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How spongy is Toronto? Global survey reveals city’s natural ability to absorb rising rainfall

Danièle Gauvin Danièle Gauvin Americas Press Office,Toronto
15 March 2023

Toronto has joined Arup’s Global Sponge Cities Snapshot – a global survey of the natural ability of cities to absorb rainfall. The survey has shown that the urban center of Toronto is far ahead of Sydney and London in terms of its “sponginess,” with its sponge rating of 30% (compared to 18% for Sydney and 22 % for London). Toronto obtained the same sponginess rating as New York, Mumbai, and Singapore.

Arup, a global sustainable development consultancy, developed the snapshot to prompt cities to ask: how spongy am I? The authors of the survey are calling on leaders to move beyond concrete interventions and instead look to nature for solutions to climate-related challenges, such as managing heavy rainfall.

Authors have studied sample areas of approximately 150km2 in ten diverse global cities with Toronto joining Auckland, London, Montreal, Mumbai, Nairobi, New York, Shanghai, Singapore, Sydney – to assess how well their existing natural infrastructure helps them absorb rainfall.

This analysis comes as the IPCC predicts that water-related risks will increase with every degree of global warming, with around 700 million people currently living in regions where maximum daily rainfall has increased.

A digital illustration of analysis using shades of green and blue to indicate sponginess. The illustration is of the map of Toronto. A digital illustration of analysis using shades of green and blue to indicate sponginess. The illustration is of the map of Toronto.

Toronto ranked third in the sponginess snapshots, just ahead of Montreal and tied with New York City, Singapore, and Mumbai. Over one quarter (29%) of the land analyzed in Toronto is covered in trees. The city’s sponginess rating is also based on the soil’s classification and run off potential.

Toronto’s sand and clay soil mix is comparable to New York City, though Toronto’s ability to absorb runoff is slightly lower. Both North American cities have the same level of green and blue spaces in their downtown core (39%), but with different concentrations and characteristics. In comparison, Mumbai and Singapore have a much higher blue-green cover of 45%, while both also have much less permeable soil than Toronto or New York. As a result, all four Snapshot cities have a sponginess rating of 30%, based on different land cover compositions.

By comparison, London and Shanghai align much more with the “concrete jungle” stereotype, with a higher percentage of hard surface in their urban centers: 69% and 67% respectively. This places them as the two least “spongy” of the cities surveyed.

In Toronto, the study area includes approximately 150km2 of the city including the high-density downtown core, a mix of medium- and low-density housing, and major urban parks. The Snapshot shows Toronto’s ravines function as a network of urban green infrastructure which mitigates flooding risks. The city’s interconnected ravine system, one of the largest ravine networks in the world, links with natural watercourses to receive, filter, and transport stormwater from urban landscapes to Lake Ontario. Toronto has already begun expanding blue-green infrastructure across the city, particularly along the lakeshore to the south. Arup has been involved in the design of many of these projects, including the Queens Quay revitalization, Corktown Common, Aitken Place Park, and Love Park (now under construction). These projects, along with the City of Toronto’s newly published Green Infrastructure Standards, led by Arup and landscape architect DTAH, are establishing green infrastructure as a City priority.

A chart of information indicating the sponginess of global cities. A chart of information indicating the sponginess of global cities.

To create the calculations, Arup’s team used an advanced digital tool, Terrain, which applies machine learning and artificial intelligence techniques to accurately quantify the amount of green infrastructure (e.g., grass, trees) and blue (e.g., ponds, lakes), versus the amount of grey (e.g., buildings and hard surfaces). The survey was based on detailed satellite imagery covering a snapshot of approximately 150 square kilometers of each city’s main urban center. Authors supplemented this analysis with insight on soil types and vegetation, enabling them to estimate how much rainwater would be absorbed in a defined heavy rainfall event.