Spring floods of 2017
"Exceptional 2017 spring floods occurred in the spring of 2017 in several Quebec municipalities. The flood waters affected 278 municipalities, flooded over 5 300 residences and forced the evacuation of more than 4 000 people as well as the closure of several roads "as recalled by the Ministry of Municipal Affairs (Mamot).
Recurrence flood ratings
By definition, floodplain represents the land surface that is submerged by a watercourse when it overflows its bed. "Many factors affect the flow of a river, and therefore the risk of flooding. Among the most important are the amount and type of rainfall, the nature and condition of the watershed, and the climate "(Government of Canada).
"Spring floods are measured in relation to their level and frequency. For this purpose, the terms "return period" and "recurrence" are used. In the Protection Policy for Shorelines, Coastlines and Floodplains (PPRLPI), it is the recurrence flood ratings of 20 years and 100 years that are taken into account to determine the limits of flood plains. They correspond to the flood limits that have respectively 1 chance on 20 (5% chance) and 1 chance on 100 (1% chance) to occur each year "(Minister of the Environment of Quebec - MDDELCC).
"Essentially, the PPRLPI prohibits all constructions, any reconstruction of buildings considered destroyed by a flood, all works and all works, with some exceptions, in the area of high current (0-20 years). Also, it makes construction and works subject to the application of immunization measures in low-current areas (20-100) "(Ministry of Municipal Affairs - Mamot).
- Flood areas:
- Flood area of 0-2 years: 50% risk of being flooded by year;
- Flood area of 2-20 years: 5% risk of being flooded by year;
- Flood area of 20-100 years: 1% risk of being flooded by year.
- Mapping Flood Areas
According to the grouping of watershed organizations in Quebec, the data concerning the flood zones are either almost inaccessible or obsolete: "the maps of flood zones are difficult to access, sometimes incomplete and they date in some cases from the 1970 years" (ROBVQ).
The government has the power to create a special intervention zone (ZIS), by order in the territory of certain municipalities affected by the floods [using Article 158 of the Planning and Town Planning Act]. A ZIS aims to "solve a development or environmental problem whose urgency or seriousness justifies, in the opinion of the government, an intervention" (Article 159 LAU).
In 1987, the government adopted its first Shoreline, Coastal and Floodplain Protection Policy (PPRLPI) and the Land Use Planning and Development Act requires municipalities to comply with this Policy in their regulations. It is therefore forbidden to build in the flood zone 0-20 years, because this goes against the protection of the environment.
Damage to buildings
The spring floods of April and May 2017 caused considerable damage to buildings in flood-prone areas.
Here are some examples of damage caused by floods:
- Slab on damaged soil due to high hydrostatic pressure;
- The appearance of star-shaped cracks in the slab on the ground;
- A structural movement in the foundations;
- The subsidence of the building, etc.
Municipalities require foundations to be water-repellent when a building is located in a flood-prone area (floodplain). Waterproof foundations imply that concrete foundations must have the samearmature necessary to resist the hydrostatic pressure that would cause a flood. The entire external surface of the vertical part of the foundation - located below the flood level - shall be covered with a water-repellent rubberized asphalt membrane with a minimum thickness of approximately 1,6 mm.
In addition, the floor of foundations must be constructed with a base slab (slab), the surface of which is covered with a water-repellent rubberised asphalt membrane with a thickness of approximately 1,6 mm.
In short, the foundations, the footings, the slab on the ground - in the system of water-repellent foundations - must be well waterproof and waterproof. This type of system must not allow water to enter the basement, despite the high hydrostatic pressure that is practiced against these elements.
After flooding a building, the authorities request that a civil engineer visit the property to assess the damage and propose the appropriate solution for the necessary corrections.
St. Lawrence River Basins
The notion of watershed is essential when it comes to flooding. Indeed, "the source of the flood comes from the entire watershed" (ROBVQ). Some experts advocate integrated management of major watersheds to provide flood protection.
Here is a definition of this one: "a watershed is a territory whose limits are determined by the peaks of the mountains and the unevenness of the ground. These mountain ridges and elevations are commonly referred to as the watershed. Each drop of water that falls within the boundaries of a watershed, no matter where it falls, will reach the same outlet at the end of its course. This outlet can be a lake, a river, a river or an ocean, depending on the scale chosen "(CREBSL).
It should be noted that the St. Lawrence is fed by numerous rivers draining 92 watersheds.
"Examples of solutions to be put in place include the restoration of wetlands - which can act as retention ponds -; the delimitation of an area of freedom for watercourses where the residential establishment would be prohibited; the delimitation of non-urbanized natural areas where it would be possible to direct floods; the realization of rain gardens in urban areas "(ROBVQ, watershed organizations).