Accelerating Landslide Hazard at Kandersteg, Swiss Alps; Combining 28 Years of Satellite InSAR and Single Campaign Terrestrial Radar Data

In summer 2018, in an area above lake Oeschinen in Kandersteg (Bernese Alps, Switzerland), significant terrain changes with indication of fast ground movements were observed. The NW dipping rock and debris slope named “Bim Spitze Stei” had been known to be under constant movement before. However, the rapid acceleration from a maximum volume prone to failure of about 20 mm³ prompted the authorities to undertake a thorough analysis of the situation and analyse primary (rock avalanche) and secondary (floods and debris-flows out of the rock avalanche debris) hazard processes and the risk they pose to the nearby Village of Kandersteg. A first assessment of the most recent Sentinel-1 satellite InSAR data confirmed rapid ground movement in the order of several mm/d up to cm/d and a rapid acceleration of the west-flank of “Bim Spitze Stei” landslide from initially 7 mm/d to few cm/d within 2 weeks in July 2018. In addition, different sectors with different kinematics could be identified by interpretation of single interferograms. In a second step, an archive analysis of historical InSAR data reaching back to 1991 clearly showed that an acceleration trend from initially sub-stable conditions up to several m/a. Finally, based on the findings from the satellite InSAR analysis, a survey campaign with a terrestrial radar interferometer was performed in order to define the current state and location of the potentially outcropping glide plane in the west-flank. The successful campaign led to the observation of the presence of two active glide planes with the lowermost encompassing the maximum estimated volume of the mass in movement thus helping for the definition of potential failure scenarios thus helping in the selection of enhanced monitoring systems and increasing the preparedness for the runout-areas.