Potential failure mechanism and tsunami risk analysis of the Longmen dangerous rock mass in the scenic area within the three gorges reservoir area, China

High and steep dangerous rock masses (DRMs) along reservoir banks pose significant threats to navigation and the adjacent towns. Taking the Longmen DRM (LDRM) in the Three Gorges reservoir area as an example, this paper analyzed the geological background and focused on the No.5 DRM of LDRM (LDRM-5) to investigate deformation characteristics and potential failure mechanisms. Furthermore, the rockslide tsunami risk of LDRM-5 was assessed. LDRM-5 is a typical pillar-shaped DRM with a distinct binary structure prone to sliding. Its boundaries are well-defined. The lower part of LDRM-5, situated within the reservoir fluctuation zone, comprises fragmented thin-layered argillaceous limestone with Karst breccia interlayer, and was considered to control the stability of the steep slope directly. In the future, the base rock of LDRM-5 might collapse due to long-term erosion from fluctuating reservoir water and constant pressure from the slope above, potentially resulting in overall slope failure. A granular flow model was employed to assess the tsunami risk posed by LDRM-5. Results show that the rockslide-generated tsunami exhibits greater propagation velocity at the water level of 175 m, while displaying a higher wave height at 145 m water level. At 145 m water level, the tsunami run-up on the opposite bank can reach 24.3 m, posing an extremely high-risk area covering 102.55 × 104 m2 and threatening a riverbank stretch of 16.03 km. At 175 m water level, the extremely high-risk area decreases to 45.31 × 104 m2, endangering a 7.85 km riverbank stretch. This study can provide a reference for research on failure mechanisms and tsunami risks of other high-steep DRMs in reservoir areas.