Rainwater and flood zones in Terrebonne
Updated September 28, 2026
In Terrebonne, some streets drain through ditches, open or piped, that pass under private driveways through culverts. Their upkeep is shared between the city and owners, who must among other things cut vegetation, remove sediment and keep their driveway culvert clear.
Terrebonne was formed in 2001 from the former towns of Terrebonne, Lachenaie and La Plaine, on the north shore of the Rivière des Mille Îles. Recent heavy rain hit different sectors: the Croisée urbaine and Lachenaie in September 2022, downtown and La Plaine in July 2025. After Debby and the July 2025 rain, the city launched its Stratégie bleue, an action plan to better prevent flooding caused by heavy rain.
Heavy rain events
What public sources report about heavy rain in Terrebonne, at sector level.
Some sectors received 80 mm of rain in an hour and a half. Downtown and the La Plaine sector were particularly affected, and flooded streets had to be closed. The Stratégie bleue report speaks of flooding in several neighbourhoods. The sources do not say how much came from runoff and how much from sewer backup.
Sources: CFNJ, 14 juillet 2025 · Ville de Terrebonne, bilan 2025 de la Stratégie bleue (PDF)
According to the city, Debby dropped close to 160 mm of rain in a few hours. La Presse reports at least one Terrebonne case where basement flooding was attributed to sewer backup, which excluded it from government aid. The mayor also reported a pavement collapse and badly damaged culverts that had to be replaced. In its December 2025 report, the city said an analysis of the affected areas was under way to pin down the probable causes of the flooding.
Sources: Ville de Terrebonne, bilan 2025 de la Stratégie bleue (PDF) · La Presse, 16 décembre 2024 · La Revue, août 2024
In the evening, radar estimated 70 to 90 mm of rain over the Les Moulins RCM. In Terrebonne, the Croisée urbaine sector, where Highways 40 and 640 meet, was the hardest hit. Firefighters responded to flooded homes in a residential sector of Lachenaie, where public works also dealt with water pooling in the streets. The source does not say what part sewer backup played.
Sources: La Revue, 14 septembre 2022
Good to know in Terrebonne
- Ditch and culvert upkeep falls under by-law 732. Owners may not pipe the ditch in front of their property, except under the vehicle entrance. The culvert under a private driveway remains the owner's responsibility, even inside the municipal right-of-way, and removing, widening or replacing it requires a permit. (Ville de Terrebonne, fossés et ponceaux)
- According to the Commission de toponymie, the name La Plaine reflects a sector on a fairly uniform surface with no notable relief, in the Montréal plain. (Commission de toponymie du Québec, fiche Terrebonne)
- Under by-law 225-3 (2014), as published by the city, a garage entrance below street level must not collect water from the street: a hump at least 7.5 cm above the crown of the street is required at the top of the driveway. (Ville de Terrebonne, règlement 225-3 (PDF))
- The same by-law requires a backwater valve on the branches serving basement fixtures and on those that drain below-grade garage entrances, outside basement entrances or French drains. It forbids any valve on the main building drain. (Ville de Terrebonne, règlement 225-3 (PDF))
- The city asks all owners to install backwater valves, whatever the year the house was built, and subsidizes replacing or upgrading valves, sump pumps and their batteries, for work done since 9 August 2024 by a holder of an RBQ licence. (Ville de Terrebonne, clapets antiretour et pompes à puisard)
- The Stratégie bleue plans the cleaning of sewers and catch basins, the inspection of the city's stormwater facilities within five years, and the cleaning of all piped ditches and culverts in front of properties by 2030. (Ville de Terrebonne, Stratégie bleue)
Rivers and lakes: what the check does not cover
Waterways in Terrebonne: Rivière des Mille Îles, Mascouche River and Saint-Pierre River.
In January 2018, anchor ice, ice jams and frazil ice raised the Rivière des Mille Îles: the city declared a state of flooding, and some riverside homes on the Rivière des Mille Îles were flooded. In February 2019, frazil and ice built up east of the Lepage bridge and raised the river: an alert covered riverside areas, and the water briefly overflowed onto a boulevard on Île Saint-Jean before the ice was cleared. The city notes that in winter, frazil ice can block the river and cause flooding.
Sources: Radio-Canada, 9 janvier 2018 · Radio-Canada, 4 février 2019 · Ville de Terrebonne, état de la rivière et risque d'inondation
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 Terrebonne
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) | L'Assomption, 21 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 | 142Culvert 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.
- Terrebonne zoning: the interactive map shows the flood zones (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 Terrebonne 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 Terrebonne
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.