Rainwater and flood zones in Saint-Constant
Updated September 28, 2026
Several waterways cross Saint-Constant: the Saint-Régis, Saint-Pierre and Tortue rivers, and Lasaline Creek. In heavy rain, water from streets and lots drains to the storm sewer, retention basins and ditches, such as the fossé des Prémontrés. In its 2026 prevention guide, the city defines resilience in part as accepting that, in heavy rain, water may collect for a while, while controlling where it collects.
On June 20, 2026, streets flooded and the city's notice speaks of both flooding and sewer backups. For a house in Saint-Constant these are two separate questions: where surface water collects around the lot, and how the basement is protected against sewer backup.
Heavy rain events
What public sources report about heavy rain in Saint-Constant, at sector level.
According to the mayor, the "line of storms" hit Saint-Constant, Delson and Sainte-Catherine directly. An Environment Canada meteorologist quoted by La Presse reported close to 120 mm of rain between the morning and late afternoon. The city's notice speaks of both flooding and sewer backups. Streets flooded and several roads were closed because of standing water. The city declared a local state of emergency that evening and lifted it the next morning. According to Le Reflet, it asked residents not to use toilets, showers or washers.
Sources: Radio-Canada, 2026-06-21 · La Presse, 2026-06-20 · Le Reflet, mesures d'urgence de juin 2026 · Ville de Saint-Constant, pluie abondante · Ville de Saint-Constant, levée de l'état d'urgence
Tropical Storm Debby left large accumulations of water and power outages across the town. The city closed several streets and the Base de plein air bike path, and asked residents whose basement had taken on water to contact their insurer. Its notice does not say whether the water came from surface runoff or from sewer backup.
After heavy rain, the city said on the morning of December 19 that the situation had stabilized overnight and that its crews were working to prevent sewer backups. Paths around the retention basins were closed. The city asked residents to check their sump pump and report blocked street catch basins. Its notice is mainly about river levels, which the check does not cover, and the risk of sewer backup.
Good to know in Saint-Constant
- Construction by-law 1530-17 requires a sump pit and a sump pump in the basement of every new main building. The pump discharges to the storm sewer or, where the lot has no storm sewer, at least 1.5 m from the foundation. (Ville de Saint-Constant, règlement de construction 1530-17 (PDF))
- According to the city's leaflet, gutters connected to the foundation drain are not compliant (by-law 503-83 on private sewer connections). They comply, among other options, if the water is released onto the lot or a rain garden, at least 1.5 m from the foundation. (Ville de Saint-Constant, dépliant sur les gouttières (PDF))
- A city permit is needed to install a backwater valve or a sump pump. The city also asks owners to grade their lot so water moves away from the foundation without being directed onto neighbouring lots. (Ville de Saint-Constant, travaux publics)
- Zoning by-law 1528-17 prohibits reshaping a lot in a way that sends runoff onto neighbouring lots or causes poor drainage. It also requires an areaway (cour anglaise) serving basement windows or a door to drain to a sump pit inside the building. (Ville de Saint-Constant, règlement de zonage 1528-17, chapitre 4 (PDF))
- The city's 2026 prevention guide plans new retention basins, improvements to the Prémontrés ditch, and campaigns to get non-compliant gutters and storm connections disconnected. (Ville de Saint-Constant, guide de prévention des inondations 2026 (PDF))
- According to the city, the sanitary sewer has no capacity problem in dry weather. In heavy rain, a lot of rainwater enters it through non-compliant connections such as foundation drains or sump pumps. The city expects the planned basins to help reduce the risk, but not to eliminate it. (Ville de Saint-Constant, bureau de projets)
Rivers and lakes: what the check does not cover
Waterways in Saint-Constant: Saint-Régis River, Saint-Pierre River, Tortue River, Lasaline Creek and Lac des Fées.
In spring 2018, after flooding, rain and thaw between March 28 and May 17, Saint-Constant was among the municipalities that could qualify for Québec's general financial assistance program (Programme général d'aide financière), run by the Ministère de la Sécurité publique. The city's flood-zone plans cover the Saint-Pierre, Saint-Régis and Tortue rivers and Lasaline Creek. New CMM maps for the Saint-Régis and Saint-Pierre were due to take effect on August 1, 2026, but Québec postponed their publication. Until then, the old maps remain in force.
Sources: TVRS, 2018-08-01 · Ville de Saint-Constant, plan d'urbanisme 1527-17, annexe C (PDF, 56 Mo) · CMM, 2026-07-30 · La Presse, 2026-09-15 · CMM, zones inondables et crues printanières, 2026-02-25
The check is about the rain that falls on the area and runs off over the ground. River and lake flooding is not part of it: in the simulation, a nearby river or lake holds no water. For a waterfront property, the official map comes first.
What we know about the ground in Saint-Constant
These values come from the public sources the check uses. They describe the area, not an address.
| 1 m terrain model (NRCan HRDEM) | Available across the whole area |
|---|---|
| Nearest rainfall gauge (ECCC) | McTavish (Montréal), 17 km awayLocal rainfall statistics. |
| Municipal street drains | Not modelled: with no open municipal inventory, the check's depths are an upper bound wherever a working network would carry water away |
| Mapped culverted road crossings | 45Culvert lines mapped in OpenStreetMap (ODbL). |
Data Source: Environment and Climate Change Canada, Short Duration Rainfall Intensity-Duration-Frequency Data, used under the ECCC Data Servers End-use Licence.
Area analysed: adapted from Statistics Canada, Census Subdivision Boundary File (cartographic), 2021 Census. This does not constitute an endorsement by Statistics Canada of this product.
Probed on September 28, 2026.
The official map and what it does not show
Québec's flood-zone maps cover rivers and lakes. They do not show rain water collecting on a lot, in a low spot or at the bottom of a sloping driveway. Check the official map for the address first: it is what says whether a property is in a flood zone.
- Saint-Constant by-laws: flood-zone plans (Annex C of the urban plan and zoning by-law, in French)
- Check a property on the Government of Québec site (in French)
FloodVerity does not reproduce these maps and does not state any regulatory zone.
Check an address in Saint-Constant for $39
We drop a 100-year, one-hour design storm on the ground around the property, route the runoff, and email you the peak depth of rain water at the address, where it collects, a map and a 3D view.
What the check does not do
- This check models rain falling on the area itself and running off over the terrain. Water arriving from a river or lake, sewer backup, ice jams and storm surge are not modelled.
- The pipe network itself, roof and lot drainage, and sewer backup are not modelled.
- Where the structures inventory records a culverted road crossing, it is lowered to a partly blocked opening rather than solved hydraulically.
- It does not replay the storms described on this page: it applies a 100-year, one-hour design storm.
- It is a screening: a simulated storm on public terrain data, not a hydraulic study, not an official flood-zone determination, not an insurance opinion.
Notaries and brokers in Saint-Constant
We run the same check over a list of addresses and deliver a spreadsheet ranking them by the modelled rain depth. Priced per address, on quote.
Email [email protected]Other towns and areas
The FloodVerity check is a screening: it applies a design rainstorm to public terrain data and simulates where the rain runs and collects. It is not a hydraulic study or an official flood-zone determination, and it covers neither river and lake flooding nor sewer backup.