Flooding is a problem. The 'sponge city' could be the solution

The type of heavy rain that hit Ottawa last week and which causes roads and basements to flood could push planners toward a natural solution with a playful name.

Modern stormwater systems not built for exceptional rain events

Ottawa councillor says preparing for emergencies 'top of mind' after Thursday's hail, flooding

4 months ago
Duration 0:55
Coun. Riley Brockington, chair of the city's emergency preparedness and protective services committee, says he's looking at ways the city can be better prepared for an increase in extreme weather events like the storm that passed Thursday.

The type of heavy rain that hit Ottawa last week and causes roads and basements to flood could push planners toward a natural solution with a playful name: the sponge city. 

More than 75 millimetres of rain was dumped on the nation's capital Thursday afternoon, a deluge so intense it temporarily overwhelmed drainage systems. 

"We had a three-hour thunderstorm, something I've never experienced," said River ward Coun. Riley Brockington. 

He said many residents told him they were convinced their storm drains were blocked. They ran to the end of their driveways to find nothing — not even a single leaf.

"It's just like cars that try and get on the Queensway at rush hour. There's only so much capacity the Queensway can [handle] until cars slow down or stop," Brockington said.

Even the best and most modern culverts would not have been able to accommodate such an unusual rainfall, he said, arguing that the city's system performed just as it should. 

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But with the city itself projecting that the volume and intensity of rainfall will increase, some experts are calling for a change in strategy.

A car submerged on Kilborn Avenue, Aug. 10, 2023.
The massive rainfall on Thursday proved too much for Ottawa's stormwater management system. Some experts point to natural solutions. (Giacomo Panico/CBC)

Natural defences are lost

This issue is common to many major Canadian cities built to support rapid urban growth in the middle of the last century, explained Alexandra Lesnikowski, the leader of Concordia University's Climate Change Adaptation Lab. 

With extreme weather events becoming more common, upgrades are a major economic challenge that will require planning, Lesnikowski said.

"They will [have to] spread the cost of updating their stormwater and sewer system in a way that makes it financially manageable for them to undertake," she said.

Left unchecked, flooding can push cities to make difficult decisions about whether living in areas prone to these sorts of disasters is preferable or even possible. 

Usman Khan, an urban hydrology expert at York University, said the central issue is that roads, rooftops and parking lots inhibit the water cycle and encourage flooding because excess water can't be absorbed. 

"The concept of a sponge city is to basically try to mimic this natural behaviour that we've taken away," he said. 

"Introducing green areas like green roofs, rain gardens, vegetated swales can help temporarily hold water in the same way that a sponge does during rain events — and then release it slowly over time." 

It's something the city has toyed with in the past, providing financial incentives for people to add a rain garden or a driveway made with permeable concrete, or adding bioswales — vegetated channels capable of holding runoff — to streets in Old Ottawa South and Sandy Hill. 

A woman stands in a park.
Alexandra Lesnikowski says flooding from major rain events can be a problem for cities like Ottawa that experienced rapid urban growth in the middle of the 20th century. (Rowan Kennedy/CBC)

Goes 'hand-in-hand' with intensification

While the ideas may seem at odds with Ottawa's moves to intensify downtown areas and address a growing housing crisis, some experts say these ideas can go hand-in-hand.

They can also be expensive — but, so are the alternatives.

"If you look at flooding, say across Canada over the last 10 years, 20 years, we're seeing every time a major event happens, it's classified as the most expensive natural disaster," said Khan.  

"It's not just flood defence infrastructure. It's not just stormwater management. It's access to green space, which has a whole variety of benefits."

Lesnikowski adds that expanding cities outward can make matters worse by eliminating natural flood plains.

line graph shows the max amount of rain recorded on a single day in a given year is projected to increase
A report from the City of Ottawa and the National Capital Commission suggested the maximum amount of rain dumped on the region in a single day is rising. (City of Ottawa and National Capital Commission graph)

Flood risks set to increase

Flood damage is already a well-recognized problem.

It was among the climate-related risks deemed to be a priority by city staff last year, with increased maintenance to stormwater infrastructure dubbed a potential vulnerability.

"All stormwater systems are designed for a particular scale of rainfall event," the city's most recent draft infrastructure master plan stated.

"As such, the capacity of any storm system will inevitably be exceeded in response to an extreme rain event that exceeds the design assumptions."

The overarching plan, set to be approved during this council session, went on to explain that intensifying downtown areas does add more pressure to the existing systems.

Brockington, the chair of the emergency services committee and a member of the environment committee, said developers must prove their plans won't push infrastructure beyond its limits or pay to upgrade them. 

Harder to identify, however, may be cumulative effects or deterioration over time.   

With clear evidence that weather-related risks are shifting, Brockington said council will continue to press for an answer to a question that many are asking: "How do we protect the city?" 


Elyse Skura


Elyse Skura is a reporter based in Ottawa. Since joining CBC News, she's worked in Iqaluit, Edmonton and Thunder Bay. Elyse spent four years reporting from Tokyo, where she also worked as a consulting producer for NHK World Japan. You can reach her at