Downtown Minneapolis adds stormwater retention

MINNEAPOLIS — Permeable pavers are helping to reduce stormwater and pollutant runoff from a mile-long stretch of two streets in the downtown business district of Minneapolis. About 15,000 square feet of permeable pavers were installed as part of the Marquette Avenue and Second Avenue South Transit Project (Marq2), which runs through the heart of downtown.

Interlocking concrete permeable pavers permit stormwater to drain through aggregate-filled voids between the pavers into subsurface detention areas, where it’s directed through a series of natural filtration processes before gradually exiting the system.

The entire Marq2 project reduces stormwater runoff from nearly 5.5 acres. Marquette and Second Avenues, which run parallel to one another, were rebuilt from building front to building front on both sides over 12 blocks. The new streetscape includes wider sidewalks paved in part with permeable pavers, new transit shelters, public art, and 190 new trees.

The trees are planted in a drainage system designed to collect the stormwater runoff that percolates through the permeable pavers and hold it underground rather than divert it to storm sewers that discharge directly into the Mississippi River.

The stormwater retention system, made by DeepRoot, comprises an underground grid of nearly 11,000 plastic-framed cells each filled with about 580 cubic feet of a bioinfiltration soil mix. A grated cap is placed over the top of the filled cells and covered with a geotextile membrane. A layer of granite infiltration stone, about 2 inches to 3 inches in diameter, is placed on top of the geotextile, followed by a layer of smaller granite bedding aggregate. Willow Creek Brickstone permeable pavers are laid on top of the aggregate, allowing runoff to drain through the aggregate-filled voids into the soil-filled chambers below.

The trees, a mix of hardwoods and ornamentals, are planted in the soil mix and surrounded by iron grates. The stormwater that drains through the permeable pavers and grates not only reduces runoff to the river, it also precludes the need for irrigation. Each cell can hold as much as 116 cubic feet of stormwater in the uncompacted soil mix around the roots of the trees. Water slowly filters out from the cells naturally and through perforated pipe.

Chris Behringer, senior urban designer with SEH in Minneapolis, said permeable pavers are an excellent site solution for many municipalities concerned about stormwater runoff. “As landscape architects, we’re very interested in sustainability and best management practices,” Behringer said. “Because stormwater management is such a huge issue, permeable pavers are becoming a regular part of our process in determining what we can do to infiltrate water rather than just drain it off. And,” she added, “there’s a higher comfort level with pavers than there are with other permeable surfaces like porous concrete or asphalt.”

Through the combined use of permeable pavers and the bioinfiltration system, as much as 21,600 cubic feet of stormwater from each rain event will be stored and kept from draining into the Mississippi River. The system’s filtration process through the soil is expected to remove more than 80 percent of the phosphorus, 60 percent of total Kjeldahl nitrogen, and more than 90 percent of the lead, copper, zinc, and iron from the stormwater.

Property owners in the Marq2 district will reap financial as well as environmental benefits. The city of Minneapolis assesses a stormwater utility fee on all properties, including nonprofits and government buildings, to pay for the costs of stormwater management. Property owners in the project area can apply for Stormwater Utility Credits, which will reduce their stormwater bill by half.