Researchers at Carleton University say a relatively simple preventative measure could have made a big difference in helping Ottawa’s Crystal Beach neighbourhood better weather the intense Canada Day storm.
Using rainfall data from the record-breaking July 1 deluge, the team ran a computer model showing the impact of engineered rain gardens on private property and on the city’s right-of-way along streets.
The result? Major reductions in stormwater pooling and hazardous flood water.
“I think what the Canada Day storm has shown us is the vulnerabilities that exist in our neighbourhoods in terms of handling extreme weather and urban flooding,” said Jennifer Drake, an associate professor and Canada Research Chair in stormwater and low-impact development.
“We will continue to have these disasters unless we do something,” she said.
A rain garden, or bioretention cell, looks like a shallow ditch with sandy soil and deep-rooted plants that can withstand very dry and very wet conditions. It slows and absorbs water and sometimes includes a drain to a catch basin.
Drake and PhD student Ali Zoghi used Crystal Beach to model stormwater drainage because it’s representative of residential construction from the 1950s up to the 1980s. It also happened to be among the hardest hit on July 1, providing the opportunity for a case study.
The storm saw a peak rainfall volume of 167 mm over five hours. A moderate storm would see 20-30 mm over a few hours.
Streetside rain gardens alone, installed on the city right-of-way, eliminate stormwater accumulation of more than 15 centimetres by 76 per cent, according to the model. Adding rain gardens systematically to private properties as well reduced the flooded surface area by 86 per cent.
Rain gardens also reduced the depth of accumulated water in the hardest hit areas in the northern part of the neighbourhood, where the model indicated water depth of about a metre and half — cutting it to a metre.
Most importantly, streetside gardens alone eliminated water flowing 0.6 metres a second or faster, which they say is fast enough to knock someone over or move a car.
“While we didn’t eliminate overland flooding entirely, we were successful in getting rid of the most dangerous [conditions] and the ones that have the potential cause loss of life. As an engineer, that’s the end game,” Drake said.
Crystal Beach was chosen for the modelling because it shares characteristics of mid-century suburbs such as Mississauga or St. Catharines, Ont.
Drake said communities of that era weren’t designed with dual drainage pipes, a second system for major events such as a once-a-century rain event along with typical stormwater drains.
“That means water may just accumulate in low spots throughout the neighbourhood. It’s really a common challenge that cities across Ontario deal with,” she said.
“That’s where you see some of the worst cases of basement flooding.”
The City of Ottawa’s Stormwater Asset Management Plan notes that older neighbourhoods inside the Greenbelt have more properties at risk of flooding due to designs pre-dating dual drainage.
The plan states that about 80 per cent of buildings across the city would not experience overland flooding during a “1-in-100-year” rain event.
The rainfall on Canada Day was even more severe than that, by the city’s assessment exceeding a 1-in-200-year event.
The modelling exercise suggested improving the stormwater pipes alone — an expensive process that involves digging up entire roads — would not have had as big an impact on July 1.
Ali Zoghi, the PhD candidate who worked with Drake on the modelling and who has been studying Crystal Beach for three years, said conventional pipe upgrades reduced standing water by less than 10 per cent in the model.
“The impact that our rain gardens made was much more significant in reducing the impact of flooding and getting the height of the storm more tolerable for the residents of the area,” Zoghi said.
He said the two systems work together: the rain garden reduces and slows runoff, while pipes carry it away.
In an Aug. 24 update to council, city staff said their comprehensive review of the July 1 storm has expanded to include two additional major storms, resident reports, and finer-detailed investigations getting down to the level of creeks, watersheds, wards and neighbourhoods.
The update noted the city’s water resources engineers are conducting their own modelling and detailed reviews with the aim of identifying potential mitigation measures.
In a statement attributed to the city’s director of asset management services, the city said “it would be premature to assess the effectiveness or applicability of any individual measure, location or approach in isolation before the broader assessment is complete.”
Updates on those studies into storm impact, infrastructure performance and resilience plans are expected in the coming months.
The city has included rain gardens in its 50-year stormwater management plan and pilot projects have been carried out on Sunnyside Avenue in Old Ottawa South, Stewart Street in Sandy Hill and Hemmingwood Way near Centrepointe.
The city also provides information and rebates for installing a rain garden on your own property through the Rain Ready Ottawa program.
Drake and Zoghi note some of what made rain gardens so effective in their model was their placement on the city right-of-way where they could take advantage of existing drainage patterns.
