Complementarity of field-works and imagery data to understand dynamics of large rockslides

Imagery techniques allow to follow the global evolution of large scale landslides and cliffs, contrary to traditional techniques. Using such techniques to study the kinematics of three instable zones, we show (1) that correlating aerial images is a fast way to get global and multi-temporal data; (2) that terrestrial laserscanner allows to acquire data in small areas undergoing higher movement rates, often not accessible to conventionnal survey; (3) that there is no satisfying method using high resolution satellite images in such studies yet; and (4) that the regular acquisition of transveral images acquired on site provide a fast, efficient and low-cost way to follow the evolution of a landslide. Regarding the sites this study focusses on, we show that the mountainside motion of the Sedrun landslide (Switzerland) has been regularly slowing down since 2002, following a 150% increase in speed since 1990. The detailed description of its kinematics combined to field geology allows to figure out the future evolution of the site. As for the frontal zone of the Séchilienne landslide (Isère, France), it translates more than it subsides; using our scanner-laser data set, we show that the structural blocks the zone is composed of are rotating downhill. Lastly, the Cliets tunnel (Arly Gorges, France) undergoes a decompression difficult to track using imaging methods. We finally suggest a guide describing which imaging techniques are best adapted to study unstable moutainsides, depending on field configuration. At last, imagery methods are shown to be a very powerful tool, complementary of traditional techniques to follow the evolution of the motion of mountainsides.