Monitoring impending rock slope failure in alpine scenarios: impact of high spatial and temporal resolution satellite SAR imagery in the investigation of the June 15, 2023, failure event in Brienz/Brinzauls, Swiss Alps

Rock slope failures are the catastrophic expression of long-term geomorphological processes occurring in alpine regions. Their impact is often limited to single slopes; however, rock and debris material can occasionally travel very long distances and affect landscape, infrastructures, as well as endanger human life several kilometers away from the source area. Monitoring the evolution of surface activity is recognized as a suitable method to timely identify changes potentially leading to such failure events. Satellite based remote sensing, and in particular Synthetic Aperture Radar (SAR), has shown to be an efficient alternative to in-situ sensors to monitor displacements, especially in situations where the area of interest is large and/or barely accessible. Despite the advent of satellite missions like the ESA Copernicus Sentinel-1, operational monitoring and early warning on single slopes exhibiting surface displacement acceleration potentially leading to failure is still not viable from satellite radars. This is mainly because of the current limitations in spatial and temporal resolution, which prevent obtaining the accuracy and the timeliness often needed for such scenarios. Here we demonstrate how high spatial and temporal resolution SAR imagery can improve monitoring and characterization of the evolution of a rock slope instability prior and after catastrophic failure. We benefit from ICEYE imagery (X-Band, SPOT mode, 5x5 km scene size, ~50cm resolution) acquired over the Brienz/Brinzauls slope instability in the Swiss Alps between March and August 2023. Among 100 SAR images, we have identified a subset of 30 datasets (ascending orbit, left looking) providing an optimal viewing of the moving slope and imaging the area of interest with revisit times ranging from 3 days to a few hours. We use digital image correlation to measure surface displacements and change detection analyses to map rockfall activity and the slope failure event on June 15th, 2023. We also applied SAR interferometry on data pairs exhibiting suitable perpendicular baselines and computed topographic models at different times and determine failed volumes. The latter have been validated with local terrain models based on photogrammetric drone flights. We discuss the results obtained with ICEYE imagery versus the possibilities with Sentinel-1 data and focus on advantages and specific problems. Our results provide an important step forward towards the use of satellite SAR imagery for operational landslide monitoring scenarios and in the identification and forecasting of catastrophic slope failure events in alpine areas.