Collision dynamics of dual tidal bores in the Qiantang River Estuary, China

Abstract Dual tidal bores occur in estuaries with specific riverbed morphology, yet the hydrodynamics of their collision remain poorly quantified. In this study, we investigate the collision dynamics of dual tidal bores in the Qiantang River Estuary using field data and a numerical model. The model successfully reproduces the collision process and quantifies key parameters including bore height, velocity, propagation speed, Froude number, and steepness. Results show that bore collision leads to nonlinear amplification: bore height increases by 24.1–35.6% above linear superposition predictions. Propagation speed increases by up to 27.9%, Froude number increases by up to 52.4% (reaching 1.86–1.93), and bore steepness increases by 9.5%. Riverbed bifurcation in the bore formation zone contributes to the generation of dual bores, with the southern branch consistently exhibiting stronger intensity. These findings highlight the significant influence of bed morphology on tidal bore dynamics. Understanding dual tidal bores dynamics is critical for ensuring seawall safety and provides a foundation for assessing the effects of tidal bores on estuarine infrastructure.