Winter Montreal skyline; Winter Montreal skyline;

Sub-zero resilience: assessing 'sponginess' can help cities manage flooding

Cities in cold climates face unique challenges from climate change effects like increasing winter rainfall, springtime snow melt and extreme temperature variations. In this article we explore the ways such cities can build natural resilience to these challenges and find sustainable ways to deal with greater annual volumes of ice and snow within the urban fabric. 

Given that Montréal was recently in the spotlight, as host city for 2022’s COP15 United Nations’ Biodiversity conference, Arup has added Québec’s largest city to its Global Sponge Cities Snapshot. This first of its kind survey was designed to get cities asking – how spongy am I? The authors originally studied the urban centres of eight diverse global cities – Auckland, London, Mumbai, Nairobi, New York, Shanghai, Singapore and Sydney – to assess how well their natural infrastructure helps them absorb rainfall using our AI and land-use analysis tool Terrain.  

Montréal emerges ‘mid-table’ with a ranking of 29%, which places it in the fourth rank among the most “sponge-like” cities. This places Montréal’s urban centre just behind Mumbai, New York and Nairobi, at 30% jointly, and well ahead of London and Sydney which rank bottom, with 22% and 18% respectively. Montréal’s ranking is largely due to the study area’s prominence of impervious surfaces and buildings: however, one fifth of the land surveyed also provided tree coverage. 

Cities in cold climates are facing increasing flooding

Montreal, alongside many cold climate cities, has seen record-breaking floods in the past few years. Snow melt is bringing flooding to waterfront areas, while more intense, more frequent rainstorms, even in the winter season, are causing flooding across its urban centre. Not only do such floods impact the ability of cities to function, but they can also result in the loss of homes and livelihoods, with an un unmeasurable human impact.

Improving natural permeability or “sponginess” is just as important a solution in cold climates as in others – it allows excess water to make its way into the ground, while also tackling urban heat effects in warmer months and bringing education and awareness about the role of nature within urban environments.

However, the interventions and tools used must be unique to the realities of cold temperatures. Blue and green assets created need to be able to withstand freezing temperatures, intense snow removal regimes throughout winter, the use of de-icing salts, and temperatures fluttering around freezing point through large chunks of the shoulder seasons.

Four essentials for introducing blue and green solutions in cold climates

1. Use native and hardy species

Delicate landscapes or non-native species have little chance of withstanding freezing temperatures and snow cover. Green spaces built with native and hardy species have the best chance of thriving through colder seasons and require less maintenance, fertilizing or manicuring from the municipality.

2. Cultivate healthy soil

More frequent winter rainstorms falling on frozen ground are a particular challenge, but well-designed, well-positioned green infrastructure can act as a sponge 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 contributing to a city’s resilience.

3. Watch for salted run-off

Nature-based solutions in cold climate cities, like Montréal, must be designed in a way that can survive aggressive de-icing practices along roads and walkways. Blue-green infrastructure must be planned, positioned, designed and built in a way that minimizes salt accumulation in their soil and on their foliage. Where salt-laden runoff cannot be avoided designs that capture water during every rainfall, not just the most intense downpours, help flush out the chlorides that do make their way into the system over the winter months.

4. Keep it simple

Grates, pumps and valves can clog, freeze and corrode in harsh winter conditions. Simple water management solutions that work in harmony with nature, without moving parts, function most reliably. For winter storms on deeply frozen ground, a strategic grading plan can temporarily contain runoff or channel it to keep the community and critical assets safe from flooding.

Cold climate cities clearly face unique challenges when it comes to deploying blue-green infrastructure, with de-icing chemicals, heavy snow equipment and deep penetrating frost just some of the additional stresses to overcome. But this should not be a barrier to implementing blue-green infrastructure. Nature-based solutions, designed with cold climates in mind, can help these cities remain “spongy” through rain, shine, snow or sleet – helping them become more resilient as they increasingly face extreme weather events.