Soak it up: How cities are handling the rise in floodwaters

When a torrential storm hit Toronto last July, it quickly became clear the city wasn’t equipped to handle the downpour. Basements became wading pools, parks became swamps and the Don River spilled over its banks, turning the Don Valley Parkway from a municipal artery into a churning waterway. That chaos — and the resulting damages, which experts estimate cost more than $1 billion — was caused by just 10 centimetres of rain falling over a period of three hours.
To be fair, those 10 centimetres were really just the straw that broke the camel’s back — a summer of record precipitation, including the tail end of Hurricane Beryl a week prior, had already overtaxed the GTA’s drainage pipes, sewer systems, dams and reservoirs. The familiar infrastructure we’ve relied on for more than a century is not only aging, it was never equipped to handle the volume of water that now inundates our streets.
Toronto is certainly not alone — to borrow a title from Margaret Atwood, 2024 was the year of the flood. From the American South to sub-Saharan Africa, storms supercharged by climate change led to catastrophic flooding, thousands of deaths and billions of dollars of property damage. While there’s little that can be done to change the course of extreme weather systems, we can change the way we deal with them. The best strategy may be the most counterintuitive one: rather than fighting to keep water out, we should figure out how to let it in.
That’s the rationale behind “sponge cities,” a concept that Beijing-based landscape architect Kongjian Yu began developing 30 years ago. Yu’s approach is to replace conventional grey infrastructure with green infrastructure — trees, gardens, ponds and the like — that can absorb large amounts of rainwater and slowly release it into our ecosystem. In the decades since he came up with this idea, Yu has built hundreds of parks — in his native China and more than 200 other cities across the world — and has come to be recognized as a visionary in using sustainable architecture for positive change.
Yu says the kernel of this idea is rooted in the fact that the conventional stormwater management techniques in place throughout Asia were never intended for tropical climates. That infrastructure has consistently failed in countries like India and Malaysia — and as climate change shifts conditions around the globe, it’s failing everywhere. “Today, the whole world is monsoonlike,” he says. With urban flooding top of mind for many municipal planners, Yu’s approach is being integrated into the urban landscape of cities from Malmo to Montreal — and is gaining currency in Toronto, where even postage stamp–sized patches of green space highlight “sponge city” principles. Here, a selection of innovative projects that showcase Yu’s natural infrastructure in action.
Yu transformed the site of a tobacco factory in central Bangkok into a singular green space bigger than Central Park.
The primary sponge feature of Benjakitti Forest Park is its three separate wetlands — cut-and-fill techniques created a series of polka-dot-like islets planted with thousands of new tree seedlings. Those low-cost, low maintenance plantings were enhanced with additional native plants to create a diverse and dynamic ecosystem — “messy,” in Yu’s words — that has attracted greater biodiversity to the city’s core.
The wetlands also remediate contaminated water from a nearby canal, which nourishes the system during the dry season, and a specially designed floodplain captures and stores excess stormwater for future use.
Sponge infrastructure doesn’t need to be big or elaborate.
A simple flower garden, a thoughtfully planned backyard hardscape or a specially designed sidewalk can redirect rainwater.
These tiny parkettes in Toronto’s Liberty Village neighbourhood, designed by PLANT Architect, replaced conventional concrete pavers with a mixture of rain-absorbing native plants, trees and permeable asphalt.
“All small public spaces can act as sponges to help relieve stress on the large system,” says Julie Ourceau, a landscape designer at PLANT.
Of course, such interventions also beautify a community, making it more appealing to visitors and residents alike.
Yu claims that in neighbourhoods where he’s built sponge parks, property values have doubled or even tripled.
When designing this park in China’s southern Sanya City, Yu was inspired again by the ancient cut-and-fill farm-building techniques used in the Pearl River Delta.
A central lake is dotted with tiny islands, each planted with a single banyan tree designed to remove nutrients from the water and, over time, to form a canopy that will cool the lake.
Yu made sure to design for extremes — the park is accessible in both rainy and dry seasons.
When the site occasionally floods, visitors can walk above the tree canopy on an elaborate elevated walkway that spans the length of the park.
Though the park is a little more than 66 hectares in size, it has the capacity to regulate a million cubic metres of stormwater a year. “If you have 20 per cent of green space functioning like a sponge,” Yu says, “you can solve virtually all the problems of urban floods.”
Leslie Lookout, Toronto’s latest park, turns a wedge of the industrial Port Lands into a kind of miniature Sandbanks.
Better yet, all the rainwater that falls on the park (and adjacent Leslie Street) is absorbed by the park’s particular sponge features: a compact Miyawaki forest full of hundreds of native plants and trees, a new kind of permeable asphalt and the beachfront itself.
“We didn’t use a single stormwater pipe on site,” says Poyani Sheth, a senior civil engineer with Arup, the firm that designed the park’s stormwater management strategy. “No manholes, no catch basins, no nothing. It’s all green infrastructure.”
Once absorbed, not a drop of rainwater enters the sewer system; instead, it slowly seeps down to the water table and into Lake Ontario.
To find out more about how sponge city principles can help municipalities withstand even the worst weather conditions, listen to the latest episode of the MaRS podcast Solve for X: Innovations to Change the World.