Numerical investigation of the dynamic response of bridge piers under debris flow impact

A coupled Arbitrary Lagrangian–Eulerian (ALE) framework is employed to numerically simulate a single-boulder debris-flow impact on a round-ended railway bridge pier. Integrating continuous slurry hydrodynamics with transient high-energy boulder impacts, dynamic responses and failure mechanisms were assessed across multiple return periods. Results show that boulder impacts dominate loading at the frequent 5-year return period, where peak forces reach 5 times the slurry magnitude, decreasing to 1.9 under the extreme 100-year scenario. Notably, from 10- to 100-year periods, slurry-driven deformation surges 5.2-fold, significantly outstripping the 2.7-fold increase in boulder-induced deformation. This disparity stems from simultaneous increases in flow depth and velocity, which expand the slurry’s vertical load distribution. Furthermore, a critical damage threshold is identified between the 20- and 50-year periods, marking a rapid transition from localised yielding to severe concrete spalling. Spatially, failure evolves upward from the pier base, highlighting the footing and the upper-lower pier junction as primary vulnerabilities prone to progressive plastic deformation and tensile cracking. These findings provide a scientific basis for optimising impact-resistance evaluations and protection strategies for mountainous railway bridges.