Paraglacial history and structure of the Moosfluh Landslide (1850–2016), Switzerland

Abstract

Rock slopes next to the tongue of the Great Aletsch Glacier, Switzerland are characterized by rapid environmental adjustment to non-glacial conditions. This study investigates and describes in detail the historic development of the largest rock slope instability in this area, called Moosfluh Landslide. We study in detail the structure, evolution and stability since the end of the Little Ice Age (LIA) until September 2016 and discuss their relationships with the evolution of the Great Aletsch Glacier since the Lateglacial period. In 2016 around 50 m of glacial ice thickness were left at the Moosfluh Landslide toe, where in 1850 glacial ice was >400 m thick. The changing stability conditions at the interface with the melting valley glacier are studied based on novel balanced cross sections and kinematic model of the Moosfluh Landslide dominated by toppling phenomena in metamorphic rock. The morphology and evolution of this landslide since the LIA are investigated with multi-temporal landslide maps based on aerial digital photogrammetry (ADP) applied to historic images since 1961. Internal deformation at Moosfluh is accommodated by shear slip along uphill-facing foliation and fault planes, and by extensional faulting forming tension cracks and graben-structures at the landslide head. Together with Digital Image Correlation (DIC) and total station monitoring (TPS) the Moosfluh Landslide displacement history was reconstructed, evidencing post-Egesen landslide displacements and an acceleration of movements since the LIA and especially since 2007. The displacement rates increase from few mm per year until the nineties to several meter per day in September 2016. Different kinematic models have been tested and changes in the Moosfluh rock slope stability in response to retreating glacial ice and changing groundwater conditions was explored with limit-equilibrium analysis of the stepped planar block toppling model of Goodman and Bray (1976). For the observed conditions and a toppling joint friction angle of 19° the simulated factor of safety drops non-linearly from the LIA maximum (1.12) to the year 2007 (1.02), when the height of ice above the valley bottom melted down to 100 m.This study illustrates with unprecedented detail the time scales, displacement magnitudes and structural evolutions of a large toppling mode slope instability in a paraglacial setting. The long-term cumulative slope displacements between the Egesen stadial and the LIA are of the same magnitude as the cumulative displacements between the LIA and the year 2016. As large portions of the studied slope underwent multiple retreats and advances of the Great Aletsch Glacier during the Lateglacial and Postglacial period, the observed onset of large slope displacements should be related to incremental damage (slip weakening and weathering) occurring along steeply dipping toppling fractures during the LIA.