In the spring of 2019, thousands of Quebecers watched the water cross thresholds they thought were safe. One question kept coming back: why here, and not the house next door? The answer often comes down to a few tens of centimetres of terrain — exactly the scale LiDAR can measure, and that the old maps ignored.
Flood-zone mapping rests on a geometric truism: water fills the hollows. To know how far it spreads at a given level, you must know the relief in fine detail. A coarse terrain model smooths the banks, erases natural levees, wipes out depressions; it is therefore wrong precisely where the stakes are highest. The 1 metre LiDAR DTM changed the game.
Why resolution decides everything
Along a river, the flood limit can hinge on a fifty-centimetre bank. A model at 10 or 20 m resolution does not “see” that bank; it averages the terrain and places the limit in the wrong spot — sometimes by several tens of metres, that is, entire rows of buildings. At 1 m, the microtopography that truly governs the flow is present in the data.
A flood map is no better than the relief it is computed on. That is why LiDAR is the first building block, before even hydrology.
Two broad approaches
Hydraulic modelling. This is the reference method for regulatory maps. You combine the DTM, statistical flows (the famous 100-year flood, for example) and the laws of flow to simulate the height and extent of the water. LiDAR relief provides the geometry of the channel and banks; the quality of the model depends directly on it.
Terrain-only approaches. Faster, they exploit the shape of the relief alone. The HAND index (Height Above Nearest Drainage) computes, for each point, its height above the nearest watercourse by following the flow. Low areas close to the network stand out as the most exposed. It is not a full physical simulation, but an excellent way to rapidly screen at-risk sectors over large territories.
What the relief reveals to the naked eye
Even without a model, a good DTM tells the hydrological story of a place: abandoned meanders, old floodplains, closed depressions, embankments that block a natural flow. Many risks become legible simply by looking at the terrain from the right angle — which a hillshade or a 3D view allows.
This reading of the relief connects to other water-related hazards, notably bank erosion and landslides, often concentrated along the same watercourses.
Reading a valley floor
Fly over a Québec river and tilt the view: the terraces, lowlands and slope breaks that govern a flood appear in volume. A preview of the risk mapper's raw material.
Explore a valley floor →Data to aid understanding, not an official map
Let's be clear: visualizing the relief is not the same as consulting a regulatory flood-zone map. Official zoning integrates hydrology, flood history, field surveys and a legal framework that Québec 3D does not reproduce. This tool helps you understand why a sector is vulnerable; it replaces neither official maps nor the opinion of the competent authorities. Our Terms of Service make this clear.
The takeaway
When it comes to flooding, the first factor is not the rain but the shape of the ground. By measuring that shape to the centimetre and at one-metre resolution, LiDAR gave hydrologists and planners a foundation finally equal to the stakes. Understanding where the water goes begins with seeing the terrain clearly — and that terrain is now open to everyone.