Monitoring and Analysis of Landslide-Glacier Interactions at the Great Aletsch Glacier (Switzerland)
Authors: Storni Enea , Loew Simon , Hugentobler Marc , Andrea Manconi Andrea 
Published: March 2020

&lt;p&gt;Valley glaciers&amp;#160;have traditionally been expected to significantly influence the&amp;#160;stability and movement rates of adjacent paraglacial landslides. However, detailed studies&amp;#160;related to&amp;#160;the&amp;#160;mechanical and displacement&amp;#160;interactions between glacier ice and&amp;#160;unstable&amp;#160;rock slopes are&amp;#160;essentially non-existing. This&amp;#160;project&amp;#160;deals with a detailed in-situ investigation of the&amp;#160;spatial variations of the&amp;#160;displacement field of the Great Aletsch Glacier&amp;#160;in the surroundings of a large active instability, called&amp;#160;Moosfluh Landslide. The goals&amp;#160;of this project are&amp;#160;to&amp;#160;assess the mechanical interactions between an active rockslide and an abutting valley glacier based on real field measurements and infer the impacts of glacier ice deformation on landslide dynamics. As most valley glaciers are currently strongly retreating due to global warming, uncovering significant numbers of pre-LIA slope instabilities, this detailed investigation has implications going far beyond academic interest.&lt;/p&gt;&lt;p&gt;The Moosfluh landslide is a Deep-Seated Gravitational Slope Deformation (DSGSD), with superimposed &amp;#160;large (1-5 million m3) secondary rockslides formed in fall 2016,&amp;#160;located near the currently retreating tongue of the Great Altesch Glacier&amp;#160;(Kos et. al. 2016, Gl&amp;#252;er et al. 2018, 2019).&amp;#160;In August 2018 we have performed repeat UAV-based photogrammetric surveys&amp;#160;during 74 hours&amp;#160;and applied Digital Image Correlation (DIC) techniques to record high-resolution surface&amp;#160;displacement vector fields&amp;#160;of&amp;#160;the&amp;#160;landslide, stable slopes&amp;#160;and&amp;#160;adjacent&amp;#160;glacier.&amp;#160;DIC results&amp;#160;show that the&amp;#160;landslide toe is composed of two sectors with significant differences in displacement mean velocities (0.5 and 1.5 m in 74 hours, excluding&amp;#160;rapid movements from detached blocks). Both landslide sectors induce&amp;#160;clear&amp;#160;deflections&amp;#160;of&amp;#160;the glacier vector field, moving with a velocity &amp;#160;of about 0.3 to 0.4 m in 74 hours. This influence tends to be higher near the ice-contact boundary and decrease within a distance of about 100 m&amp;#160;and 200 m&amp;#160;from the rock slope instability.&amp;#160;We investigate the viscous forces at the landslide/glacier contact using the multiphysics simulation software COMSOL&amp;#160;and simplified analytical solution, assuming a vertical interface. These forces are then applied to a limit equilibrium landslide stability model representing the real geometry at the interface boundary, and quantitatively explore the true buttressing effects of valley glaciers on a fully developed slope instability. We show that a slope in critical stability conditions can respond strongly to a minor buttressing effect posed by a valley glacier occupying the landslide toe.&lt;/p&gt;