Hydrodynamic Characteristics and Pier–Flow Interaction Mechanisms in a Mountain River Bridge Section: A Case Study of the Jinqian River “7·19” Bridge Collapse Event

Mountain rivers are characterized by steep gradients, high flow velocities, and flash flood regimes, and the interaction between in-stream bridge pier groups and flood flows is a persistent challenge in bridge hydraulics. This study investigates the flood hydrodynamic characteristics at a mountain river bridge section and clarifies the coupled pier–flow–debris interaction mechanism, using the 19 July 2024 collapse of Yanpingcun Bridge No. 2 over the Jinqian River in Shangluo, Shaanxi Province, China, as a case study. High-resolution topographic data (0.5 m) derived from three-dimensional laser scanning point clouds were used to construct a two-dimensional hydrodynamic model (IFMS) and a three-dimensional air–water two-phase flow model (volume-of-fluid method, approximately 8 million cells) of the bridge site. Six scenarios were simulated to quantify the effects of pier arrangement, tie beam elevation, and flood protection works on water level rise, velocity distribution, and main flow direction, and the models were validated against surveyed water marks and video-based velocity measurements. The results show the following. (1) The pier group produced a marked flow-blocking effect: the maximum water level upstream of Pier 10 rose by 1.28 m relative to the natural channel, the main flow direction deflected clockwise by about 18°, and a triangular confluence zone formed between the pier group and the right-bank retaining wall. (2) The existing skewed pier arrangement raised the upstream water level by 0.78 m compared with a flow-aligned arrangement and produced a disordered flow pattern unfavourable to debris passage. (3) Three-dimensional analysis showed local water level rises of 0.52–1.20 m on the upstream faces of the piers, with maximum local velocities of 5.5–6.3 m/s along the pier flanks; the maximum velocity occurred at a water depth of 1.0–2.0 m rather than at the free surface. (4) Tie beams exposed above the bed intensified local flow blockage and increased inter-pier velocities (0.3–0.8 m/s), so that debris was readily trapped beneath the tie beams, forming a blockage–backwater–load positive feedback loop. The findings indicate that the large angle between the piers and the main flow, the dense pier group, exposed tie beams, and debris blockage were the key hydrodynamic factors in the collapse, and that the amplifying feedback among piers, flow, and debris was the core mechanism of structural failure. The results provide a scientific basis for the design optimization and flood safety assessment of bridges aligned along mountain rivers.