Characteristics induced by collapse of Diaozui unstable rock mass in Qutang Gorge, Three Gorges Reservoir area under different failure modes

Rockfall-induced impulse waves in the Three Gorges Reservoir pose a significant threat to navigation safety and waterfront infrastructure. Taking the B3-1 unstable rock mass at Diaozui in Qutang Gorge as a representative case, this study systematically analyzed the dynamic characteristics and propagation patterns of impulse waves induced by rock instability under different failure modes, aiming to provide a theoretical basis for risk assessment and prevention of impulse disasters in the reservoir area. Based on geological surveys and historical collapse data, a fluid solid coupling model incorporating the k-ε turbulence model and the GMO collision module in FLOW-3D was established to simulate the generation and evolution of impulse waves under three instability modes: sliding, toppling, and sliding-fragmentation composite, at water levels of 145 m and 175 m. Wave height demonstrates a positive correlation with the impact velocity of the unstable rock mass, while showing negative correlations with both the cross-sectional area at water entry and the fluid-structure interaction efficiency. Sliding failure occurs when the bedrock can no longer support the gravitational load of the rock mass. This failure mode generates low-frequency long waves characterized by horizontal water displacement, with measured wave heights of 8.9 m and 5.1 m under different water level conditions. It presents the highest energy transfer efficiency (38.0% and 41.2%) with the slowest wave attenuation rate. Toppling failure is triggered by complete penetration of rear tension cracks under sustained gravitational loading. The rock mass impacts the water surface in a slapping manner, producing the maximum wave heights (11.5 m and 14.6 m). However, this failure mechanism shows the lowest energy transfer rates (17.3% and 37.2%) with relatively rapid attenuation characteristics. The composite sliding-fragmentation failure, constrained by the mechanical properties of argillaceous limestone and the structural features of joints J1 and J2, produces intermediate wave heights and energy conversion efficiency between the two aforementioned failure modes. Notably, it demonstrates the most rapid attenuation rate, consequently generating the minimal impact on surrounding areas. Furthermore, the study reveals that impulse wave propagation and attenuation characteristics are collectively governed by three primary factors: failure mechanism, reservoir water level, and channel topography. Therefore, differentiated disaster prevention strategies should be formulated based on the failure mechanisms. This study provides an important theoretical foundation for risk assessment and engineering control of impulse disasters caused by unstable rock masses in the Three Gorges Reservoir area.