Susanna Büsing
Supervisor: Prof. Michel Jayboyedoff
Internal Experts: MSc. Antoine Guerin, Dr. Marc-Henri Derron
External Expert: Dr. Marcia Phillips
On 31 July 2011, a rockfall with an estimated volume of 7000 m3 occurred at an altitude of 3050 meters on the southeastern side of the Lischana mountain, located in the Lower Engadin valley (Grisons, Switzerland). Luckily the rockfall event was filmed and ice could be observed on the failure plane after analysis of the images. Due to the fact that another crack was opened next to the Lischana summit and to protect the about 1500 mountaineers who climb the mountain in-between the months of July to October, the access to the summit was prohibited by the municipality of Scuol and the official mountain peak displaced of 50 meters. In autumn 2014 the expected rockfall occurred, as well as two other rockfalls situated more north and about 300 meters lower.
In order to characterize and analyze the two events situated under the summit, three 3D high density point clouds have been made by photogrammetry and LiDAR, one before and two after the rockfall of September 2014. 120 photos were taken during a helicopter flight in July 2014 to produce the first photogrammetric point cloud, and more than 400 terrestrial photos were taken at the end of September to produce the second photogrammetric point cloud. In July 2015 a third point cloud was made with three LiDAR scans.
The point clouds were georeferenced with a 2-meter digital elevation model (DEM) and compared to each other in order to calculate the volume of the two past rockfalls. A structural analysis of the two rockfall was made and compared to the geological structures of the whole eastern face in order to increase the understanding of the past rockfalls and their failure mechanisms and the probability of future rockfalls. Moreover, valuable information about the velocity from the filmed rockfall event using a Particle Image Velocimetry method (PIV) could be extracted. These analyses combined with two thermal panoramas and the analyses of triggering factors (permafrost, freeze-melt cycles, thermomechanical processes, rainfall, radiation, glacier decompression and seismic) improved the understanding concerning the recent rockfall activity on the Lischana summit and underline the influence of permafrost warming on rock instability.
In a second time, a comparison of the LiDAR and photogrammetry techniques was made in order to increase the knowledge concerning the limits and the advantages of the photogrammetry technique. The point clouds have been analyzed regarding their general quality, the quality of their meshes, the quantity of instrumental noise, the point density of different discontinuities, the structural analysis and the cinematic tests. Results show that the photogrammetric technique is of adequate quality to make a structural analysis and that a good choice of the parameters allows to almost reach the quality of the LiDAR point cloud, but several factors (focal length, variation of distance to object, image resolution) may increase the uncertainty of the photo alignment. This study confirms additionally that the coupling of the two techniques is possible and provides reliable results.
