A flood occurs when an shoreline area is submerged because the volume of the body of water has surpassed the capacity of its bed. Major floods usually happen when snow melts in the springtime, during periods of heavy rain, but also in wintertime when frazil accumulates (see Types of Floods). A quick and major increase of the water level can also occur when a dam breaks.
Preparing and protecting yourself
Risks in Montréal
Boroughs and towns on the shores of Rivière-des-Prairies are the most likely to be flooded. The primary areas are:
The Des Prairies river is primarily fed by Deux-Montagnes lake and the Outaouais river, the main tributary of the Outaouais watershed. The northern part (about 33%) of this basin is controlled by dams, which retain a certain quantity of water for energy purposes. The southern part of the basin (around 67%) flows directly into the Outaouais river, and its flow is directly related to precipitation and melting snow in the springtime that runs into the upstream tributaries. Higher water levels in the Des Prairies river are caused by three distinct phenomena: thawing, dam formation or frazil.
The risk of flooding is not as great for towns and boroughs south of the island of Montréal (along the St. Lawrence river).
Types of floods
Springtime floods usually occur from early April until the end of May. The increase in the water level is directly related to the flow of the Outaouais river. The water flow depends on a number of interrelated factors, primarily the amount of snow that fell over the winter and how quickly it thaws in the springtime.
Dams form during periods of major thawing, mainlyly from the beginning of April to the end of May. For the past several years, climate change has contributed to periods of thawing in January and February. Dams form when ice detaches from the ice cover at a certain point in the river, creating an obstacle to normal flow. Dams can also be caused by human phenomena, such as the breakage of booms that cause ice covers to form.
Frazil forms when very cold temperatures follow a period of moderate temperatures. This phenomenon can be observed between the end of December and the end of January.
In narrower sections of the river, water speed is greater and no ice cover forms. The water is in constant contact with cold air, which forms frazil. Frazil may look like slush, a mixture of ice crystals and water.
Frazil crystals mix with water and travel with it. When frazil arrives in wider sections of the river, the flow slows down. The ice cover enables frazil to accumulate in layers as thick as eight metres. The frazil condenses and forms a barrier that obstructs water flow.
A study on dam failures indicated that certain areas of the southern island of Montréal, especially around the Lachine Canal and in the port of Montréal, would be flooded if a hydroelectric dam upstream were to break. This study was conducted after the Saguenay flood in 1996. New regulations about dam safety have since been created, reducing the likelihood of such an event.
Impacts on the population and infrastructures
- Problems with water system and sewers, such as sewer backup
- Damage to flooded homes and buildings
- Flooded roads that limit access to certain areas or impede traffic
- Damage to roads and bridges
Measurement and alert system
Since January 2008, 18 telemetric stations monitor water levels for the Des Prairies and St. Lawrence rivers. These stations were installed at strategic locations around the island of Montréal and were calibrated to data around the areas where floods have had the greatest impact historically.
These stations make it possible to monitor the evolution of water levels and quickly detect a situation that could become problematic. The system identifies three alert levels according to established thresholds. These levels define actions to take and preventive measures to implement before the flood even happens. The alerts are sent to the appropriate response workers via text message.
Special response plan for floods
In certain parts of the island of Montréal, rising water levels can cause floods of varying severity. The objective of the special response plan for floods is to anticipate and plan city and borough responses in case of flooding to protect the population, their goods and the environment.
Criteria to determine the level of mobilization
The levels of mobilization are determined by trigger levels of telemetric stations (see the section above on measurement and alert systems). Levels 1, 2 and 3 are indicators that correspond to water levels in the river. These levels are not the same for all stations and enable implementation preventive and preparatory measures during a flood.
Levels of mobilization, indicators and actions
(March and April)
- Monitor weather conditions that could cause frazil formation in wintertime and flooding in springtime.
Fewer than 3 criteria
At least 3 criteria
- Begin surveillance and assessment operations such as inventory of response equipment, review of procedures and intensified surveillance.
At least 3 criteria
- Preparing response equipment:
- Submersible pumps
At least 4 criteria
- Begin installing response equipment and implement borough emergency measures for flooding
- Emergency measures coordination centres and emergency operation centres open in boroughs and neighbouring towns.