Slope instability mapping in glacier forefield environments of the Alps using standard and advanced DInSAR techniques

Global atmospheric warming and associated deglaciation effects lead to the increasing development of slope instabilities in glacier forefield environments. The primary drivers are debuttressing effects due to retreating glaciers, exposure of previously contained rock masses and thawing of permafrost. Such effects can lead to a decrease in slope stability and possible resulting failure in the generally rough and steep terrain encountered in high mountains, such as the densely populated European Alps, calling for extensive hazard analyses of such features.

Within the framework of the ESA Regional Initiatives for the Alps, the AlpGlacier project analyses capabilities to monitor glaciers in the Alps from the Copernicus Sentinel satellites. The derived products are: (a) snow cover and (b) flow velocities, both on glaciers, as well as (c) pro-glacial lakes and (d) slope instabilities, both in glacier forefields. Synergies through a combination of sensors, with focus on Sentinel-1 and -2, are tested and sensor limits identified. The presented work focusses on the slope instability product. The specific challenges to detect slope instabilities are the highly variable rates of movement combined with the long-lasting presence of snow cover, requiring the application of diverse DInSAR processing approaches and a detailed visual analysis of the observed changes. We present a comparison of standard and advanced DInSAR methods using Sentinel-1 data to evaluate the possibilities and limitations to detect slope displacements in three selected study regions in the European Alps. The Mattertal region in Switzerland, Mont Blanc region in France/Italy and Ötztal Alps in Austria are characterised by steep relief, the occurrence of permafrost, dense infrastructure and known slope instabilities. All sites experience glacier retreat since the Little Ice Age that leads to the potential formation of glacier lakes and slope instabilities. Continuous Sentinel-1 acquisitions exist for each site, which are used in multiple DInSAR techniques to detect and map slope instabilities, allowing an assessment of the variability of detected surface motion and suitability of the applied methods.

Results show a widespread distribution of slope instabilities moving at <2 cm/year to >30cm/year. Movements include soil and rock slides, rock falls, rock slope deformations as well as permafrost-related movements and rockglaciers. Time series extracted for exemplary movements occurring along retreating glaciers in each study region show distinct accelerations in the last 5 years that may be related to deglaciation effects. The results are validated with optical and SAR offset tracking methods.