While it may not have been “feasible” to prevent 300 million litres of untreated water from flowing into the Ottawa River during a record rainstorm earlier this month, cities are still catching up with the natural protection systems that can make a difference when engineered infrastructure is overwhelmed by severe weather.
When sudden storms amped up by the climate emergency get severe enough, they can and will swamp the best local defences. But the highly-touted Combined Sewer Storage Tunnel in Ottawa that opened in 2020 and hit its limit earlier this month, leading to a “mayhem” of flooded basements and health risks, was just one of the tools in the city’s toolbox for keeping floodwaters under control.
Stormwater management expert Jenn Drake, an associate professor in the Carleton University school of engineering, told CBC the system has reduced the volume of sewage flushed into the river “by orders of magnitude”, but it would cost billions and take decades to rebuild that infrastructure to account for extreme storms—if the system could be rebuilt at all.
“When you think of the environmental implications of that polluted water that was just going directly into one of the most important rivers in Canada,” Drake said, “we’ve achieved something quite significant.” But in many cities, “it is not feasible, affordable, practical” to engineer a stormwater system so big that no storm will push it beyond its capacity.
“There often isn’t the space left,” she explained. “We’ve got power, we’ve got our communications infrastructure, we have transport infrastructure. We have a lot of stuff already happening underground that it would take billions of dollars and probably decades to actually separate that stormwater infrastructure.”
The C$232-million Ottawa tunnel has the capacity of 18 Olympic-sized swimming pools. But it still overflowed massively when a record storm dumped more than 75 millilitres of rain on the city earlier this month. And it isn’t as if city planners and engineers didn’t see it coming.
“It was always known that if there were very large storms the city would be faced with a choice of either flooding sewage into people’s basements or into the river,” said Toronto councillor Dianne Saxe, a veteran environmental lawyer and former Environment Commissioner of Ontario. (In the end, Ottawa ended up with both.) Knowing that the storms “are going to get worse and worse,” she added, communities can and must do better with a problem that she summed up with one choice adjective: disgusting.
“You’re talking about human poop washing up on the beaches, contaminating the fish, contaminating the sediment, everything that lives in the water, everyone who might want to swim in the water or sail on the water,” Saxe said. “It’s filth. And it’s dangerous filth that spreads disease.”
Green Infrastructure Reduces Water Flow
Saxe first raised the issue during her term as commissioner, CBC notes, in her 2018 Environmental Protection Report titled Back to Basics [pdf]. She determined that combined sewer overflows can indeed be stopped by increasing the capacity of sewage treatment systems, keeping stormwater and groundwater from mixing with sanitary sewage, reducing the amount of stormwater flowing into combined sewers, and conserving water (the subject of an earlier report from the Environment Commissioner’s office, Every Drop Counts [pdf]).
Options to reduce the flow of water toward sewage systems include disconnecting downspouts and weeping tile systems, building green infrastructure that absorbs more water, and stormwater area charges to create an incentive for property owners.
“Green infrastructure or low-impact development is an important, but underused option,” the report explained. “It replaces impervious surfaces (like concrete and asphalt) with permeable materials that can absorb water, such as rainwater gardens, vegetated highway medians, and green roofs. Green infrastructure filters and stores stormwater, cleaning it, slowing it, and reducing the amount of water entering the sewers. As well as reducing combined sewer overflows, green infrastructure helps to reduce flooding and adds green space, which can improve both physical and mental health.”
In 2021, a CBC documentary looked into the more than 700 “secret gardens” in Toronto, with green roofs supplying space and food while helping to reduce flooding.
‘Sponge Cities’ a Work in Progress
Ottawa has also considered the concept of sponge cities, “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,” CBC writes. The idea goes hand in hand with intensification in core neighbourhoods—especially because the loss of wetlands and other natural flood defences is one of the many, multiple drawbacks of continued urban sprawl.
(Given the opportunity to get that decision right, Ottawa’s previous city council opted to extend the urban boundary before elected officials were even given a reliable budget to guide their decision.)
“The concept of a sponge city is to basically try to mimic this natural behaviour that we’ve taken away,” explained York University urban hydrologist Usman Khan. The right kind of green infrastructure “can help temporarily hold water in the same way that a sponge does during rain events—and then release it slowly over time.”
Sponge cities are slowly taking hold. Montreal is turning a green space in its Lachine borough into a “sponge park”, and the Netherlands has emerged as a global leader in anticipating and adapting to future flooding, sharing its experience and offering advice to visitors from as far away as Bangladesh, Indonesia, Vietnam, and the United States, the New York Times reported in 2017.
The country’s design and preparedness philosophy is “to let water in, where possible, not hope to subdue Mother Nature: to live with the water, rather than struggle to defeat it,” the Times wrote at the time. “The Dutch devise lakes, garages, parks, and plazas that are a boon to daily life but also double as enormous reservoirs for when the seas and rivers spill over.”
The underlying principle is that “environmental and social resilience should go hand in hand, officials here believe, improving neighbourhoods, spreading equity, and taming water during catastrophes. Climate adaptation, if addressed head-on and properly, ought to yield a stronger, richer state.”
Early this month, Bloomberg reported that China has run up against the limits of sponge cities, after spending billions of dollars in recent years to protect citizens from killer storms. The rising death toll from flooding in late July and early August “has sounded the alarm of whether these tactics are fit for purpose as rising global temperatures supercharge more intense rainfall events,” the news agency wrote.
“Worldwide it’s becoming increasingly difficult for communities, from Vermont villages to London neighbourhoods, to keep up with the fast changes in precipitation.”
The ’Sponginess’ Scale
Even so, cities may yet have a long way to go to maximize the preventive impact and positive benefits of sponge cities. In 2022, the United Nations climate secretariat’s Race to Zero program used artificial intelligence to compare cities’ tree cover and lakes to their volume of concrete, as one measure of their ability to withstand climate shocks.
“As climate change brings increasing flood threats, cities need to be designed like giant sponges that allow water to drain away safely,” Race to Zero wrote. But the first-of-its-kind assessment of seven cities—Auckland, London, Mumbai, Nairobi, New York, Shanghai, and Singapore—found that the best among them rated only 35 for “sponginess” on a scale of 1 to 100.
“New Zealand’s Auckland came out top with a 35% sponge rating—largely thanks to its stormwater systems, many golf courses, green parks, and good-sized residential gardens,” Race to Zero said. “It was followed by Nairobi at 34%, while New York, Mumbai, and Singapore tied third with 30%, and Shanghai fourth with a 28% sponge city rating. In last place was London, at 22%, mainly due to high levels of concrete and poor soil absorbency.”
There’s no suggestion in the Race to Zero summary that sponge cities can absorb the worst flooding the climate emergency can dish out. But “natural ways to absorb urban water are about 50% more affordable than man-made solutions, and are 28% more effective,” the research summary stated. And “digital mapping tools can allow cities to quickly gauge the best use of their available space—from rainwater harvesting to ponds and inner-city gardens—as well as the risks in not doing so.”