Abstract: The Luotongba-Duigou debris flow in Pingwu County, Mianyang City, Sichuan Province, poses significant risks to infrastructure safety following a recent large-scale event that transported substantial sediments downstream, endangering National Highway G247 and the newly constructed Jiuzhaigou-Mianyang Expressway. This study combined field investigations with UAV aerial surveys to analyze the formation conditions and characteristics of the debris flow. Through physical model experiments and numerical simulations, the dynamics of debris flow movement and its impact on bridge piers were reconstructed. Results indicate that the total source material volume is approximately 2.2641×10⁶ m³,with a density range of 1.40~1.68 t·m^-3 and peak discharge between 85.39 m³·s^-1 and 309.99 m³·s^-1,classifying it as a large-scale dilute valley-type debris flow. Under a 10-year return period rainstorm scenario, the debris flow reaches the accumulation zone at the gully mouth about 10 minutes after breaching the blockage area, threatening bridge piers and other structures. Physical modeling shows that impact forces concentrate at the pier base and attenuate rapidly with increasing distance from the debris flow outlet and pier height. Piers located farther from the outlet may experience multi-phase strong impacts during debris flow events. Numerical simulations reveal that the debris flow fully envelops the pier upon impact, exhibiting circumferential climbing flow. The maximum principal stress (0.81 MPa) and principal strain (2.18×10^-5 m·m^-1) are concentrated at the pier base, with surface pressure distributed in a semi-elliptical pattern propagating upward and inducing a top displacement of 0.33 mm. For engineering practice, it is essential to reinforce pier bases against debris flow impacts and avoid direct collision between flow-transported debris and piers. These findings provide actionable insights for debris flow risk mitigation and support structural safety and disaster resilience planning in the region.