Toronto Ahead of the Curve on Tree Growth and Stormwater Management

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Monday, 20 April 2009 18:12

by Leda Marritz

Stormwater “Proof of Concept” Streetscape Installation on The Queen’s Way, Toronto, Canada

Toronto Water, the city’s water authority, was trying to solve the problem of managing stormwater runoff from ordinary rain events. They had been unable to meet municipal water volume and quality treatment requirements because there was no system for detention, retention, or cleaning. Toronto Water wanted to test a new combined tree growth and stormwater management infrastructure product, the Silva Cell, on its capacity to manage surface runoff.

In conjunction with Ryerson University and Deep Root Canada Corp., the city installed a proof-of-concept installation on The Queensway, a commercial area between Moynes and Berl Avenue in Toronto.

Pave-Al, the contractor, excavated two trenches for two Silva Cell systems, each two frames deep and with spots for two tree openings, that straddled the sidewalk area and the parking bays.

The Silva Cells were filled with a bio-retention soil mix (80% sand, 20% soil) that has a 20% water holding capacity. All of the rainwater runoff is collected in the city’s standard stormwater catch basin. In effect, the Silva Cell is being used to create a giant bio-retention swale underneath the sidewalks and parking bays.

A bio-swale can keep the surface runoff out of the stormwater system for 24 hours and filter out pollutants from the water. A typical rain event in Toronto is 0.09-0.12” (2-3 mm) in 24 hours, and 50% of Toronto’s annual precipitation events are less than .19” (5 mm). The Silva Cell system was sized and laid out to manage the runoff from a 2” (5 cm) rain event in 24 hours.

The system captures all of the run-off from the crown of the street to the building face and from one end of the block to the other. All of this water is cleaned, retained and detained by the bio-retention soil. The system also meets AASHTO H-20 loading requirements to support the weight of vehicle parking.

The entire installation took 3 days, with a crew of 5 workers, and provided a total of almost 600 ft3 (16 m3) of bio-retention soil per tree.

An 8” (20 cm) PVC pipe runs from the street catch basin into the top layer of the Silva Cell system, delivering all of the surface runoff from the roadway and adjacent sidewalk into the bio-retention soil. A perforated PVC pipe then distributes the surface runoff evenly throughout the soil and the water infiltrates through the soil until it reaches the bottom of the system.

At the bottom of the trench is a perforated drain line that will carry any excess overflow into the existing city stormwater system.

Depending on the site, the runoff that percolates to the bottom of the Silva Cell system can be infiltrated into the subsoil to help replenish the aquifer.

As the two trees in each of the trenches mature they strengthen the efficiency of the stormwater management system by evapotranspiring large volumes of rainwater out of the soil through their roots systems and up out through their canopies.

The City, with assistance from Ryerson University, is installing monitoring equipment to track the flow quality of infiltration and quality of water in and out of the Silva Cells. The first set of performance data is due to be available in Fall 2009. With the help of the Silva Cell they are hoping to meet their regulatory requirements in areas where they couldn’t before.

Installation Summary