FIELD AND LIDAR BASED MAPPING OF LANDSLIDE-INDUCED LIQUEFACTION DEPOSITS IN THE VORDERRHEIN RIVER VALLEY, EASTERN SWISS ALPS

The Flims rockslide, located in the eastern Swiss Alps, is the largest postglacial landslide in Europe. About 9400 years ago, 10-12 km³ of Cretaceous limestone detached from the northern wall of the Vorderrhein river valley. The rock mass rapidly fragmented and, upon impact with the valley floor, liquefied approximately 1 km³ of late-glacial and postglacial sediments. A slurry of liquefied sediment traveled down the Vorderrhein and 16 km up the valley of the Hinterrhein, its largest tributary. Huge fragments of rockslide material (tomas), up to 100 m across and 50 m high, were rafted up to 11 km on the liquefied slurry. The sheet of liquefied sediments, referred to as the “Bonaduz gravel”, is locally >60 m thick and fines upward from cobble gravel at the base to sand at the top. A digital map of the Bonaduz deposit, produced from 2-m-resolution LiDAR imagery acquired by SwissTOPO, reveals previously unrecognized aspects of the event. Together with field observations, the DEM elucidates the three-dimensional extent and emplacement mechanism of the Bonaduz gravel. Details of other events that preceded and immediately followed the Flims landslide also became evident from study of the new DEM — a large rockslide (Tamins landslide) slightly older than the Flims landslide, and outburst flood deposits resulting from draining of the lake in the Vorderrhein impounded by the Flims landslide. The Tamins landslide also impounded a lake in the Vorderrhein into which part of the Flims landslide fell. We hypothesize that liquefaction induced by the Flims landslide fragmented the Tamins landslide mass, producing many of the tomas that were rafted down the Vorderrhein. In this presentation, we showcase a large-scale map of the area at the confluence of the Vorderrhein and Hinterrhein that illustrates the new insights obtained from the digital terrain map.