Taylor says that doesn’t even take into account situations where the elimination of a retention pond provides more buildable space. He recently did a project in Kentucky where eliminating the retention pond in favor of putting the stormwater under the streets allowed the developer to build an additional building.

“The engineer placed the value of the space at $400,000,” Taylor relates. “The developer said it was actually worth a million dollars to him, because he got to put a building on it.”

Cambridge IMG 0325Click photo to enlargeA just-completed project in Minneapolis won’t have quite that level of financial payback, but property owners in what’s referred to as the Marq2 district will be able to apply for stormwater utility credits from the city.

Marq2 (short for the Marquette Avenue and Second Avenue South Transit Project) incorporates approximately 15,000 ft² of pavers from Willow Creek Concrete Products in Oakdale, Minn., and installed by Glacial Ridge Inc., of Willmar, Minn.

Chris Behringer, a senior urban designer with S E H in Minneapolis says that city’s Stormwater Management Group asked the design firm to consider a structural-cell component for the project, and S E H suggested the use of the pavers.

Behringer, whose firm has been looking at permeable and pervious products for several years, says the pavers were a better option for parts of the sidewalk than porous concrete, which she says isn’t as aesthetically pleasing.

“Because this is in the sidewalk area, the bituminous asphalt really wasn’t an alternative,” she says. “And, because we were using colored concrete and plain concrete as well, the price difference was just about a wash.”

The pavers were hand-laid in the non-pedestrian area between 179 street trees, which will be the recipients of stormwater recovered from the sidewalk. In this case, the stormwater is retained in an underground grid of plastic-framed cells filled with a bioinfiltration soil mix after passing through a geotextile membrane and two layers of different-sized aggregates.

“We did work with the city’s head forester on making sure the tree species we picked would do well within the corridor,” Behringer says. “The salt put on the streets shouldn’t be a problem, as long as we have some good rainfalls to flush the system. And, the soil is going to remove the phosphorous and nitrogen and other minerals that come through the stormwater.”

Since the project was just completed late last fall, Behringer adds that it’s probably too early to get good feedback on how well the system deals with the up to 20,000 ft³ of water it’s supposed to handle with each rain.

“However, what we’re hearing from people is that the look of the transportation corridor is a big improvement,” she says. “The aesthetics of the improvements are very positive.”

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