Debris flows in alpine canyon regions pose serious threats to infrastructure and human settlements due to their abrupt onset and high mobility, yet the interactions among topographic complexity, sediment supply, and mitigation performance remain insufficiently understood. This study integrates field investigations, hydro-meteorological analysis, and dynamic numerical simulations to investigate the “8·03” event in Ridi Gully, eastern Qinghai–Tibet Plateau. The results indicate that the initiation and subsequent evolution of the debris flow were jointly controlled by intense localized rainfall, abundant unconsolidated sediment, and steep channel gradients, with local blockage-breaching processes further amplifying flow velocity and impact energy. Mitigation simulations further show that the single slit-dam configuration reduced the debris flow deposition volume and area by 14.29% and 16.23%, respectively, whereas the combined slit-dam/rigid-check-dam configuration achieved reductions of 65.54% and 44.97% relative to the no-protection scenario. These results highlight the potential contribution of temporary blockage-breaching to downstream hazard amplification and demonstrate the different regulating effects of the tested mitigation configurations on flow intensity and sediment deposition. The findings provide case-specific insights into debris flow hazard evolution and staged structural mitigation in alpine mountainous environments.