Silva Cells installed along section of 61st Avenue Greenway prove the effectiveness of soil volume and green, at-source stormwater management
A well-trafficked corridor in southwest Calgary, the four-block stretch of 61st Avenue SW from Centre Street to Macleod Trail underwent a pedestrian-centered overhaul in 2016 with the intention of improving the public-space experience. The city also partnered with Urban Systems to embrace the benefits of green infrastructure, planting a series of new trees along the streetscape. The two-block span of 61st Avenue (from 3rd Street to the railroad tracks) installed Silva Cells along the north side of the street in the fall of 2016 — and the trees are thriving, in stark contrast to the frail and/or dying trees planted in traditional pits along the rest of the project area. The Silva Cell system is simultaneously performing at-source stormwater treatment, slowing and cleaning runoff before it enters the sewers and watersheds.
Number of Silva Cells: 150 (2x) and 438 (3x)
Amount of Soil Volume Per Tree: 15 m3
Number of Trees: 17
Type of Project: Municipal, Streetscape, Stormwater
Project Designer/Engineer: Urban Systems
Project Contractor: Foothills Landscaping
Installation Date of Silva Cells: Fall 2016
How might we transform this corridor into a thriving urban boulevard?
This was the question asked by project planners as they embarked on the 61st Avenue SW improvement project in 2015. To meet the pedestrian needs of the area, a number of objectives were outlined, including making the streetscape more “safe, accessible, enjoyable, and adaptable.” This was accomplished with a series of modern, welcoming upgrades such as custom street furniture, walkway treatments, widened sidewalks, improved lighting, and a landscaped center median.
Likewise, the “greenway” project sought to improve the sustainability of the corridor, assessing the preexisting condition of both the street trees and the area’s stormwater management functionality. As noted in the initial plan’s assessment: “Currently, none of the landscape on the corridor manages stormwater flows or treatment,” going on to say that “The trees on the north side can be planted in Silva Cells to manage stormwater.”
This is precisely what happened, with the DeepRoot Silva Cell system being in installed in the autumn of 2016.
The team at Urban Systems tasked the Silva Cells with providing both lightly compacted soil volume for new tree plantings and at-source stormwater treatment along two blocks of the north side of 61st Avenue: an affordable dual-purpose tool that works in tandem with the natural landscape. A total of 588 Silva Cells (both 2x and 3x configurations) are providing an optimal soil environment for the 17 newly planted trees.
In contrast to the rest of the street trees in the project area, which were placed in traditional concrete pits, each one of the Silva Cell plantings are receiving access to at least 15 cubic meters of uncompacted soil volume — and the difference in tree health is remarkable. The flourishing trees in the two blocks planted in Silva Cells offer a stark visual opposition to those in the rest of the project area, many of which have already died and been replanted — and none of which show the health and vitality of their Silva Cell counterparts.
Calgary city officials also wanted to utilize the Silva Cells to clean and slow stormwater runoff on location. Rainwater from 61st Avenue drains into catch basins where it flows through a pre-treatment sump before then entering the Silva Cells by way of an 8” (200mm) perforated pipe. Bioretention is performed as the water seeps into the soil, cleaning it of pollutants and mitigating peak-flow flooding concerns before it drains into the sewer system.
To address the slope of the street, and the fact that the distribution pipe needed to stay flat, a unique arrangement was implemented: the 200mm distribution pipe was set on a 2x Silva Cell system at the back of the sidewalk. The rest of the Silva Cells in the sidewalk area then utilized the taller 3x configuration, which allowed the distribution pipe to remain flat and to “rise” in the Silva Cell as the street sloped down (without getting higher than the 3x system itself).
This project, like many other Silva Cell installations around the world, underscores the critical importance of optimizing the urban soil for healthy root expansion, an objective accomplished by depositing loamy, uncompacted soil into the Silva Cells’ void space. The multi-purpose functionality of the system is also demonstrated by its treatment of stormwater at-source, reducing the burden on the municipal sewer system, even in a somewhat challenging sloped environment. Silva Cells’ value is likewise realized by circumventing the cost associated with replanting dying trees every few years, as appears necessary on the south side of 61st Avenue.