Is Ottawa ready? More high-volume rain events in our future

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When a storm rampaged though the Toronto area last week, dumping more rain in one day than the city usually receives in a month, floodwaters overwhelmed the Don Valley Parkway, stranded a GO train and forced first responders to rescue residents in a long-term care home by dinghy.

Could something similar happen in Ottawa? Spring river flooding is one thing, but the most recent example of flooding caused by heavy rainfall was last Aug. 10, when a torrent flooded several streets and basements, stranded motorists and left thousands without electric power.

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And it will get much worse. The City of Ottawa has warned that no stormwater system could be engineered to handle every extreme event.

Older neighbourhoods were designed to the standards of their time, according to a report presented last March to the city’s environment committee. Stormwater management is a relatively new field of design. Ottawa’s topography couldn’t accommodate the pipe size required now, and the expense would increase development charges, the cost of new home construction and the city’s capital renewal budgets.

Current infrastructure doesn’t have the capacity to handle large volumes of rainwater, but that isn’t infrastructure failure, said River Coun. Riley Brockington, chair of the city’s emergency preparedness and protective services committee.

“You’ll never build for a 100-year storm,” Brockington says.

There are other factors besides infrastructure at play, including the nature of the storm and the topography of the neighbourhood. “The cause of flooding is different. The answer is different in each case,” Brockington says.

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More big storms are inevitable, according to climate projections for the region commissioned by the National Capital Commission to help plan climate change resiliency initiatives. That includes an increase in precipitation volume and intensity during all seasons — with the notable exception of summer — according to the predictions.

“We are told (rainstorms) will be in greater frequency, with greater volumes of water. We have to prepare the infrastructure as well as the homeowners,” Brockington says.

Adams Avenue in Elmvale Acres is more than three kilometres from the nearest large body of the water, the Rideau River. Still, on Aug, 10, 35 Adams Avenue houses had some flooding, resident Rosemary O’Reilly says. Two residents had to write off vehicles damaged in the flood. One neighbour reported basement flood waters more than a half-metre deep, she adds.

Aug. 10 marked the sixth time her basement had flooded, says O’Reilly, who has lived in her house for 37 years and estimates that repairs and replacing contents have cost $10,000 overall.

After six floods, weather reports of heavy rainfall are triggering for her.

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“We didn’t have a flood since 2017. I had time to recover. I thought I had done everything, I thought I was equipped. I did everything on the checklist,” she says.

O’Reilly installed a sump pump and backup valve after the 2017 flood with a subsidy from the city, but it didn’t work last Aug. 10 because she had failed to clean it the previous spring. She went with a broom to try to clear debris from the sewer grates on the street, but found there was nothing to clear. Inside, water bubbled from a basement drain and the drain of a bathtub. O’Reilly had to enlist her brother to help bail water.

There are plans to remediate sewers in that neighbourhood, but the project is expected to take two years and hasn’t started yet, O’Reilly says. Meanwhile, she has purchased a portable sump pump and a generator in case the power fails, and she has hired a plumber to check the backup valve.

Alta Vista Coun. Marty Carr says more than one-third of the 487 reports of flooding the City of Ottawa received about the Aug. 10 storm were in her ward. Afterwards, Carr heard from residents that there were no simple mechanisms to report a flood to the city.

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“The way we manage floods has to change, and the way we assist residents,” Carr says. “There’s anxiety every time we have a major rainstorm. People have financial concerns. People have mental health concerns. Some residents have lost their flooding insurance or find it too expensive to maintain.”

Understanding why sewers of different designs back up takes a lesson in history and civil engineering.

Some of the sewers in Carr’s ward date to the post-Second World War period, and some neighbourhoods still have combined sewers that accommodate both household sewage and surface runoff created by rainfall and melting snow. Those systems are prone to “surcharge” during large rainfall events, causing sewage to back up into some basements.

From 1948 to 1961, the city built sanitary sewers and surface runoff was conveyed using ditches and shallow storm sewers. In 1961, city council passed a bylaw requiring all new sewer systems to be fully separated — one sewer for domestic sewage and another for runoff — Carr says.

Partially separated systems were built before the early to mid-1960s, with weeping tiles connected to sanitary sewers. However, when a high volume of rain falls over an extended period of time, that water infiltrates the ground and reaches the weeping tiles, then enters the sanitary system, which exceeds capacity and backs up. When the flow from the weeping tiles exceeds the capacity of sanitary sewers during heavy rainfall, backup results.

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“We know more is coming. We have been hit very hard with extreme weather,” Carr says.

Aging infrastructure lacks the capacity to deal with volumes of intensification as more surface areas are paved over, says Kathryn Bakos, managing director of finance and resilience at the Intact Centre on Climate adaptation at the University of Waterloo.

At the same time, wetlands, grasslands and forested areas are under threat from development and intensification. “We have removed so much natural infrastructure,” says Bakos, who argues that, along with upgrading infrastructure, cities need to maintain 30 per cent of natural infrastructure.

“You can build resilience. It is easier to build resilience than to retrofit. We need to make that cost-effective argument,” she says.

House prices have been devalued in neighbourhoods impacted by floods. There are insurance and mortgage implications for homeowners without mortgages and insurance polices, Bakos says. In Quebec, for example, Desjardins Group announced it would no longer offer new mortgages in high-risk flood zones or provide flood insurance in those areas.

