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.
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. »
At the City’s public meetings, however, Longueuil residents pointed out that at home, they had taken all possible measures and that, in their old neighborhood, there was no space to create parks.
“We want to identify specific issues sector by sector, to be able to act on certain central points,” replies Mme Normandy.
She gives the example of an intersection prone to sewer backups in an area where it would not be possible to intervene. “Are we able to build green infrastructure elsewhere to reduce water pressure? We will have to carry out precise analyzes to act on particular points of vulnerability; we have a team dedicated to this. »
Unlike floods caused by rivers, pluvial floods are still poorly documented.
There is “no citizen alert system based on weather forecasts”, scientific knowledge on the sectors “more affected than others in terms of rainfall” nor “the same level of documentation of the phenomenon” , mentions Mme Normandin, who carried out research on river flooding at the National School of Public Administration (ENAP).
Do not wash your hands
After the floods of September 13, 2022, Longueuil received requests for compensation from approximately 380 citizens.
“I certainly have citizens who have suffered damage and have not made a claim to the City, so I do not have the complete data. And I have the data from the City of Montreal because there was an article published recently in The Press (920 requests), but I do not have the data for the entire metropolitan region,” explains Mme Normandy.
Longueuil, like Montreal, has rejected all these requests for compensation, but the cities cannot wash their hands of this.
“What are we going to do in architecture, in town planning, in citizen risk communication, in civil security? One must use different types of scientific knowledge and disciplines at the same time. »
The scientific advisor cites the Auditor General of Ontario, who devoted a 70-page report to the risks of flooding in urban areas.2
“Insured losses attributable to extreme precipitation in urban areas are 10 times higher than losses caused by river flooding,” we read in particular in this report.
Quebec cities are increasingly interested in this issue, notes Mme Normandy.
“All cities are likely to be affected because almost all of us have a sewer network, and we are going to have episodes of rain over which we will not be able to put a dike! So, we will have to find other types of solutions, and this will involve several combined actions. »