Rainwater and flood zones in Longueuil
Updated September 28, 2026
In Longueuil, torrential rain is the city's main flooding concern, more than the St. Lawrence. Combined sewers, where one pipe carries wastewater, foundation-drain water and street runoff, are found mostly in Le Vieux-Longueuil; the city says they cope less well with high-intensity rain. Pseudo-separate networks, where street water goes to a separate storm sewer, cover much of Saint-Hubert and Greenfield Park; they cope less well with long-lasting rain.
At the surface, the city talks about cuvettes: low areas where water collects naturally, formed by the terrain or by raised boulevards. Some streets are themselves a cuvette, because the houses were built higher than the roadway. The territory also has ditches, which the city plans to clean and reshape.
Heavy rain events
What public sources report about heavy rain in Longueuil, at sector level.
During Debby, Longueuil received as much rain in 24 hours as in an average August. The city says the rain caused a lot of damage to many properties. Since then it has deployed a dedicated recovery coordination team for major events. Its documents do not separate sewer backup from surface runoff.
Sources: Ville de Longueuil, stratégie de résilience aux fortes pluies (PDF) · Ville de Longueuil, inondation résidentielle : quoi faire
Close to 70 mm of rain fell in under an hour. The sources mainly describe sewer backups: according to the city, most calls received across the agglomeration concerned backups and flooded basements, and water collected at certain points of the network. FM 103,3 names part of Le Vieux-Longueuil and the Laflèche neighbourhood, in Saint-Hubert, among the flooded areas.
Sources: Ville de Longueuil, 2022-09-14 · FM 103,3, 2022-09-14
Good to know in Longueuil
- By-law CO-2021-1141 requires a reverse-slope driveway to have a hump of at least 300 mm at the property line, raised sides and a catch basin at the bottom draining to a sump pit with a pump. The hump rule also applies to existing buildings. (Ville de Longueuil, règlement CO-2021-1141 (PDF))
- To adapt future construction, the city has adopted new rules that ban reverse-slope garages for single-family homes, duplexes and triplexes to prevent runoff flooding, according to its 2025 heavy-rain resilience strategy. (Ville de Longueuil, stratégie de résilience aux fortes pluies (PDF))
- The same by-law requires every owner, including the owner of an existing building, to install a backwater valve on each branch serving plumbing fixtures and floor drains. It prohibits connecting downspouts to the foundation drain or to the municipal sewer. (Ville de Longueuil, règlement CO-2021-1141 (PDF))
- The city has divided its territory into drainage sectors, prioritized partly by the extent and frequency of past flooding. The first ones analyzed are Greenfield Park and the Laflèche neighbourhood, Le Moyne, Centre-Ville (Victoria) and Normandie Saint-Sylvestre Chambly. (Ville de Longueuil, stratégie de résilience aux fortes pluies (PDF))
- Besides cleaning and reshaping ditches, the city's 2025-2027 action plan provides for replacing those on a section of chemin de Chambly with planted swales. It also plans information tools on sector topography and cuvettes. (Ville de Longueuil, plan d'action 2025-2027 (PDF))
- On June 8, 2026, Québec announced in Longueuil an adaptation component of the Rénoclimat program, due to start on October 1, 2026. It will offer help for simple work such as a backwater valve, a battery-backed submersible pump or removable flood barriers, wherever the house is. (Gouvernement du Québec, Rénoclimat, 2026-06-08)
Rivers and lakes: what the check does not cover
Waterways in Longueuil: St. Lawrence River.
Commenting on the CMM's preliminary map in 2024, the city said it is relatively sheltered from river floods: it lies downstream of the Mille Îles and Ottawa rivers and was not hit by the spring floods in the west of the region. The St. Lawrence floodplain map in force, in the agglomeration land-use plan, is taken from a 1985 report. The river is in the first batch of new maps, whose publication Québec has postponed.
Sources: Le Courrier du Sud, 2024-10-07 · Agglomération de Longueuil, schéma d'aménagement, annexe 1, carte 41 (PDF, 25 Mo) · CMM, 2026-07-30 · La Presse, 2026-09-15
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 Longueuil
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) | Saint-Hubert (Longueuil), 1.5 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 | 82Culvert 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.
- Longueuil agglomeration land-use plan, Annex 1, Map 41: St. Lawrence floodplain (449-page PDF, 25 MB, 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 Longueuil 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 Longueuil
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.