Monitoring and analysis of interactions between the retreating Aletsch glacier and adjacent rock slope instabilities

Abstract

The valley flanks around the Great Aletsch Glacier (Valais, Switzerland) contain several rock slope instabilities, concentrated around the current extent of the glacier tongue. These alpine rock slopes have been exposed to cycles of glaciation and retreat, leading to changing slope geometries and stress conditions. Processes leading to the formation of rockslides, rock avalanches or progressive slow movements with time since deglaciation, the impact of cyclic ice-loading and the dominant driving mechanisms have been studied for many years but are partly still unexplained. Additionally, the triggering events and causes of failures of landslides in deglaciated or currently deglaciating valleys are poorly understood. Often, the absence of observable triggering mechanisms presents a significant problem for hazard management and science. Better knowledge of subsurface processes, even if only reconstructed from surface observations and deformations, is essential to improve our understanding of paraglacial rock slope processes. During the course of this PhD thesis the extraordinary activation of the deep-seated gravitational slope deformation (DSGSD) of Moosfluh was witnessed and failure mechanisms, spatial and temporal displacement patterns and paraglacial conditioning factors were studied in great detail. During the first stage of this PhD thesis, a very accurate displacement monitoring system was developed for the high alpine environment of Aletsch. This monitoring system consists of two automatic total stations coupled with Global Navigation Satellite System (GNSS) sensors, meteo sensors and a webcam. It is designed to work all year in harsh conditions with extreme temperature changes, heavy rainfall and snowstorms. The reflectors and the stations have to be protected against rockfall and snow avalanches. In the first phase of this PhD the focus was set on the design and implementation of this total station monitoring system and the processing of high accuracy 3D displacement data. We identify key monitoring goals, discuss stability of pillar foundations, the influence of protective housing and compare calculated to observed accuracies. The selected total station monitoring is based on a lightweight but stable foundation and a total station instrument protection without optical refraction of the light beam and mechanic protection of reflector prisms. The selected monitoring sites presented in this study do not only focus on instabilities, like the large and rapid rock slope instability of Moosfluh and the slow‐moving instability of Driest, but also on seasonally reversible rock slope deformations along several profiles in stable rock across the Great Aletsch Glacier valley. During the second stage of this PhD thesis, the paraglacial history and structure of the Moosfluh Landslide from 1850 to 2016 was investigated. The geomorphological evolution of the Moosfluh slope was reconstructed with photogrammetric models generated from historical aerial photographs. This multi-temporal landslide analysis showed that the bulk displacement and internal deformation at Moosfluh is accommodated mainly by toppling composed of shear slip along the steeply dipping Alpine foliation and extensional faulting in the crest area. Numerous uphill-facing scarps, scarps, tension cracks, toe bulging, graben-structures and displacement of moraine deposits, evidence post-Egesen landslide displacements and an acceleration of movements since the LIA and especially since 2007. Together with digital image correlation and total station monitoring, an increase of displacement rates from a few millimeter per year until 1990, to several meter per day in September 2016 was retraced. Balanced longitudinal sections through the Moosfluh Landslide are based on observed ground surface displacements and geometrical and mechanical constraints. A simple limit-equilibrium analysis helped explore changes in rock slope stability caused by glacial ice retreat and changing groundwater levels, showing that the simulated factor of safety drops non-linearly from the LIA maximum (1.12) to the year 2007 (1.02), when the ice thickness at the landslide toe melted down to 100 m. During the third stage of my PhD thesis, the unexpected acceleration of the Moosfluh DSGSD, which started in September 2016 and posed hazards to a nearby cable car station and an adjacent dammed lake 4 km downstream of the Moosfluh slope, was studied in detail. With our high accuracy displacement monitoring system, installed in 2013 and 2014, it was possible to record the progressive evolution of the Moosfluh Landslide from toppling to sliding in unprecedented detail. The formerly slowly moving toppling mode DSGSD of about 100 Mio m³ and up to 170 m depth developed into a fast moving landslide through the formation of several shallow (30-50 m) secondary slides. These secondary landslides started their development at the toe of the slope, and evolved upslope with time, as evidenced by time series of surface displacement vectors. Digital image correlation (DIC) of webcam images and helicopter-borne photogrammetry allows for detailed mapping of landslide boundaries and active morphological features. For kinematic analyses of toppling and sliding block a new method called ‘plunge angle analysis’ was developed. This approach allows to quantify the underlying failure mechanisms (toppling, sliding and mixed mode deformations) and assign depths and inclination angles of toppling/sliding planes from measured surface displacement vectors. The analysis of displacement vector time series allows the identification of landslide volumes and rupture plane depths. The results and developed method provide a better understanding not only of the Moosfluh Landslide but of all slopes affected by mixed toppling and sliding failure mechanisms. This study assembled a unique dataset documenting the development of a 'paraglacial' landslide from a slow moving DSGSD into an active and hazardous landslide. Combining multiple methods for reconstruction and current surface deformation monitoring strengthened our understanding of mechanisms and long-term controls occurring in currently deglaciating environments. In addition this thesis provides a new basis for implementing and operating a surface deformation monitoring system in high alpine environment, understanding the role of glacial ice load on rock slope instabilities and assessing present day hazards of mass movements in which toppling and sliding mechanisms coexist.