How can we adapt our living environments to torrential rains?

Hundreds of residences flooded. Closed roads. A metro station in the fleet. The torrential rains that fell on Quebec on Tuesday evening caused significant damage. How can we adapt our living environments to avoid the worst consequences of these extreme climatic events, which promise to multiply?

Events that will multiply

Torrential rains like the one that fell in the Montreal region on Tuesday evening are exactly the kind of extreme events that will increase in frequency and intensity with climate change.

“It is a certainty that events like yesterday [mardi] will be more and more frequent,” says Ursule Boyer-Villemaire, climate risk specialist at Ouranos, a research consortium on adaptation to climate change. Since warmer air can hold more moisture, it can then dump more rain.

It is difficult to establish with precision the increased risk of very heavy rains in southern Quebec. The natural variability of the hydrological cycle is already high in the province. However, in some regions, extreme precipitation could become up to 20% more frequent, according to the researcher.

“If we agree to no longer bury our heads in the sand, we must absolutely act to adapt our use of the territory and our infrastructures,” argues Boyer Villemaire.

Ironically, the torrential rains fell just hours after Quebec’s major cities asked the provincial government for a $2 billion « green pact » to help them adapt to climate change. The outgoing prime minister, François Legault, immediately refused this request.

Last April, the CAQ government unveiled its 2022-2027 implementation program for its Plan for a Green Economy. Adaptation to climate change does not play a large role: only 5% of the budget envelope is earmarked for “strengthening resilience”.

Transforming the sewer network

According to engineers, a storm sewer system consists of two parts: the minor network and the major network. The first includes underground pipes. It can accept water from very heavy rains, which occur every 2 to 10 years, without overflowing. The second, the major network, is used during extremely heavy rains, which occur for example only once a century.

The major network is in fact “the streets and the ditches”, explains Jean-Luc Martel, a professor at the École de technologie supérieure, specialist in urban hydraulics and the repercussions of climate change on infrastructures. It is therefore necessary to intelligently design the districts so that this overflow into the “major network” takes place in a controlled manner. These precautions were not taken a few decades ago.

In new real estate developments, the streets and their surroundings are designed to better accommodate the water from these extreme precipitation events. Municipalities can, for example, create retention basins — sometimes a simple depression in the ground, or otherwise, in more densely built environments, an underground structure — to contain the water from ultra-rare events.

It’s all about trade-offs: are these infrastructures worth it, given the risks involved? As the climate changes, the assumptions of yesterday’s designers are shattered. « Perhaps the damage will be greater than we thought, and it would have been worth the cost to invest more in our networks to make them more resilient, » said Mr. Martel.

Greening and naturalizing cities

The islands of greenery act like sponges. When heavy rain hits a city, vegetated surfaces can slow water runoff and give storm sewers time to evacuate water before it builds up.

One of the best ways to slow down this water is to create “bioretention cells”. Sarah Dorner, a drinking water specialist at Polytechnique Montréal, collaborated on a project in Trois-Rivières where such plant cells were set up along a street. Sumps now direct water there.

« It’s not that expensive to make bioretention cells when you have to rebuild a street and its sewers anyway, » explains Dorner. These cells also have the advantage of filtering rainwater before it returns to the environment. They also provide a more pleasant environment for citizens and reduce heat islands.

Obviously, an all-out greening of cities is beneficial to protect them from torrential rains. In a city like Montreal, explains Dorner, flat roofs intercept a good part of the rainwater, and return it directly to the sewer. Green roofs can slow this flow. Trees can also deflect rain to permeable soils.

“The government could send a clear signal to encourage demineralization among private owners,” adds Boyer-Villemaire d’Ouranos.

Adapting the Montreal metro

Rainwater made its way Tuesday evening to the Square Victoria-OACI station of the Montreal metro. The service was interrupted there, but the water accumulated on the rails did not prevent the trains from circulating to serve the other stations of the orange line.

The metro is designed so that water infiltration is directed towards certain “safe places” on the track, explains Philippe Déry, spokesperson for the Société de transport de Montréal (STM). Pumps then evacuate it. When major floods occur, auxiliary pumps can be deployed.

For now, the STM is focusing on infrastructure maintenance — repairing cracks in the concrete, replacing waterproofing membranes, etc. — to protect the metro from heavy rain. A recently formed committee is also working on developing a climate adaptation plan.

One thing is already certain: the future stations of the extension of the blue line will be higher above street level. As for the existing stations, the costs to lift them would be very significant, warns Mr. Déry. This option is therefore not considered for the moment.

“Currently, we are not making requests [aux gouvernements]but it is certain that adaptation is a very important element that will guide our actions in the coming years,” concludes Mr. Déry.

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