Long-Term Payoffs

Spring 2014

Permeable pavers help Minneapolis handle stormwater and extend the life of urban trees

Elizabeth Millard

By

Long-Term Payoffs

Numerous surveys rank Minneapolis as one of America’s best places to live. Forbes magazine recently ranked the City of Lakes as the healthiest in the nation, in part because of its green spaces and walk-friendly downtown. So, when dealing with stormwater, it’s no surprise the City chose permeable interlocking concrete pavement that boasts a major benefit: extending the longevity of urban street trees. “This project was unique because the City wanted to treat rainfall where it fell while capturing nearby runoff, and it realized that tree health could benefit from that process,” says Bob Kost, Landscape Architect Director for Short Elliott Hendrickson (SEH), who worked on the project. “This was one of the first projects of its kind in the country, and I think other cities should consider this kind of system.”

New Vision

Named Marq2, the project extends along Minneapolis’ major downtown corridors of Marquette Avenue and Second Avenue South and covers about 15,000 sf (1,400 m2) along 48 blocks of city sidewalks. Marq2 rebuilt streetscapes, from building front to building front, widening sidewalk space and incorporating 190 new trees, public art and new transit shelters. Permeable pavers enabled a substantial reduction in stormwater and pollutant runoff on a mile-long stretch of downtown.

The stormwater retention system consists of an underground grid of nearly 11,000 plastic-framed cells filled with about 580 cf (16 m3) of a bioinfiltration soil mix. The cell groups, which resemble milk crates stacked on top of each other, provide pavement support while preventing soil compaction in order to maintain infiltration. Perforated pipes in the cells convey excess water out of the system. Based on research by Prince George’s County, MD, the filtration by the soil inside the cells removes 80 percent of the phosphorous; 60 percent of the nitrogen; and over 90 percent of the lead, copper, zinc and iron from the stormwater.

A grated cap on top of the cells is covered with geotextile, granite infiltration stone, followed by a layer of smaller granite bedding aggregate. Permeable pavers were placed over that, allowing runoff to enter the soil-filled cells beneath. Tree grates and guards were designed by a local artist and fit well with the natural gray-colored pavers. The grates allow water to filter down to the tree roots. The combined use of the permeable pavers and the bioinfiltration system can receive some 21,600 cf (610 m3) of stormwater from each rain event, keeping it from entering the Mississippi River.

“This system protects the waterway,” says Kost. “Many cities face this exact issue because plenty of them have old stormwater systems connected to sewer systems. Managing each of these is important if you want to protect your water resources.”

Role of Pavers

Utilizing the permeable interlocking concrete pavers led to greater sustainability, believes Chris Behringer, principal at Behringer Design, who worked as senior urban designer on the Marq2 project. She notes that the permeable pavers are becoming a regular part of planning for landscape architects because they allow for better stormwater management. “There’s a higher comfort level with pavers than with other permeable surfaces like porous concrete or asphalt,” she says. “The pavers help alleviate stress on the whole system because they disperse water in a larger capacity [than other options].”

That’s important not just for summer and fall rainstorms but also for spring thaws. Minneapolis receives an average of 45 in. (114 cm) of snowfall annually, and as the huge mounds of plowed snow melt, the permeable pavers make runoff management more effective.

Like any well-traveled surface, the pavers require maintenance, she adds, but even then, they seemed a much stronger choice than other options. Unlike asphalt, which would have to be cut out in a chunk, for example, the pavers can be replaced on a smaller scale, limiting disruption and saving on maintenance costs. “You mainly have to replace some of the infill as well as vacuum out some of the substrate to prevent clogging,” she says. “These are very affordable, minimal maintenance tasks, though.”

Leafy Return on Investment

Another key factor for improved ROI for the City of Minneapolis is just above pedestrians’ heads. Typically, urban trees need to be replaced about every five to seven years, Kost notes. Trees begin to decline in health due to soil compaction and/or limited availability of suitable soil, or a city might replace them to control irrigation costs. Because of Marq2’s innovative system, the mix of hardwoods and ornamentals planted in 2009 are still growing strong and aren’t up for replacement in the near future. The permeable interlocking concrete pavers and the system beneath prevent soil from compaction while stormwater draining through the permeable pavers significantly reduces the need for additional irrigation. That means the City saves money that would have been spent for watering. Also, each tree costs about $450, so extending the lives of all 190 trees means major savings.

Beyond those short-term savings lie longer-term benefits. As trees mature and expand their canopies, they provide more oxygen, urban island heat reduction and sidewalk shade. “You don’t get these benefits from younger trees; it’s only when trees reach a certain size,” Kost says. “Many cities are forced to replace trees just before they mature so they don’t reap these huge advantages.” By extending tree longevity and controlling stormwater management — with the help of permeable pavers — Minneapolis isn’t likely to lose its healthiest city title anytime soon. “This whole system is part of creating a healthier environment,” says Behringer. “It creates benefits for everyone.”

 

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