Flood resilience - MMC - Plan Archipel

Following the last two major flooding events in 2017 and 2019, the Montreal Metropolitan Community (MMC – 82 municipalities) developed a flood resilience portal, with the objective of proposing an approach for aligning and updating the data on flood-prone areas in the MMC. This a new way to manage flood-prone areas. The Plan Archipel flood plan has two main components: the first phase (late 2020) is aimed at carrying out floodplain mapping; proposing risk-based regulations; proposing new Quebec regulations in collaboration with the Government of Quebec; installing 29 real-time water level measuring stations; building a flood forecasting and monitoring website; and conducting pilot projects to characterize the risk in municipalities prone to flooding. The second component of the plan (late 2023) will consist primarily of characterizing the risk to the area as a whole. Its objectives are to ensure the safety of individuals; minimize damage to the built environment; minimize material losses; reduce the vulnerability of elements exposed to flooding; and avoid the addition of new vulnerable elements to flood-prone areas.

Understanding and Assessing Impacts

Currently, floodplain management is based on a single variable: the probability of a flood occurring that would lead to flooding of the area. This is the well-known 0–20 years / 0–100 years approach, which has been used in Quebec for many years.

On the other hand, a risk-based approach to flood management involves taking into account not only the probability of flood occurrence but all factors affecting the risk level. Characterization of these factors is necessary to measure the level of risk and identify options to reduce it.

The MMC would therefore like to implement a risk-based approach throughout the area under its jurisdiction. This approach is internationally recognized by flood experts, including the French organization Cerema.
The following are the main steps involved in measuring flood risk level: First, characterization of the probability of flooding is calculated based on the flood hazard and the effectiveness of preventive and protective measures. Vulnerability, defined as the susceptibility to damage, is then characterized. Vulnerability is determined by the significance and nature of the elements that are exposed to flooding and their degree of exposure. A flood risk assessment matrix is used in this step. Lastly, actions to reduce the probability of flooding and/or the vulnerability of the exposed environment are developed.

Identifying Actions

FLOOD MAPS: The MMC Council amended an interim control by-law (ICBL) at its June 18 meeting in order to integrate nearly 200 new flood maps produced in the past year and which complete the mapping of watercourses in the archipelago of Greater Montreal.

WATER LEVEL STATIONS: A total of 29 gauging stations will collect water level data across the metropolitan area, monitor the behaviour of the main watercourses and determine 3-day forecasts, which will be disseminated on a website.
In conjunction with the data now included on the new flood maps, the data collected will be used to implement preventive measures adapted to periods of flooding, better direct the efforts of public safety teams and optimize the management of temporary protection structures (sandbags, floodwall, temporary dike, etc.). Prioritizing actions via risk analysis. Once there is a better understanding of the risks linked to flooding in the MMC, it will be easier to identify actions to improve flood resilience. The MMC recognizes the importance of providing a comprehensive and integrated flood response: relying on several tools to ensure sustainable development of the areas concerned—where vulnerabilities will be minimized and where land use changes will increase flood resilience—while improving the attractiveness of riverside municipalities.

Examples of response measures aimed at reducing the probability of flooding and/or the vulnerability of the exposed environment:

  • Dike and valve
  • Retention basin
  • Pumping system
  • Water regulation
  • Other anthropogenic measures

Examples of response measures aimed at reducing the magnitude of risks:

  • House raising
  • Resilient structures
  • Renaturalization
  • Flood proofing

Implementation

The MMC has developed a methodology that integrates geomatics tools and expertise. The methodology makes it possible to produce a detailed map illustrating the boundaries of areas at risk of reaching flows standardized by the Quebec Protection Policy for Lakeshores, Riverbanks, Littoral Zones and Floodplains (PPRLPI) as well as the highest known high water levels. To do so, it performs geomatics processing comparing the terrain model directly with the 2D hydraulic modelling grid network. The methodology also makes it possible to more accurately forecast water levels at the shoreline for a given flow rate and illustrates the potential severity of a flood event using a coloured gradient that represents flood elevation. It is the first of its kind in North America.

Hydrological and hydraulic modelling of watercourses in the archipelago are carried out in order to better characterize flood hazard. In addition, 29 water level data collection stations will enable the watercourses to be monitored. Together, these models and measures will enable the production of water level forecasts out to 3 days. These data will be shared on a website to better inform decision making with respect to preventive measures and public safety interventions.

Flooding is one component of the MMC’s Plan Archipel. This wide-ranging plan, which aims to group together water-related issues in the MMC, includes two objectives:

  1. Transform the archipelago into an attractive environment by implementing Trame verte et bleu projects and improving water quality.
  2. Strengthen the resilience of Greater Montreal by ensuring better flood risk management and by taking actions that promote renaturalization, requalification or sustainable land use.

Outcomes and Monitoring Progress

Flood risk mapping and 3-day water level forecasts. A support program for municipalities, the general public (outreach) and specialists. Developing land use and urban planning measures to promote resilience in flood-prone areas. Support for renaturalization and reforestation projects. Propose better ways to adapt to climate change through actions that ensure a more effective response in the event of a major spill and maintain a high-quality water supply. The MMC monitors the implementation of the Plan Archipel using the table included in the action plan, which summarizes the main elements of the action plan and the selected monitoring indicators. It is important to note that the purpose of several actions is to put in place programs that will support a variety of actions taken by partners. In these cases, the programs developed will themselves include a follow-up procedure and indicators to monitor changes in phenomena with which they are associated.

PILOT PROJECT IN THE DEUX-MONTAGNES RCM: Prior to adopting a risk-based approach throughout the area under its jurisdiction, the MMC will implement a pilot project in the Deux-Montagnes RCM, where frequent flooding occurs. The RCM includes the municipalities of Saint-Marthe-sur-le-Lac, Deux-Montagnes and Pointe-Calumet. The MMC will conduct risk characterization using methodologies developed and tested in France by Cerema. A number of elements will be measured throughout the area of the RCM to determine its flood risk: recurrence, severity, winds, routes of entry for water, protective measures, location of buildings and infrastructure, property values, etc. Upon completion of the pilot project, the MMC will characterize the risk to the area as a whole, which will greatly assist decision-makers and emergency preparedness experts in better managing flooding and its impacts.

Next Steps

The Bureau de projet de gestion des risques d’inondation (BPGRI) of the MMC has a project that consists of two phases. The first phase, which will be completed in late 2020, is aimed at:

  • carrying out floodplain mapping (completed in June 2020, except for the Richelieu River)
  • proposing risk-based regulations
  • proposing new Quebec regulations, in collaboration with the Government of Quebec
  • installing 29 real-time water level measuring stations
  • building a flood forecasting and monitoring website
  • conducting pilot projects to characterize the risk in municipalities prone to flooding.

The second component of the plan, expected to be completed in late 2023, will consist primarily of characterizing the risk in the area as a whole.

The BPGRI intends to establish a variety of measures to better manage flood risk in the Greater Montreal Area. The objectives are to:

  • ensure the safety of individuals
  • minimize damage to the built environment
  • minimize material losses
  • reduce the vulnerability of elements exposed to flooding
  • avoid the addition of new vulnerable elements to flood-prone areas.

Resources

Link to Full Case Study (in French only)

Additional resources: