At COP15, Arup releases Montréal’s Sponge City Snapshot, highlighting that nature-based solutions help cities build resilience as they face climate change

In Montréal, data shows the city’s “sponginess” ranking is boosted by the fact that its green spaces contain a high proportion of trees, including the iconic Mount Royal Park. This park, along with many others in the city, is a naturally spongy green space that goes a long way in mitigating the impact of more frequent summer rainstorms. The urban forest also provides numerous co-benefits ranging from provision of natural habitat to reduction of urban heat island effects, promotion of recreation to nurturing wellbeing.

The increase in wintertime rainy days is a particular threat to Montréal and other northern cities. Global warming is causing temperatures in cold climate cities to increasingly hover around the freezing point. Instead of snow, rain is beginning to fall more regularly on densely compacted frozen ground. Water absorption is impeded in these types of winter conditions, causing rainwater to flow across the surface, presenting a new winter flood risk for northern cities. Implementing nature-based solutions that actively manage stormwater can improve the absorbing capacity of compacted frozen ground.

“Cold climate cities must design nature-based solutions to better absorb water, acting like a sponge even when temperatures drop below freezing. Well-designed blue-green infrastructure can absorb water even in cold conditions. Healthy soil and root ecosystems create a layer of heat, trapped between the soil and snow, becoming a more effective 'sponge' and contribute to the city’s resilience,” added Ormsby.

In addition, blue-green infrastructure can be precisely designed to overcome the burden of salted run-off from winter de-icing practices. Proper siting, inlet protection and salt-hardy plant selection are but a few of the measures designers can adopt to ensure nature-based solutions can survive and thrive despite harsh winter conditions.

To create the Sponge City Snapshot calculations, Arup used the firm’s advanced proprietary digital tool, Terrain, applying 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) from satellite imagery. 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. The cities were ranked based on these factors.

The study area includes approximately 150km² of the Island of Montréal, spanning from the Saint Lawrence River in the south, including Verdun and the Southwest Borough; the Olympic Stadium in the East; the Trans-Canada Highway in the North; and the CN Rail intermodal yard in the West. Within this area is notably Mont Royal Park and the Central Business District.

Global Sponge Cities Snapshot

Arup's Sponge City analyses in cities across the globe have provided unprecedented data to help cities assess their “sponginess” and prepare for flood events and climate change. We developed the Global Sponge Cities Snapshot, initially looking at the sponginess of Auckland, London, Mumbai, Nairobi, New York, Shanghai, Singapore, and Sydney. Arup also published an Africa Cities Sponge Cities Snapshot during COP27, including Cairo, Durban, Kigali, and Lagos.