Silva Cell’s Support 167 trees in Downtown Minneapolis: A Case Study

– Silva Cells solved the 2009 CSO stormwater transition during the Marquette and 2nd Avenue (MARQ2) busway project, mitigating stormwater runoff.

– Over 167 trees were installed along 48 blocks of downtown Minneapolis each with 588 ft3 (16.6 m3) of soil volume.

– Above ground, the trees provide shade, clean air, and beauty. Below ground, during rain events, Silva Cells are removing phosphorous, nitrogen, lead, copper, and zinc.


Like many older downtowns, Minneapolis utilizes a combined sewer overflow (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 find a new way of dealing with stormwater.”

Seeking solutions that would prevent the system from overflowing, project designers at Short Elliott Hendrickson Inc. (SEH) and AECOM chose to integrate Silva Cell to manage stormwater to meet the City’s goals.
The Marquette and 2nd Avenue (MARQ2) busway project covered 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 were replaced with pervious pavement, which allows for infiltration 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” rainfall 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. Silva Cell’s perforated piping convey excess water out of the system. Based upon data values from research done by Prince Georges County in Maryland, the filtration 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 benefits of highly accessible soil volumes. 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 first time in my thirty years of design practice we finally have a product that allows street trees to function as a measurable, long-term component of civic infrastructure.”

– Bob Kost, Landscape Architect Director, SEH


All of this was accomplished without compromising existing urban infrastructure, like utility lines. 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. 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.  10 years later we are seeing an abundance of greenery along 2nd avenue, as the trees grow into maturity.

 

Installation Summary
Total bioretention soil volume per tree: 588 ft3 (16.6 m 3)
Number of Silva Cells: 4,900 2X
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 Designers: Short Elliott Hendrickson Inc. (SEH) and AECOM