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In 2019 and 2020, the Intact Centre examined the preparedness of 16 major Canadian cities. Ottawa, which had scored an A- in 2015, dropped to a B-, one of six cities that were less prepared for flooding than they had been in 2015. The average score was C+.

Edmonton was among the cities that moved up, from C to B+. Among other measures, it provided free home flood assessments through a utilities provider, mandated backwater valves and offered financial subsidies. It had also completed full safety assessments from the standpoint of public safety and the risk of drowning, a risk framework that considered health, safety and the environment, as well as financial and social-service impacts.

“Nothing offers 100 per cent protection from extreme heavy rainfall. You can’t fully get risk out of the system. But you can change risk from 80 per cent to 20 per cent,” Bakos says.

“This is not the worst event we’re going to see. The good news is that, across Canada, we know what we need to do. In the past two or three years, resilience and adaption has really started to pick up speed.”

Ottawa has about 3,000 kilometres of storm sewers and about 3,000 kilometres of sanitary sewers, 55 wastewater pump stations, 14 stormwater pump stations, up to 6,000 kilometres of roadside ditches, three wastewater storage tanks, a combined sewage storage tunnel, 167 stormwater ponds and 95 other other stormwater facilities.

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All stormwater systems are designed for a particular scale of rainfall event. “As such, the capacity of any storm system will inevitably be exceeded following an extreme rain event that exceeds the design assumptions,” says the city’s infrastructure master plan, which was approved by city council in June and aims to support the city’s planned growth until 2046, when Ottawa’s population is projected to hit 1.4 million with 195,000 new dwellings to be built.

The risk that system capacities will be exceeded under extreme wet weather conditions could increase even more as population and employment rise above original design specifications, the master plan says.

“The impacts of intensification are cumulative and may not be apparent until years after redevelopment has occurred. They are also difficult to remedy after re-development,” it says.

Intensification project proposals sometimes involve altering or filling open drainage ditches, which can alter stormwater system capacities when they’re not properly planned and designed, the master plan notes.

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On the plus side, intensification can often improve existing system performance, such as removing foundation drains from the sanitary system and regrading properties.

“More and more land is getting drained. But the water has to go somewhere. It doesn’t just disappear,” says Angela Kelly-Herzog, executive director of Community Associations for Environmental Sustainability (CAFES). “If everything becomes impermeable, there’s a price to pay.”

Ottawa is behind other major cities when it comes to green infrastructure, she says. Vancouver has a target to capture and treat 90 per cent of its average annual rainfall by using green rainwater infrastructure tools and design guidelines on public and private property. Vancouver’s rain city strategy aimed to use green infrastructure as a form of water management that combines engineered systems and nature-based systems.

“It is really important to understand that this is engineering, not landscaping,” Keller-Herzog says.

Green infrastructure can include such features as bioswales, urban trees and naturalized stormwater ponds along with engineered assets such as permeable pavements and cisterns. Grey infrastructure includes features such as roads, bridges, culverts and parking lots. Streets can act as temporary reservoirs for rainwater during extreme weather.

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Ottawa’s master plan, meanwhile, does not stipulate that green and grey infrastructure must work together and actually forbids substituting green infrastructure for grey infrastructure, Keller-Herzog says. “There needs to be an integration of the green and the grey infrastructure. They work together.”

Sponge parks, which are designed to retain water to nourish trees and other vegetation, are one example. Last fall, the city of Montreal, which already had seven “sponge parks” and 800 “sponge sidewalks” built since 2022, announced 30 more parks would be built by 2025. Sponge parks are located at lower points in neighbourhoods and are designed to retain water to nourish trees and vegetation. Montreal’s latest park project in Verdun will cover 4,300 square metres.

There’s no mention of sponge parks in Ottawa’s infrastructure management plan, although the city has done projects in the past, including a grassed surface stormwater storage area used for rare rain events coupled with a vast underground tank beneath the Sandy Hill Community Park, completed in 2009.  When there’s heavy rainfall inundating the nearby roads, flooding is directed into the park, preventing basement flooding.

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The tank has two cells, which are used infrequently. The first is intended for use once every two years on average. The second is intended for storm frequencies greater than once in five years.

The city also did two small bioswale pilot projects on Stewart Street and Sunnyside Avenue. A bioswale is a shallow, open channel with vegetation designed to move and treat stormwater runoff, particularly from roadways. Grass swales can reduce runoff volumes and reduce pollution by filtering sediments and nutrients.

But both of these pilot projects are now more than a decade old, and it’s time for a followup, Keller Herzog says.

Meanwhile, rainwater tree trenches retain runoff and store rainwater, treating the water as it filters thought the soil and the roots. Trees not only help to absorb water, but they also provide shade. But there’s only one proposed pilot of a tree trench on the south side of Glebe Avenue just west of Bank Street — and that project has only two trees, she says.

Ottawa is aware of what will happen with more episodes of intense weather, Keller-Herzog says.

“We know this is coming. There will be more and more of this. We shouldn’t be shy to talk about why this is happening. It’s climate change. And we also have to address the root causes of climate change.”

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