25. The Use of LiDAR Airborne Data for Retrogressive Landslides Inventory in Sensitive Clays, Québec, Canada

Since 2003, the Québec Government uses airborne lidar surveys to map landslide-prone areas. Hillshade maps greatly helped in obtaining a detailed inventory of old scars of retrogressive landslides in sensitive glacio-marine clays. This technique allowed a much better identification of scars than with conventional air photo interpretation. It has also made possible to distinguish different types of landslide, even when scars are very old and forested. Although only 66% of the area has been covered by lidar so far, about 3,500 scars of retrogressive landslides have already been counted in post-glacial Champlain, Laflamme and Goldthwait seas. The 50 largest scars identified so far were split into three groups of dimensions. The seven cases included in the first group have surface areas comprised between 6.5 and 20 km², and most were triggered by earthquakes. The second group is characterized by landslide scars with a surface area between 2.5 and 5.3 km². The only event of this group documented in historical records is the St. Alban landslide, with a surface area of 4.5 km². The third group includes the last 23 cases that have surface areas comprised between 1.0 and 1.9 km². The second largest event in recorded history is the 1896 spread of Grandes-Bergeronnes, with a surface area of 0.54 km².