Beneath much of the St. Lawrence Lowlands sleeps a delicate geological legacy: the clays of the Champlain Sea. Stable most of the time, they can, under certain conditions, abruptly lose their cohesion and flow like a liquid, carrying away houses and roads. Anticipating these failures means monitoring the terrain very closely. LiDAR has become one of the best tools for that.
A hazard written into the relief
A landslide leaves a recognizable topographic signature: an amphitheatre-shaped scar, a sharp scarp at the head, a mass of debris at the toe, a flow downstream. These shapes, often masked by vegetation, are preserved in the bare-earth terrain model. A well-chosen hillshade brings them out immediately, including for old slides that no archive has documented.
Mapping these scars is already drawing a hazard map: where the ground has slid, it can slide again, and neighbouring sectors with the same geometry of slope and soil deserve attention.
The power of the multi-temporal
The real shift comes from repetition. When a territory is flown several years apart, you subtract the two terrain models to obtain a difference map: each cell shows how much the ground rose or fell. Stable areas come out neutral; active sectors light up.
You are no longer looking at a photograph of the terrain, but at its movement. A few centimetres of subsidence, otherwise invisible, become a warning.
This approach detects the retreat of a scarp, the bulging of a slope toe, the opening of cracks — sometimes early signs of more serious instability. It is the same principle as tracking forest growth, applied this time to human safety.
Bank erosion, in slow motion then all at once
Along rivers, banks retreat: slowly through continuous erosion, abruptly during floods. Comparing successive LiDAR surveys quantifies this retreat metre by metre — which banks are degrading, how fast, which structures or buildings are threatened. This information feeds protection decisions (riprap, revegetation, managed retreat) and monitoring after an event.
Unstable banks and landslides often combine: a watercourse undercutting the toe of a sensitive-clay bank sets the stage for failure. Crossing erosion and slope on the same LiDAR data illuminates these chains of events.
Recognizing a scar
Look along a watercourse for amphitheatre-shaped hollows and sharp scarps: the mark of past slides. The 3D view makes their geometry obvious.
Examine a slope →Understanding is not assessing
Spotting a landslide shape on a relief is not a risk assessment. Characterizing an unstable site is a matter of geotechnical expertise: boreholes, soil analyses, instrumented monitoring, a regulatory framework. Québec 3D lets you observe and understand these terrain forms; it provides no assessment of the stability or safety of a place. In case of doubt about a site, only the authorities and qualified professionals are authoritative — as our Terms of Service recall.
The takeaway
By turning the terrain into a surface you can measure, compare and replay over time, LiDAR gave the management of ground movements a new power: to see the ground move. For a Québec built in part on capricious clays and along living rivers, this ability to detect change — sometimes before failure — is anything but trivial.