Study on the Rainfall–Reservoir Water Interaction Mechanism of Wading Landslides in Metamorphic Rock Areas

Rainfall and fluctuations in reservoir water levels are critical external factors influencing the stability of landslides along reservoir banks. This study investigates the Yetan Primary School landslide, which serves as a representative example of a metamorphic rock landslide influenced by reservoir conditions in Shiyan City. We quantitatively investigate the evolutionary characteristics of slope seepage and stress responses by integrating GNSS field monitoring data (2021–2025) with FLAC3D numerical simulations. The analysis encompasses five distinct working conditions, including groundwater level rises of 1 m, 3 m, and 5 m, as well as periodic fluctuations in reservoir water levels ranging from 236 m to 242 m. The results indicate that rainfall is the dominant triggering factor for the landslide, with monitoring data revealing two stages of stepwise deformation that are synchronized with heavy rainfall, and a maximum average displacement rate of 0.302 mm/d. Simulation results show that when the groundwater table rises by 1 m, 3 m, and 5 m, the maximum displacement at the landslide front is 0.51 m, 1.26 m, and 2.63 m, respectively, with the plastic zone extending from the front to the rear, exhibiting a composite failure mode characterized by front traction and rear thrust. The drawdown of the reservoir water level (from 242 m to 236 m) generates outward hydrodynamic pressure, resulting in a displacement of 12.8 cm at the front, which is greater than the 2.7 cm observed during the rise. However, the impact of reservoir water alone on overall stability is limited. Landslide instability can be attributed to a coupling mechanism of ‘easily sliding metamorphic rock—hydraulic driving (water-rock softening + hydrodynamic pressure)’: rainfall triggers the composite failure of front traction and rear thrust, while fluctuations in reservoir water level primarily exacerbate deformation at the front. This study provides theoretical guidance for the early warning and prevention of similar landslides in the metamorphic rock region of northwestern Hubei Province.