OK, but the conversations need to start as soon as we can.”

Cities will never be able to build infrastructure to fully handle a 10-inch rainstorm, but they might be able to adapt to more rain more often, Desotelle noted. The June storm was a tragedy, “but we hope to build on this, only because people are talking about climate change because of it.’’


John Myers is a reporter at the Duluth News Tribune


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