The Big Idea: Investing in “sponge cities” can mitigate the toll of climate change

Serious flooding is in our future—here’s one way we can take it on
Usman T. Khan
Photo-illustration Maclean’s

My fascination with water began as a child, when I saw rain for the first time. I spent the first 10 years of my life in the United Arab Emirates—one of the driest countries on Earth. My sister and I had eagerly anticipated the first rainfall of our lives, excited to put our new rain jackets to the test. When the first drops spattered on the windows of our house, I felt the excitement and joy bubble up inside me as we ran outside. The cool, calming sprinkle was unlike anything I had ever experienced. 

Since moving to Canada in 2002 for university, I’ve been studying—and now teaching—urban stormwater management. Our relationship with water is crucial, and I decided early in my career that working in water management would allow me to use my skills to make a direct impact on people and society. 

In the time since, storms, floods and other climate events have only grown more severe, overloading the systems built to handle them. Canadians from Nova Scotia to B.C., where flooding has devastated communities in recent years, have seen this toll firsthand. One thing has become clear: many Canadian cities aren’t built to withstand the intense rainfall and flooding that is in store for our future. Experts report that “once-in-a-century” catastrophic storms will now occur every 20 years—or more frequently, if temperatures continue to rise above pre-industrial levels. The solution? We need to invest in turning our cities into “sponge cities.”

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Traditionally, our cities are designed to resist water and remove it very quickly: water is directed by hard surfaces like concrete into drains, which take water into underground pipes that lead from cities into rivers, lakes, or the ocean. This design disrupts the natural water cycle, as the water isn’t able to soak into the ground or evaporate. The size of these pipes could manage the average rainfall of cities when they were installed, but recently, more frequent and serious storms have overloaded this outdated infrastructure.

Instead of repelling water, the concept of sponge cities (also called blue-green cities) actively embraces it. This kind of city is designed to soak up water up and release it back over time into the ecosystem. The idea is to mimic the natural water cycle that would occur if there was no city there at all, reducing the volume of water after rain events and reducing the scale of flooding during extreme downpours.

To accomplish this, cities can use technologies and materials that mimic the behaviour of a sponge. This includes using absorptive materials to build public spaces, or introducing nature-based infrastructure, such as green roofs or pocket parks. Many types of sponge infrastructure hold on to stormwater and allow it to slowly drain or permeate back into the environment—instead of allowing all the water to drain into a city’s limited stormwater infrastructure, which quickly reaches capacity, causing flooding.

Some cities, such as Toronto, have begun to build public parking lots with permeable asphalt, a more porous form of concrete. While this looks no different from any other parking lot, water is able to seep through the surface, collect underground and seep away slowly over time back into the environment. 

Each element of a sponge city takes careful planning and consideration—from choosing the right basins to maximize water retention while maintaining the structure’s strength, to finding the sponge infrastructure—from green roofs to rain gardens—that best suits the size and needs of the area it will occupy.

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Cities across Canada have begun welcoming some forms of sponge city infrastructure. In Vancouver, some trees along the sides of roads are actually a type of infrastructure called “tree boxes,” which divert rain from the roads. Rainwater trickles down from the street, through the soil at the base of each tree, and into a holding tank below, where it slowly drains over time into the environment. The city of Toronto implemented a bylaw in 2009 that mandates new developments or building additions with more than 2,000 square metres of roof area must have a set percentage of the roof set aside as a green roof. 

It’s expensive (not to mention time consuming) to dig up and replace an entire city’s drainage system. By installing sponge technology, cities can add new capacity to their drainage systems without having to remove outdated infrastructure. These technologies offer a lot of other benefits, such as having more green spaces for residents to enjoy. Little parkettes in highly dense neighbourhoods can act as sponges during storms, while people can otherwise use them as public spaces.

Currently, Canadian cities are significantly lacking the infrastructure needed to handle the changing climate—let alone qualify as a sponge city, which refers to areas that can retain as much water as the natural, non-urbanized environment would. Cities in Denmark, the Netherlands, Germany and China have become trailblazers in the sponge city concept. In Copenhagen, city engineers have found ways to make sponge technology—such as the underground storage tanks that hold stormwater—both useful and fun for residents to interact with. The tops of the tanks are interactive floor panels that children can bounce and play on, and the energy generated from their jumping helps to pump water through subterranean pipes. 

A fully realized sponge city could have these elements of whimsy and purpose, through a mix of creativity, function and fun. It could include a variety of lush green spaces to soak up and repurpose water, parking lots that wick away rain, and playgrounds that allow for residents to contribute firsthand to the city’s stormwater outflow. There’s something for everyone.

So what’s holding Canada back? Because this technology is new and not yet widely used, it can be expensive—although there is a significant potential return on investment, especially as infrastructure like a park serves multiple purposes for communities. 

And many people, including community leaders, simply lack awareness about the existence and role of this technology. Every time there’s a significant flood, reporters call me from across the country to talk about sponge technology. I’ve been sharing my perspective on this issue for over 10 years, but I find it disheartening that, each time, I still have to explain what the sponge city concept is. Most people just aren’t aware of this infrastructure, or the impact it could have in their neighbourhoods.

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Walking around cities like Toronto in the summer, I see dozens of infrastructure projects happening downtown every single day. Whether it’s streetcar tracks being redone or water mains being replaced, crews are always hard at work. But watching them fill everything back up with impermeable surfaces again, it feels like a wasted opportunity to make smarter decisions about how we shape our city. For example, instead of just repaving an intersection, what if they added a rain garden to capture all the runoff? I think these are the types of conversations that are missing—and it’s a Canada-wide issue.

To me, sponge cities present an elegant solution to a very difficult problem. The excess capacity for flood management from sponge technology could help to meet some of Canada’s climate adaptation goals. Protecting our cities from floods in this way has the potential for immeasurable benefits—in times of good weather and bad.

—As told to Emily Fagan