Rock slope instabilities in Sogn and Fjordane County, Norway: a detailed structural and geomorphological analysis

Large rock slope instabilities affect river channels both due to catastrophic failures and long-term creep. The relationship between rock slop instabilities and processes in the adjacent river system are typically assessed in terms of channel profile perturbations and cross-sectional morphology, e.g. excess topography. However, such relationships can also be evident in planform changes of the channel alignment, e.g. in landslide dams and long-term channel migration. Large scale creeping rock slope instabilities can be considered point sources which introduce sediment laterally to a river channel. In cases in which sediment production from one side of the channel exceeds that of the opposing side, the course of the river can be shifted towards the less active hillslope. The deviation of the channel from its original course may therefore be used as a proxy for relative sediment input of the two opposing hillslopes.In order to characterize the planform morphology of the river channels, we treat them as signals fluctuating around a smoothed channel and use a fast Fourier transform to extract characteristic wavelengths and amplitudes of the stream network. We observe a consistent increase in amplitude of planform deviation with increasing wavelength with a variability of two orders of magnitude at the shortest wavelength (101 m) and less than one order of magnitude at longer wavelengths (103 m).When comparing characteristic channel morphologies based on these analyses to the deviation of channels adjacent to mapped landslides, the amplitude of the deviation appears higher than those naturally occurring in the river system at wavelengths similar to twice the landslide width.

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Numerical Investigation of the Stability of Toppling Rock Slopes Subjected to Glacier Retreat

10.5194/egusphere-egu2020-6568 ◽

2020 ◽

Author(s):

Nikola Toshkov ◽

Jordan Aaron ◽

Simon Loew ◽

Franziska Glueer ◽

Valentin Gishig

Keyword(s):

Theoretical Analysis ◽

Water Table ◽

Rock Slope ◽

Structural Model ◽

Numerical Models ◽

Back Analysis ◽

Glacier Retreat ◽

Glacial Retreat ◽

Slope Instabilities ◽

The Stability

Glacial retreat is often cited as a cause of rock slope instabilities in mountain regions. Until recently, glacial debuttressing was thought to be the main mechanism by which glaciers influence slope stability, however recent work has questioned the efficacy of this mechanism. &#160;It appears that other mechanisms, including slope kinematics and hydro-mechanical interactions between the glacier and slope are important drivers of paraglacial rock slope instabilities. &#160;In the present work, we use discontinuum numerical models to investigate the interaction between rock slope kinematics, slope/glacial hydrology and glacial retreat.&#160; We perform both a theoretical analysis using a simplified slope geometry, as well as a back-analysis of the Moosfluh Landslide. &#160;For the theoretical analysis, we investigate the response of both toppling and sliding slopes to two factors: the weight of the ice, assumed to be applied as a ductile load acting normal to slope topography, and the variation of the slope water table, which is linked to the ice level and lowers as the glacier retreats. We then apply the insights from the theoretical analysis to investigate the Moosfluh Landslide. &#160;This landslide, which is located at the left flank of the Great Aletsch Glacier Valley (Valais, Switzerland), at the present-day glacial terminus, underwent a dramatic acceleration in 2016 in response to glacier retreat. &#160;The landslide was extensively monitored during this acceleration, and analysis of this data has revealed that the kinematics of movement changed from toppling to secondary sliding. &#160;We simulate the behaviour of the Moosfluh Landslide by implementing a structural model determined from field mapping, and systematically lowering the ice level and slope water table, to simulate glacial retreat. We find that the interaction between slope kinematics and glacial retreat leads to a complex slope response. &#160;For sliding slopes, the stability of the slope is relatively insensitive to glacial ice loss. &#160;For toppling slopes, the slope response is highly sensitive to ice loss, and the slope is the most unstable at a critical ice level, before ice has completely retreated. &#160;For the Moosfluh instability, we are able to simulate the initial toppling kinematics of this landslide, as well as the transition to sliding triggered by the ice reaching a critical elevation. &#160;Our analysis has important implications for understanding rock slope response to glacial retreat, and highlights the disparate behaviour of toppling and sliding slopes.&#160;

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