One of the biggest tasks facing Duluth in the aftermath of last week's historic flash flooding will be repairing the city's 400-mile storm-water removal system.

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.

How big is big enough?

The task facing Duluth, with its combination of steep hills, clay-over-rock geology, and the need for an immediate fix -- is "daunting," said Jesse Schomberg, a coastal communities educator for Minnesota Sea Grant, a research and information enterprise funded by the National Oceanic and Atmospheric Administration (NOAA) and the University of Minnesota.

"The big question is: Do you build it the same way, or build it to somehow manage for bigger events, like we seem to be seeing more and more often?" Schomberg said. "But then the question is: How much bigger? That's something we don't really know yet."

In the Twin Cities metro

In the metro area, more than two dozen communities in the Minnehaha Creek Watershed District have embarked on a study, funded in part by NOAA, to brainstorm new stormwater management strategies -- bigger pipes, more absorbent surfaces, underground storage -- in the face of increasing precipitation.

Many of those 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.

In the aftermath of last week's storms, Shaffer wasn't ready to say what sort of strategy Duluth might take.

"We would like to make sure, where [structures] have been torn out, that we do put them back large enough to make sure they don't get ripped out again," he said. "We don't have the money to replace them every 25 years."

Bill McAuliffe • 612-673-7646