Green roofs helping cities manage floods

Recent research into green roof technology has produced promising results for optimising flood management techniques in high-risk urban areas.

Through studies conducted at the University of Toronto’s gritlab – an interdisciplinary teaching and research facility focused on green roof innovation – researchers have optimised green roofing to help manage stormwater runoff in Toronto’s flood-prone urban centre.

University of Toronto’s Dr Jennifer Drake said Toronto’s urban design made green roofs an obvious option for stormwater mitigation.

“Toronto downtown is built with a combined sewer system. We’ve got old infrastructure and really limited space,” Drake said.

“When the city was looking at different options for infrastructure that would help mitigate the issues we have with flooding in the city, green roofs stood out because we have a lot of roof space in the core.”

Although green roofing is nothing new in Toronto, gritlab set out to help design options that are more efficient and effective at capturing stormwater, Drake said.

“The lab was created in 2010 because the city adopted a green roof bylaw. Any new construction with a roof larger than 2000m² has to put in a green roof. The bylaw created a mandated industry overnight,” she said.

“We have a lot of green roof construction in our city and the bylaw is fantastic but it doesn't actually identify how to construct a green roof for best performance. The only criteria you have to have is that vegetation still needs to be green after two years.”

Researchers considered soil and plant type, soil depth and irrigation while constructing 33 miniature green roofs, measuring temperature, soil moisture and discharge rates.

“From that work, we have been able to identify the combinations that function really well, the parameters that are important for stormwater management or urban heat island mitigation,” Drake said.

The benefits of green roofing for flood mitigation are two-fold. Drake said the infrastructure offers a combined benefit for stormwater mitigation, as it retains water and releases it at a slower rate, but also removes a significant portion of water through transpiration.

“By putting a vegetation system on the surface of a roof, part of the water it captures will be restrained in the soil structure and then transpire. The amount it captures will depend on your climate, obviously. From our lab in Toronto, we were able to capture more than 50% of our rainfall over the course of a growing season,” Drake said.

“The second benefit is that the water has to go through the system, which slows it down. The water might still become runoff eventually, but the risk of flood is minimised because it’s not entering your network as fast.

“If you are working with a combined sewer system, delaying stormwater in this way can stop it from overwhelming your network and flooding.”

Drake said that while green roofs can’t help solve urban flooding in all scenarios, they can mediate the intensity of flood waters in many contexts.

“There are different types of flooding and different types of solutions, but they certainly can help especially when you are dealing with small, isolated, urban rainfall. They can provide that on the spot retention that you need so that your infrastructure doesn't get overwhelmed,” she said.

“Creating a green roof automatically gives you benefits, regardless of small details. In many climates, you will automatically capture more than half of the annual rain and you will automatically reduce your peak runoff rates by about 80%.”