Critical Initiation Mechanism and Multi-Field Response Characteristics of Coal Gangue Debris Flow Under Different Slope Angles

Coal gangue deposits may develop debris flows under hydraulic loading, yet how slope angle affects initiation and internal responses remains unclear. Flume experiments were conducted using coal gangue from western Beijing at 12°, 16°, 20°, and 24°, with large-scale particle image velocimetry, pore-water pressure, earth pressure, and MEMS measurements. Coupled PFC–FiPy simulations at 16°, 20°, and 24° examined particle motion, stress, porosity, and seepage. Initiation was progressive and spatially asynchronous, evolving from localized rolling and sliding to widespread sustained movement. Critical initiation discharges were approximately 132.19, 42.56, 59.00, and 29.10 m3/h, respectively, showing an overall decline with increasing slope angle despite a higher threshold at 20° than at 16°. Simulated high-velocity zones initially developed near the slope toe and extended inward, with maximum local particle velocities of approximately 0.347, 0.387, and 0.412 m/s, respectively. Pore-water pressure and effective saturation were relatively high near the toe, while vertical stress and porosity varied spatially. The findings suggest that increasing slope angle generally favors initiation by increasing downslope gravitational loading and reducing normal constraints, while local seepage and particle-contact variability may affect instability propagation. These findings support debris-flow hazard assessment and monitoring at coal gangue disposal sites.