a green landscape, utilizing trees in urban spaces. Photo credit Leonardo Da via shutterstock.

You may have noticed that flash floods are becoming more common and more intense in Canada. On a single day in early June for example, Fort McMurray received almost 5 cm of rain. About month later on July 13, flash flooding hit Calgary and shortly afterwards, states of emergency were declared in four Saskatchewan communities. On July 17, Swift Current got more than 6 centimetres of rain in less than an hour.

When heavy rain falls, most stormwater systems either struggle to manage the flow or are completely overwhelmed. The government of Ontario’s manual on the topic lists several serious consequences: “Without proper stormwater management, reduced baseflow, degradation of water quality and increased flooding and erosion can lead to reduced diversity of aquatic life, fewer opportunities for human uses of water resources, and loss of property and human life.”

The problem is only getting worse with continued development in urban areas, which means more surface area than ever covered with impervious materials such as pavement and cement. Climate change and changing patterns of precipitation are also having an impact on stormwater volumes.

The good news is that there is a simple, cost-effective and extremely environmentally-friendly solution for stormwater woes – strategically-planted trees.

“Trees can be a critical tool in integrated urban stormwater management,” says Julie Taylor Schooling, a landscape architect with McElhanney Consulting Services Ltd., a firm with offices in many locations in Alberta and BC. Schooling recently completed a Masters degree in Environmental Science at Thompson Rivers University in Kamloops, BC with supervisor Darryl Carlyle-Moses.

“Consciously choosing the right tree for the right place means stormwater can be a resource instead of a nuisance or hazard,” Schooling says. “Planted in the right site with the local storm regime in mind, urban trees can potentially self-irrigate, evapotranspire the diverted runoff and even recharge depleted groundwater.”  


Front Street, Toronto ON. Silva Cells - 20m3 of soil per tree. Photo credit DeepRoot Canada

Using Research Findings

New urban trees are a big investment, so scientific evidence of their benefits is critical. Not all trees are equally effective in stormwater management, and different trees provide different capabilities.

Schooling has discovered, for example, that stemflow volume (rainfall that is intercepted by a tree and flows down the trunk) varies among deciduous trees. She points to a separate study in North and West Vancouver which found that coniferous species such as western red cedar can intercept up to 60 per cent of rain falling on them. European beech is a species that does both interception and funneling stemflow extremely efficiently. As one would expect, sparse, horizontal-branching species like honeylocusts let more rain through the canopy to the ground.

“Good interceptor trees reduce the amount of water reaching impervious surfaces under the canopy, which is great,” Schooling notes, “but if canopies are sparse and soils permeable, we’d prefer that water to flow down the trunk to infiltrate at its base.”

Engineers and urban planners are welcome to use the ‘Tree Canopy Module’ of The Water Balance Model, a non-profit decision support tool initiative of BC’s Partnership for Water Sustainability, funded through provincial government grants and subscription fees. It provides information on the benefits of various trees in stormwater management at both the site and watershed scales.



East Bayfront, Toronto ON. Silva Cells - 20m3 of soil per tree. Photo credit DeepRoot Canada

Size Matters

Trees provide more stormwater and other ecological services as they grow and mature, and therefore stormwater management through the use of trees is a matter of the larger, the better. 

A US-based research firm called the Kestrel Design Group has developed a formula that calculated how much storm runoff can be evapotranspired by trees of different sizes. It’s been added to the ‘Tree Stormwater Control Measures’ section of the state of Minnesota’s Stormwater Manual (see link below).

At the request of Toronto Water (that city’s water authority), Kestrel has also applied its formula to a local streetscape.

With a typical rain event in Toronto being 2 to 3 mm over 24 hours, Kestrel found that eight five year-old trees can evapotranspire all the runoff from a 5mm rain event over three days, and eight mature trees can evapotranspire all the runoff from a 13mm rain event over three days.  

However, for trees to become large in the urban jungle, they need enough soil to support a large root system, notes DeepRoot Canada General Manager Michael James. In addition, the same soil must be protected from excessive compaction because it’s very difficult for roots to grow (and also more difficult for water to be absorbed and held) in compacted soil.

“Many communities plant trees and their growth is stunted or they die because they don’t have large amounts of lightly-compacted soil around them,” James notes. “The trees die, you have to re-plant every 10 to 15 years and you never gain all the benefits that large trees provide.”

Enter DeepRoot’s flagship product, the Silva Cell, an underground framework for containing lightly-compacted soil that supports the loads of sidewalks, parked cars and so on. Silva Cells enable trees to access a vast quantity of soil even in an urban core, while preventing their roots from interfering with sidewalks, water pipes and other infrastructure.

DeepRoot currently has more than 1000 Silva Cell installations worldwide, with many of those in Canada.

Large trees and their underground root systems also provide water quality improvement through more rain and melt water percolating down through the soil. James points to research by North Carolina State University scientists (published in the Journal of Ecological Engineering) which found that Silva Cell systems provide stormwater water quality benefits equal to or better than traditional bioretention systems in removal of both pollutants and run-off nutrients such as phosphorus.

Toronto Water staff are currently testing the Silva Cells’ capacity to manage surface runoff in conjunction with scientists from Ryerson University. They have installed a proof-of-concept giant bio-swale installation on the Queensway, a commercial area between Moynes and Berl Avenue. The new Queen’s Quay project also features Silva Cells supporting a double row of trees alongside the Martin Goodman Trail, which James says will manage almost 50 per cent of surface run-off.

Schooling believes the potential of using trees in stormwater management is large. “But tree selection and planting site design must use the best available data,” she says, “and also anticipate the local storm regime to maximize benefits and minimize risks related to stormwater runoff.”

Sugar Beach, Toronto ON. Silva Cells - 30m3 of soil per tree. Photo credit DeepRoot Canada

For Further Reading

Treena Hein is a regular contributor to many publications in Canada and beyond, concentrating on environmental issues, agriculture, manufacturing, technology, business, food and more. Treena is the proud winner of the ‘International Federation of Agricultural Journalists’ STAR Prize, and the Canadian Farm Writers' Federation Gold Feature Award. She is also a two-time Kenneth R. Wilson National Magazine Awards finalist.

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