Water flowing into the Mosquito Creek is clean and safe thanks to the DeepRoot Silva Cell System
Developers for the Remix Condo project in North Vancouver, which began construction in the spring of 2013, faced a unique stormwater obstacle: the sidewalk and road runoff drains into nearby Mosquito Creek via the downstream property downspout — and, by city mandate, it had to be treated at source. As the city would not allow a raingarden in the boulevard, due to the need for pedestrian access, Vector Engineering turned to the Silva Cells, whose void space allows for stormwater management below the surface while maintaining a flat walking surface on the grass boulevard above.
Number of Silva Cells: 120 decks, 230 frames
Amount of Soil Volume Per Tree: 16.28m3
Number of Trees and Type: 4
Type of Project: Integrated: Trees and Stormwater
Project Designer: Vector Engineering
Project Contractor: Anderson Creek Site Development
Installation Date of Silva Cells: October 2013 and April 2014
The Mosquito Creek Watershed in North Vancouver is a unique natural retreat nestled within a large urban footprint, a popular area for bikers and walkers featuring the largest hiking trail in Canada. Home to regional wildlife, the creek has become compromised with non-source water pollution — and the city has responded by requiring local developers to treat their stormwater runoff at source.
The Remix Condominiums, situated on West 14th Street in North Vancouver, broke ground in the spring of 2013. As with most of the area’s properties, the adjacent road and sidewalk surface runoff drains directly into Mosquito Creek.
The team at Vector Engineering wanted to design a tree and lawn area between the curb and sidewalk to serve as a bioretention swale, slowing down and treating stormwater runoff prior to its entering Mosquito Creek; however, because the ground-level retail space area was sure to be heavily foot-trafficked, the city mandated an alteration to the simple lawn proposal for fear of creating a muddy trip hazard.
To eliminate the possibility of ponding and soil settling, while still maintaining bioretention, Vector turned to the DeepRoot Silva Cell system.
The Silva Cells provide void space for soil and stormwater treatment, as they are designed to do — but instead of being topped with aggregate and paving, the system is topped by additional soil that creates the tree lawn. The structure of the Silva Cells prevents soil settling while allowing an open, pedestrian-friendly design above ground. By removing the chance of settlement, pedestrians have a standard, safe, flat lawn to cross — and the city and development team retain all the benefits of a bioretention swale below grade.
Silva Cells were installed in a two-later system under the tree lawn and extend from the back of the curb to the sidewalk. Approximately 300mm (or one foot) of topsoil and sod was added above the decks, where paving would typically go. This topsoil drains through the decks of the Silva Cells to the soil below to provide a flat lawn from curb to sidewalk.
Water is collected from the street in a catch basin and then runs into the Silva Cells through a distribution pipe to ensure that it reaches the entire soil column. As the water moves through the soil column of the Silva Cells, there is significant peak flow. The soil in the Silva Cells is also enhanced as the nitrogen-phosphorous and other carbon-based pollutants are removed. An under-drain line at the bottom of the system takes the excess water to the downstream catch basin, which then drains into the Mosquito Creek. The site has a total catchment area of 350 square meters (3,767 square feet).
At the surface, the site looks like a normal tree lawn that is easily accessible to foot traffic. Below ground, the underground bioretention portion of the design removes pollutants and improves water quality, making it safe for the excess water to drain into the Mosquito Creek habitat and watershed. Residents of the Remix development and visitors to the retail area alike can stroll easily and safely across the tree lawn while, unbeknownst to them, bioretention is happening below. Likewise, the Silva Cells also helped Remix developers meet the city’s minimum requirement of 10 cubic meters (353 cubic feet) of soil volume per tree, yet another green infrastructure benefit of the system.
As one of more than twenty total projects in North Vancouver that use the Silva Cell system to manage stormwater on site, the Remix development demonstrates not only how private owners and cities can work together to meet shared development goals but also that there are new areas where stormwater can be treated without giving up pedestrian-friendly grass areas.