Exploring scour mechanisms of bridge group under tidal Conditions: In-situ survey, laboratory experiment, and numerical analysis

Scouring under tidal conditions threatens closely spaced bridge groups along rivers, yet this issue remains inadequately addressed. This study investigates scour at pile foundations of bridge groups under ebb tide conditions through in-situ survey, laboratory experiments, and numerical analysis. In-situ survey was conducted at the bridge group along the Wulong River to assess tidal flow characteristics. Laboratory hydrodynamic experiment and computational fluid dynamics (CFD) simulation were performed to analyze flow behavior and scour patterns. Results reveal that closely spaced bridges induce complex hydrodynamic interactions due to the formation of turbulent wake zones. Upstream bridges decrease flow velocity near downstream bridges while intensifying turbulence, thereby exacerbating local scour around downstream foundations. The most severe scour was observed at a bridge located 220 m downstream of another bridge, positioned at the end of the upstream wake zone. Based on along-channel flow velocity and local scour effect, a scour depth prediction method for closely spaced bridge groups is proposed and validated. While the model provides a comprehensive framework for scour assessment under tidal influence, it was developed under ebb tide conditions with a specific bridge pier layout. Further calibration is recommended for application to varying tidal conditions and structural configurations.