The Effect of Pier Placement on Large Wood Accumulation Probability and Local Scour at Bridge Piers and Abutments: Laboratory Flume Experiments

ABSTRACT The position of bridge piers relative to abutments plays a critical role in controlling large wood (LW) accumulation and the resulting local scour, which are among the leading causes of bridge instability. Previous studies have mainly examined isolated piers or abutments using simplified, solid debris shapes, overlooking the interactive effects of pier–to–abutment distance, and the natural porosity of wood accumulations. To address this gap, a comprehensive series of flume experiments was conducted, systematically varying pier–to–abutment distance, log length, flow intensity ratio, bed condition (fixed, movable‐static, movable‐dynamic), and temporal evolution of scour. The results show that LW accumulation probability (LW AP) is primarily controlled by relative log length, bed mobility, and pier–to‐abutment distance. Accumulation reached up to 98% for longer logs at large pier–abutment distances, as these positions aligned the piers with the main LW transport path. While flow intensity ratio and pier placement significantly influenced scour depth and geometry, porous, and transient LW accumulations produced only minor additional scour effects (< 6.5%) compared to solid accumulations reported in earlier studies. This study, by highlighting pier–to–abutment distance as a governing factor in LW accumulation and scour, simulating realistic LW with variable porosity instead of idealized debris shapes, and comparing LW AP under fixed, static, and dynamic bed conditions, provides novel insights into flow–structure–wood interactions and demonstrates that strategic pier placement can offer more effective risk mitigation than conservative design assumptions based on non‐representative debris models.