File this in the “if you don’t think climate change will cost money you’re crazy” department. In Duluth, where at least 770 homes are flood damaged, officials will now begin contemplating just how big to make their rebuilt stormwater system after last week’s flash flood.
The northern Minnesota city’s network of sewers, culverts, ditches and basins, in some places more than 100 years old, suffered “extensive damage all over the city,” said Eric Shaffer, Duluth’s chief engineer of utilities.
But building and rebuilding a sewer system these days means making an educated and possibly expensive guess on a changing climate. Many communities are studying what steps they might take to accommodate increasing precipitation, but for Duluth, it will be a full-immersion process.
“Duluth is maybe in the first wave of cities to adapt to climate change,” said University of Minnesota Extension climatologist Mark Seeley.
Climate scientists say increasing precipitation, particularly from intense thunderstorms, is a symptom of ongoing climate warming, because warm air holds more water vapor than cooler air.
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 Climatic Data Center. Last Tuesday and Wednesday’s Duluth rainfall, measuring from 7 to more than 10 inches across the city, was in some places nearly double what’s regarded as Duluth’s 1 percent-chance rainfall. That made it “next to impossible to plan for,” Shaffer said.
“An event of this magnitude in 24 hours cannot be handled no matter what system we design,” he said.
Duluth’s deluge came in the same one-week period in which Cannon Falls, in southern Minnesota, received 8.83 inches (on June 14) and 3.31 (on Monday). The 8.83 was the most ever recorded by a National Weather Service observer on a single June day in Minnesota. (The Duluth area rains fell overnight, thus on two calendar dates.)
But it’s the smaller, increasingly frequent downpours that cities now need to plan for, many climatologists and community leaders say. In Minnesota, the frequency of 2-inch rainfalls doubled across the state from 1991 to 2010 over the previous long-term rate, even in the north, where cooler weather generally tempers severe storms, Seeley said.
It’s probably time for all Midwestern communities to reassess whether their stormwater systems are adequate and where the greatest risks lie (emphasis mine).
Many of those [Twin Cities metro] communities, like Duluth, have wastewater systems designed for 100-year rainfall standards that were established in the Rainfall Frequency Atlas of the United States, published in 1961. An overhaul of those numbers is expected soon. Latham Stack, a consultant working with the Minnehaha Creek project, said expanding storm-water capacity more than two and a half times would not be extreme for most communities.