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Throughout April UrbanToronto is featuring a special State of Environment editorial series to explore critical sustainability issues across our region.
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As cities contend with more frequent, intense rainfall and aging infrastructure, the challenge of managing stormwater is becoming increasingly visible across streets, parks, and neighbourhoods. In Dorval, Québec, a recent climate resilience study led by Lemay in collaboration with CIMA+ moves the focus from isolated fixes to a coordinated, city-wide strategy. By mapping flood risk, infrastructure capacity, and vulnerable populations across the entire municipality, the study outlines how targeted, small-scale interventions within public spaces can relieve pressure on overstretched systems, an approach that carries implications for cities like Toronto. Climate projections for Southern Ontario point to more frequent short-duration, high-intensity rainfall events, placing additional strain on aging stormwater systems across the region.
UrbanToronto recently spoke with Mylène Carreau, Landscape Architect at Lemay, to learn more. “Part of the mandate was not only to find solutions for a specific site, but to assess the risk on the entire territory of the city,” said Carreau.
Rather than approaching flooding as a series of isolated problem sites, the Dorval study begins with a layered analysis of the entire municipal territory, combining topography, stormwater modelling, and documented flood events to build a clearer picture of how water moves through the city. Civil engineering simulations were used to identify overloaded pipes and potential overflow conditions, while historical claims from residents helped ground the analysis in lived experience. These datasets were then cross-referenced with climate indicators and the location of vulnerable populations and community infrastructure.
“We used the hydraulic model to identify the vulnerable sections of the stormwater network and to simulate probable overflows,” noted Carreau. “When you overlay all these risks, you can highlight where flooding is most likely to happen and where solutions will have the greatest impact. We tried to locate the risks... and find the areas in the city where a small gesture would make the most difference.”
This type of territory-wide analysis is gaining relevance in Ontario, where municipalities are increasingly required to prioritize limited infrastructure funding toward areas of highest risk.
Translating these findings into built interventions required working within the constraints of a fully developed urban landscape, where available space is limited. In Dorval’s post-war neighbourhoods, established tree canopies, active parks, and tightly configured streets leave little room for large-scale infrastructure changes, while the project mandate restricted interventions to publicly owned land. This ruled out widespread upgrades to underground systems or private property solutions, changing the focus to precise, surface-level adjustments within streets, parks, and other municipal spaces. The approach is particularly relevant as Toronto continues to intensify its existing neighbourhoods, where limited available space and established streetscapes constrain conventional infrastructure upgrades.
“The lack of free space was the biggest challenge,” shared Carreau. “The goal was to provide a toolbox of different solutions that could be applied within the city’s public domain.”
The strategy entails redirecting excess stormwater flow away from overloaded pipes and into carefully designed green infrastructures that can absorb, store, and release water gradually. Rather than relying on costly upgrades to underground infrastructure, the approach introduces localized depressions, planted basins, rain gardens, and landscaped bioswales that temporarily hold runoff during peak rainfall events. In parks, these systems can be tied into existing collectors, allowing water to overflow into designated areas that are shaped to accommodate periodic flooding without disrupting everyday use.
Across streets, interventions work with existing grades and drainage patterns, removing excessive asphalt where possible, and using gravity to move water through a network of visible, distributed green features that collectively reduce strain on the system. With infrastructure replacement costs rising across Ontario, these lower-cost, surface-based interventions are gaining attention as municipalities look for more feasible ways to improve system performance without large-scale reconstruction.
“We sculpt the land to create areas that can flood temporarily, relieving pressure on the rest of the network,” said Carreau. “Everything works with gravity, so we design overflows and surface interventions that fit within the existing system.”
Introducing planted systems across streets and parks increases tree canopy and vegetation cover, helping to moderate temperatures, improve air quality, and support biodiversity, while also improving the usability and comfort of public space. In many cases, stormwater features are paired with new pathways, seating, and landscape elements, turning functional infrastructure into amenities that encourage more frequent use. Narrowed roadways and added greenery can contribute to slower traffic speeds, while upgraded parks become more attractive and accessible community destinations.
“Wherever we introduced infrastructure to collect water, we planted trees… it reduces heat, improves comfort, improves air quality, and supports biodiversity,” Carreau told UrbanToronto. “It becomes something people can see and understand, not just infrastructure underground.”
Rather than relying on large, singular infrastructure projects, the strategy is built around the cumulative effect of smaller, targeted interventions distributed across the city. Each adjustment (whether a reconfigured street edge, a landscaped basin in a park, or a reduction in paved surface) addresses localized conditions while contributing to system performance. In Ontario, where full stormwater system upgrades can be prohibitively expensive, this incremental approach offers a practical alternative that can be implemented alongside existing capital programs.
“They’re more like numerous small gestures combined,” expressed Carreau. “Excavating underground and installing piping is very expensive… Creating a landscaped basin, on the other hand, costs much less, and the benefits are huge."
Moving from strategy to implementation introduces a different layer of complexity, requiring coordination across municipal departments, elected officials, and ongoing capital projects. Because interventions are distributed across multiple neighbourhoods, decisions around where to invest first must balance technical need with political considerations and community expectations. The study’s territory-wide analysis provides a framework for prioritization, helping to direct funding toward areas with the greatest combined risk while maintaining a fair geographic distribution. At the same time, proposed interventions must align with planned street reconstructions, park upgrades, and infrastructure works already in progress.
“A clear analysis of the targeted territory helps to balance the investments and intervene where the need is greatest," said Carreau.
For a city like Toronto, where aging infrastructure and intensifying rainfall events are already shaping development pressures, the study points to a scalable, adaptable framework. While the most direct applications are found in residential neighbourhoods and areas with some available green space, the underlying approach (mapping risk at a broad scale, then applying targeted interventions) can be translated across different urban conditions. As municipalities across Ontario balance growth, infrastructure renewal, and climate adaptation, this framework offers a way to integrate these priorities within existing urban environments. Even in more constrained environments, opportunities remain to reallocate space within the public realm, whether through street redesigns, reduced asphalt coverage, or integrated landscape systems.
“Wherever you can remove a lane, you can have a green solution,” said Carreau. “Every project should have a green solution. It should not be an afterthought.”
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UrbanToronto's research and data service, UTPro, provides comprehensive data on construction projects in the Greater Golden Horseshoe—from proposal through to completion. Other services include Instant Reports, downloadable snapshots based on location, and a daily subscription newsletter, New Development Insider, that tracks projects from initial application.
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