Sponge cities in the age of the flood

On July 13, 2024, Toronto was soaked with 126mm of rain in about 90 minutes. This amount of rain is the 5th most rain ever recorded for Canada’s most populous city. This qualifies as a 100-year storm. Except, we just had one of these storms, equally terrible, 11 years ago. Just like in 2013, streets were underwater, highways closed, and drivers and passengers were caught in dangerous situations in partially submerged vehicles, trains, and streetcars. Water flooded the entrance hall and down the formidable stairs of Union Station. Houses across the city were without power, flooding from sewage backups AND torrents of rain simultaneously.

On top of it, an estimated 1.3 billion litres of raw or partially treated sewage flowed into the city’s waterways, making its way into Lake Ontario: the drinking water source and beloved recreation space for millions of people.

I live in the flood plain of Hurricane Hazel, which hit Toronto in October of 1954. Over 80 Torontonians lost their lives in the flooding caused by the 200mm of rain Hazel dropped on the city. The deluge transformed the Humber River and other city creeks into roaring nightmares. In 1954, no one was prepared for this much rain and what it was capable of.

But in 2024, compared to the rest of the city, my neighbourhood was virtually unaffected, even as the Humber River spilled its banks again.

In the aftermath of Hurricane Hazel Toronto enacted new regulations that restricted new development in flood plains. This was meant, at least on paper, to allow Toronto’s rivers to flow naturally and reduce the risk to people and infrastructure during flooding. Since the 1950s, more impressive and ambitious steps have been taken towards flood protection, such as green roofs, spongy sidewalks, and the renowned Port Lands Flood Protection project at the mouth of the Don River, which is creating a naturalized river valley.

Despite all this, Toronto still remains a city of mostly impervious surfaces: concrete and buildings. And Toronto is still flooding, more frequently and more intensely than ever. Like many cities big and small, Toronto has done away with most of its natural features. This city was literally built on rivers and wetlands, most of which are now lost or buried.

These lost natural features provide something we desperately need in order to cope with flooding: sponginess.

Sponge city vs concrete jungle

When weighing the causes and impacts of these mega-storms, saying that Toronto, or any other city for that matter, “isn’t designed for this” is true in the urban planning sense. But climate change isn’t the only factor to blame here. The way we design our cities and our traditional infrastructure play leading roles in the inevitable flooding that happens when it rains. (Impervious surfaces + torrential rains of course leads to flooding). To anyone stuck under an overpass on Dundas Street during a storm, it’s as plain as day.

Sponginess is a city’s ability to absorb and hold onto rainfall with it’s natural features, such as trees, wetlands, uncovered creeks and rivers, parks, and gardens.

The concept of the Sponge City was first coined in China, as a urban planning model. China also suffers from flash floods and their costly impacts. Improving sponginess was adopted as a solution to temper the blow. The Sponge City model emphasizes green infrastructures over grey infrastructure (drainage, pipes, concrete retention, etc) in flood management planning. The goal is to limit run-off and maximize absorption into the ground. Absorbing the water for later use is also important, as droughts are as concerning as these extreme precipitation events.

According to ARUP’s Global Sponge Cities Snapshot, Toronto has about 30% “sponginess.” Not so bad, you might say. However, many of these areas of absorption are concentrated in places like our ravines, while huge parts of the city are completely impervious, meaning water can’t be absorbed. Moreover, the study found that “Toronto also has a moderately high runoff potential (less than 50% sand and 20-40% clay), meaning water infiltrates more slowly into the soil.”

We have work to do.

Contribute to sponging up your city

Looking for ways to contribute to green infrastructure and sponginess in your hometown or city?

1. Support municipal policies and plans for sponginess and green infrastructure: If you want to see smart and spongy updates to your infrastructure, you need to get vocal about your support. Reach out to your local city councillors to learn more about green infrastructure plans and flood management.
2. Create a rain garden: If you have a garden or green space, create a rain garden. A rain garden, according to the Toronto Regional Conservation Authority, is “a landscaped feature that replaces an area of your lawn to collect the stormwater (rain and melted snow) that runs off your grass, roof and driveway. This shallow depression has loose, deep soil that absorbs and naturally filters the runoff, preventing it from entering the storm drain system and, eventually, our waterways.” Learn more here.
3. For apartment or condo living, support green roofs: Green roofs, also known as living roofs or vegetated roofs, support the growth of vegetation. Among their many benefits for cities, green roofs help with the management and absorption of rain.
4. Install (or support the installation of) spongy infrastructure, like parks, sidewalks, and parking lots: Cities need to be spongy everywhere, not just in concentrated areas. Modifying the above-surface infrastructure to be spongy is a great way to make more of the urban landscape absorbent.