Like many older downtowns, Minneapolis utilizes a combined sewer over ow (CSO) system to handle large storm events, discharging contaminated water directly into the Mississippi River when it exceeds capacity. Today, to protect the river, Minneapolis requires landowners to disconnect all rain leaders (gutters that collect or funnel rainwater) from the sanitary sewer system and to explore every opportunity to slow the rate of runoff, reduce the total volume and improve water quality. To this effect, Minneapolis implemented a stormwater utility fee to both incentivize and add consequences to this policy.
Mitigating stormwater runoff, therefore, is one major goal for the City. “We have long had capacity problems with stormwater management downtown,” said Lois Eberhart, the Water Resources Administrator for the City of Minneapolis. “We needed to nd a new way of dealing with stormwater.” Seeking solutions that would prevent the system from over owing, project designers at SEH and URS chose the Silva Cell integrated tree and stormwater management system as a natural t to meet the City’s goals.
The Marquette and 2nd Avenue (MARQ2) busway project covers 48 blocks of downtown Minneapolis in a mixed-use stretch of town that is a transit-way streetscape renovation. A portion of the impervious sidewalks are being replaced with pervious pavement, which allow for in ltration and ltration of stormwater within the Silva Cell system. Currently the project is only collecting runoff from about 1.1 acres, but it has the potential to store the 1” rain fall event from a 5.7 acre watershed. Water enters the Silva Cell system through pervious pavers and tree pit openings.
Project engineers and landscape architects designed the streetscape to route stormwater to the soil in the Silva Cells to eliminate an irrigation system and reduce runoff. The Silva Cell groups have perforated piping to convey excess water out of the system. This water is rst ltered through aggregate and 3’-4’ (.9 m – 1.2 m) of bioin ltration media in the Silva Cells. Based upon data values from research done by Prince Georges County in Maryland, the ltration offered by the soil within the Silva Cells will remove over 80% of Phosphorous, 60% Total Kjeldahl Nitrogen, and over 90% of heavy metals such as Lead, Copper, Zinc and Iron.
“We were immediately attracted to Silva Cell because of its holistic nature of providing heavy-duty structural pavement support, stormwater treatment and the horticultural bene ts of highly accessible soil volumes,” says Bob Kost, the landscape architect director for SEH who worked in conjunction with the city on the project. “Unlike structural soil alternatives, Silva Cell makes large volumes of uncompacted soil available to soak up urban storm water run-off while making this resource available for uptake by the street trees, completing the hydrologic cycle. For the rst time in my thirty years of design practice we nally have a product that allows street trees to function as a measurable, long-term component of civic infrastructure.”
All this was accomplished without compromising existing urban infrastructure, like utility lines. Lois Eberhart was pleased with the outcome as well, explaining “one of the beauties of the Silva Cell system is that existing utilities to a great degree could stay in place. It de nitely gave us the best solution.”
The streetscape design places each of the 167 trees in a Silva Cell group. Each tree holds 588 ft3 (16.6 m3) of bioretention mix soil and can store 118 ft3 (3.3 m3) of stormwater. Over the entire project site, more than 19,000 cubic feet (0.45 acre feet, or 558 m3) of stormwater can be treated within the Silva Cells. The Silva Cells are able to capture and treat well over the “P” storm, which is 90% of rain events (which in Minneapolis is less than or equal to 1.03”/2.62 cm in 24 hours) from their watershed.
The project was installed from May through November 2009. It utilized almost 5,000 Silva Cell decks and 10,000 Silva Cell frames to help the trees reach mature growth and to ful ll stormwater management goals.
Total bioretention soil volume per tree: 588 ft3 (16.6 m 3)
Number of Silva Cells: Over 9,800 frames
Installation Date: Spring-Summer 2009
Installation type: Large trees and stormwater management
Bioretention soil stormwater treatment capacity: over 19,000 cf (0.45 acre feet, or 558 m3 or 147,400 GAL)
Project Designer: hort Elliott Hendrickson Inc. (SEH) and URS Corporation
Owner: City of Minneapolis and Metro Transit
US: Nav Kaur ([email protected] or 415-746-1557).
Canada: Mike James ([email protected] or 604-687-0899).
UK: Steve Chatwin-Grindey ([email protected] or 02038484230).