Storm Debby: Laval Releases Its Reports

This article written in french by Stéphane St-Amour was automatically translated using AI

Ten months after the August 9, 2024, deluge that flooded approximately 7,200 private properties in Laval, the Boyer administration has released two reports produced by the Municipal Resilience Office and the Engineering Department.
One of them, the strategic report on crisis management, aimed in particular to identify the lessons learned from this painful experience and to propose recommendations to improve risk management and make the city more resilient.
The technical report on the performance of municipal infrastructure focused on analyzing the impact of this historic rainfall on the city’s drainage and sewer systems.

“This assessment demonstrates our willingness to learn, adapt, and above all, take action,” said Mayor Stéphane Boyer in a statement issued on June 2. “We must ensure that our citizens are met with a level of preparedness and response that reflects the new climate challenges.”
He reiterated that the City will invest $350 million by the end of 2027 to strengthen its water infrastructure—$100 million more than what had been budgeted in the previous Three-Year Capital Investment Program (PTI).

Aging Infrastructure
The engineering report notes that most of the buildings flooded by Storm Debby are located in areas served by combined and pseudo-separated sewer systems.
Built up to around 1965, the combined system collects both sanitary and stormwater in the same pipe. Notably, this type of system serves a quarter (24%) of the Île Jésus territory.
The pseudo-separated sanitary system, on the other hand, was mostly built during the 1970s and early 1980s. Made up of a sewer line dedicated exclusively to transporting stormwater collected from street catch basins, this network channels wastewater and runoff from buildings to the treatment plant.

However, these older systems—which make up 60% of Laval’s sewer network—become overloaded during heavy rainfall, causing sewer backups and wastewater overflows into the environment.
Still, “replacing a vast proportion of Laval’s sewer network would likely not be a socially, temporally, or financially acceptable solution,” the Engineering Department concludes.

Natural Basins
It is also learned that the runoff generated by Storm Debby would have exceeded by about double the capacity of the retention structures, whose protection recurrence is 100 years. In plain terms, this means that these basins were designed to withstand a rainfall event likely to occur only once a century.
Unsurprisingly, the areas where the most damages were reported are, for the most part, located in these natural basins into which surface water and overflows from the overloaded sewer system flow by gravity.

Furthermore, during the August 9, 2024, deluge, the team at the pumping stations of the Water Management Department had to deal with power outages in addition to noting that some pumping stations had reached their mechanical pumping capacity.
That being said, the authors of the report note that the “operation of gravity overflows” at certain pumping stations “made it possible to evacuate the water despite power outages or reaching pumping capacities.”

Measures Put Forward
In light of the observations and recommendations made by internal teams, the City will deploy in the short term a series of measures, including “the addition of generators to pumping stations, the improvement of stormwater network maintenance, especially in vulnerable areas, as well as the implementation of innovative infrastructures such as sponge streets, rain gardens, and an underground basin” to strengthen overflow management.
An integrated stormwater management plan is underway to optimize the entire water cycle in Laval, it is also stated.

Risk Management
On the other hand, the strategic report on crisis management, written by the Municipal Resilience Office, provides a precise assessment of the interventions carried out in the wake of Hurricane Debby. It highlights in passing the “proactive response” in civil security and the 25,000 hours of work provided by municipal staff in support of affected households.
In terms of improvements to be made, it notes nonetheless “a difficulty in quickly estimating the number of victims and issues in the collection and analysis of real-time data,” summarizes the press release published by the municipal administration, which also commits “to implementing additional mechanisms to correct the identified vulnerabilities, strengthen governance in civil security, increase the territory’s adaptive capacity, and ensure rigorous follow-up of actions.”

Shared Responsibility
While acknowledging its “crucial role in sustainable urban planning [and] the implementation of resilient infrastructure,” the City emphasizes that “the engagement and mobilization of citizens is just as important in achieving greater municipal resilience.”

In this regard, the engineering report illustrates that when an extreme weather event exceeds the capacity of the sewer networks, reliance must be placed on the “internal plumbing protection devices of buildings” to avoid the worst.
“Property owners must ensure that their plumbing system complies with the various current standards and must perform regular maintenance on protective devices.”

The drainage of private land is also part of the equation.
“Gradients must direct runoff water away from foundation walls, and mitigation measures should be implemented in cases where there is low-permeability soil (such as clay soil, for example),” suggests the Engineering Department in the conclusions of a 50-page report.

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