Classifications and revealing long-time deformation trends of retrogressive, progressive, and complex landslides in the Xiluodu reservoir area using InSAR technologies

To identify the driving force of the landslide, the deformation modes of landslides can be classified as retrogressive, progressive, and complex. The fluctuation of the reservoir water level has different effects on retrogressive and progressive landslides. Under the long-term action of the reservoir water level, the deformation trends of landslides in different deformation modes are also dissimilar. Therefore, classifying the modes of landslides and discovering their deformation trends not only possess academic significance but also have implications for reservoir prevention and control. Interferometric Synthetic Aperture Radar (InSAR) technology, known for its all-weather capability, traceability, and high resolution, is widely utilized for monitoring surface deformations in reservoir landslides. However, the dynamics of these landslides are significantly influenced by fluctuations in water levels; thus, results obtained at a single time point often fail to accurately represent long-term deformation trends. Consequently, this study utilized ALOS PALSAR-1 (2006–2010), ENVISAT ASAR (2010), and Sentinel-1 (2014–2024) satellite SAR data to identify deformed landslides before and after the first impoundment of the Xiluodu Reservoir using an enhanced InSAR methodology based on the average of multi-SAR data fusion. Five deformation modes based on the magnitude of deformation in different areas of the slope were summarized, and deformation areas corresponding to each cycle of water-level fluctuation and annual variations were extracted to investigate correlations between different deformation modes and water-level fluctuations, thereby inferring the long-term deformation trends. The results revealed: (1) The initial impoundment of the reservoir had a significant impact on retrogressive landslides primarily during the first three years post-impoundment; (2) The influence of water-level drawdown on landslide deformation in the Xiluodu Reservoir area was more pronounced than that caused by water-level rising; (3) Over the long term, all five deformation modes exhibited predominant decreasing trends in their respective deformations; (4) The retrogressive landslides with continuous deformation accounted for 47% of all observed landslides, suggesting that this mode demonstrates greater activity persistence and heightened sensitivity to water-level fluctuations compared to other modes.