SBAS DInSAR and in situ monitoring of the Šumljak landslide (SW Slovenia) dynamics driven by rainfall and piezometric-level fluctuation

Abstract This paper presents the first attempt at establishing the dynamics of the Šumljak debris slide by using a multidisciplinary approach, including long-term inclinometer and piezometer measurements and a time-series analysis of SAR images for displacement vector estimation. The Šumljak landslide is located in the Rebrnice area, SW Slovenia, where the regional highway Razdrto-Nova Gorica runs through and poses a threat to the highway viaduct’s stability. By combining both ascending and descending Sentinel-1 data, we have estimated 3D displacement vectors for the landslide, providing insights into the usability of the technique for this kind of slow-moving landslides. Furthermore, we have combined and compared the SBAS DInSAR results to the displacements measured by inclinometer to evaluate the rate of displacement of the landslide and the viaduct. The inclinometer measurement results show that the landslide has a single sliding plane between the flysch colluvium and the slope deposits and provide the proof of the enlargement of the sliding area above its main scarp. The displacement vectors are spatially heterogenous, suggesting that different sites have different velocities and slightly different directional vectors. These results partly comply with the InSAR results. We further discussed the estimated vectors of displacement and the possible mechanism of movements. By using the piezometric data, we studied the groundwater level fluctuation in relation to daily rainfall and its influence on the displacements. Our findings show a correlation between the movements and the wet/dry season, indicating that the landslide is driven by rainfall and consequently groundwater level fluctuation. In 2020, the groundwater level rise was smaller than in the years before due to less rainfall, resulting in lower velocity of the landslide (5.5 mm/year) than in 2019 (7 mm/year).