Integrating Multi-Temporal Sentinel-1 InSAR, GIS-Based Susceptibility Modeling, and Geotechnical Monitoring for Sustainable Dam Safety: Capabilities and Limits at the Yaprak Dam Reservoir, Türkiye

Reservoir slope instability threatens the safe and sustainable operation of dams. This study tests what open access data can reveal about a reactivated landslide on the left-bank reservoir slope of the Yaprak I Dam (Amasya, Türkiye) by combining multi-temporal Sentinel-1 InSAR, SAR-derived reservoir surface area, a GIS-based weighted overlay susceptibility model, and site-investigation data. Ascending and descending 2017–2024 interferograms were inverted with a small-baseline approach and decomposed into vertical and east–west components. In the instrumented sector, mean line-of-sight velocities ranged from −1.1 to +1.6 mm/yr and the east–west component was about −2 mm/yr, all below the local detection threshold of about 5 mm/yr (2σ). This agrees with the 2.8–7.9 mm recorded by six inclinometers over 4–12 months in 2022–2023, whereas the 20.6–24.9 mm recorded by two older casings in July–November 2022 were not resolved. Correlations between reservoir surface area and displacement were not significant after correction for multiple testing, and InSAR anomalies were distributed among susceptibility classes in proportion to their area, irrespective of the factor weights. Open access InSAR thus bounds east–west and vertical slope motion at a few millimetres per year at low cost, but is insensitive to north–south and small-scale movements and cannot replace in situ monitoring.