Canadian development stores runoff in below-ground soil system
The city of Edmonton, Alberta, Canada, faced two development challenges. First, pollutant-laden storm water runoff that feeds into the North Saskatchewan River through storm sewers was a major concern for the city and downstream municipalities. Second, the city’s neglected downtown area was in need of revitalization.
A recent effort to update Edmonton’s infrastructure, in combination with storm water runoff considerations, led the city to nature-based development practices to bring environmentally friendly development to its downtown.
In 2013, in the process of revitalizing the Quarters District, a neighborhood located at the eastern edge of downtown, the city invested $12 million to upgrade area drainage services. As part of this project, Edmonton made land and streetscape improvements on 96th Street from Jasper Avenue to 103A Avenue, creating a pedestrian-oriented streetscape corridor. The corridor connects the districts that make up the Quarters Downtown, and invites residents to visit other areas, including the River Valley and Louise McKinney Park. This area, referred to as the Armature, is the heart of redevelopment and is expected to propel growth in the Quarters, reduce storm water runoff and improve resiliency.
ISL Eng. conducted the conceptual, preliminary, and detailed design and construction services for the Armature, with a recognition that nature-based systems could make a significant contribution to the development. Russell Barth, senior water resources engineer for ISL, explained the storm water considerations for the project.
“Redevelopments of this nature that provide storm water contributions to the river need to be treated. So the city has decided to do a sewer separation, which would take the storm water runoff and put it in a new pipe system and rehabilitate the old combined sewers to carry new sanitary flows coming from the development,” Barth said. “The city put in a large storm tunnel deep below the roadway and it is temporarily draining at a controlled rate into an existing combined storm water system.” In the future, this storm tunnel will be extended to a river outfall, which makes treatment of the storm runoff a priority.
Underground Soil System
Almost 4,400 Silva Cells were installed at the Armature. The cells provide storm water management through interception and evapotranspiration via the 90 trees planted in the system and absorption and cleaning via the soil the system contains. The nearly 1,238 cu meters (43,740 cu ft) of soil have the capacity to handle 25 mm of storm water runoff, which is equivalent to a 2-year, four-hour storm event.
“Silva Cells were used for their ability to achieve water quality treatment goals as well as to grow large and mature trees. Nature-based systems are, by far, the best ones available to us,” Barth said.
Following the nature-based approach, Barth and his team of engineers designed the road surface to convey flows without spilling into adjacent developments. The cross-section of the road, as well as the sidewalk, slope toward the system, and water is directed toward traditional catch basins, where the total suspended solids are captured in the sumps. A perforated pipe set above the sump then evenly distributes storm water into cells at the top of the soil profile, where it infiltrates down through the soil. Any excess flows are diverted via an underdrain and into the storm sewer system. The soil mix is three parts unscreened top soil, three parts sand and one part peat, which provides the appropriate drainage capacity and organic material required to sustain the trees and allow for storm water infiltration.
At the end of the project, the responsibility for monitoring the underground infrastructure will be transferred to the city. The monitoring group will monitor water quality and discharge via a flow monitor at the underdrain discharge and from samples off the street.
“The Silva Cell system works to the storm water design objective right away, but as the trees mature, they add resiliency to the system in the form of increased storage through evapotranspiration, increased porosity in the soil due to root growth, and increased water quality that can be attributed to the symbiotic relationship between tree roots and the microbial column in the soil,” said Michael James, general manager for DeepRoot Canada Corp. “The trees become a bio-utility and an integral part of the system.” As the elm and maple trees planted in the Armature grow and mature, they will become defining elements of the landscape in the community.
The city of Edmonton’s recent Street Tree Soil Volume standard, set in the 2016 Volume 5 Design and Construction Standards, also will support the integration of soil volume for trees and storm water management. Future phases of revitalization in Edmonton have an estimated cost of $110 million and will see additional infrastructure investments focusing on park development, sidewalks and further streetscape and public space improvements. With these projects, the city of Edmonton aims to attract growth and enhance the resiliency of its landscape.