Climate trauma | Longueuil tackles its rain floods

Basements and garages flooded repeatedly, damage that is no longer insurable, citizens at the end of their rope. Aware that episodes of extreme rain are likely to increase, the City of Longueuil began looking for solutions.

A little over a year ago, on September 13, 2022, Longueuil received almost 70 millimeters of rain in less than an hour. A flood. In several areas, the municipal sewer network was not sufficient for the task. Water infiltrated homes, businesses and even schools.

“We must recognize that for the citizen, it is a great source of stress, a great source of loss,” admits the City’s new chief scientific advisor, Julie-Maude Normandin.

The South Shore municipality was one of the first in Quebec to have such a function. When she arrived on the job last summer, the scientific advisor found rain flooding on top of her pile.


Julie-Maude Normandin, chief scientific advisor to the City of Longueuil

Serial sewer backups, claims that have become uninsurable, concerns about the value of property, exasperation over clogged cesspools: the citizens who came to testify at the public meetings organized by Longueuil a few months earlier had a lot on their hearts.

The floods of September 13, 2022 were “the great trigger for reflection and action in the City”. Notre-Dame-de-Lourdes college, in Longueuil, even had to close temporarily, as did Durocher college, in the neighboring municipality of Saint-Lambert.

Slow down the drop of water

In Quebec, “good practices” provide that sewer networks be built for extreme rain every 10 years, reports Mme Normandy.

But with climate change, the average total height of extreme rains as well as their maximum two-hour intensity are expected to increase. The season should also lengthen. 1

Part of the solution lies in “rethinking the network”, but that cannot be the only answer. Financially, we will not provide, and increasing the size of the network, in certain places, we will not be able to do it.

Julie-Maude Normandin, chief scientific advisor to the City

“Green infrastructure” (parks to divert water, grassy borders as retention basins) must be part of the “adjustments”. Citizens can also “do their part” by installing backwater valves and rain barrels.

Like the Dr Arruda who wanted to “flatten the curve”, Longueuil wants to “flatten the speed” of the water.

“It’s the speed at which it arrives in our networks that causes our overflow. So, if I can store it in certain places temporarily, or even absorb it in grass, so that it arrives later in my network, when the intensity of the rain has lessened, that will help me. We need to slow down the water, that’s really it. »


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