[Climate change will often bring changes in weather patterns that may add stress to already-aging infrastructure - see the December 2007 report When it rains, it pours. Creativity will be required to solve these problems with limited resources; solutions such as porous pavements and other pervious surfaces, green roofs, promoting rain gardens and other runoff mitigation techniques may be the most useful tools in the toolkit. This article is an EXCERPT from a longer article, which you can read here. -Ed.]

A threat builds deep beneath the Twin Cities

By David Shaffer

Deep beneath the Twin Cities, aging storm water tunnels are splitting apart under the pressure of heavy rains, posing a risk of collapse that could flood streets and buildings above.

Minneapolis' 15 miles of tunnels -- some more than 100 years old -- need $75 million in repairs to prevent such failures, according to engineers studying the problem.

The tunnels, mostly built with unreinforced concrete or masonry, collect storm water from streets and rooftops and take it to the Mississippi River. Increased urban runoff and heavy rains frequently fill some tunnels to capacity, creating pressures they weren't designed to handle.

In many places, excessive water pressure has cracked or burst tunnel walls, undermining them, Krumm and other engineers said. If such a tunnel collapsed, the falling debris could block it, backing up storm water and flooding the neighborhood that the tunnel is supposed to protect, engineers said. Most of the water comes from city streets, filling the tunnel to capacity and creating enormous pressure.

Minneapolis officials concede that tunnel maintenance has been a low priority but insist that's changing. "I think the way folks managed in the past was in the reactive mode," said Rhonda Rae, who was appointed director of the city's storm water programs last year. "The checkbook's getting smaller, and as things get older they need more maintenance. So where do we put our priorities?"

Photo credit: Dan Thompson