Controls of Sediment Grain Size and Bedform Dynamics on Scour Variability in a Strait‐Influenced Tidal Delta

ABSTRACT Scour development around offshore monopiles in the Eastern Mouth Tidal Delta (EMTD) of the Qiongzhou Strait was investigated based on field observations of bathymetry, hydrodynamic conditions and sediment grain size. The results reveal pronounced spatial variability in scour depth ( S D ) and scour volume ( V D ), with scour intensity increasing systematically towards the direction of the Qiongzhou Strait. This spatial pattern indicates a strong hydrodynamic gradient across the study area. Quantitative analysis of wave‐ and current‐induced bed shear stress shows that spatial differences in near‐bed forcing are primarily controlled by tidal current velocity, whereas wave‐induced shear stress remains relatively uniform and acts as a secondary contributor. This suggests that tidal currents are the dominant driver shaping the observed spatial distribution of scour intensity, particularly in zones of enhanced scour development. Sediment grain size is shown to covary with hydrodynamic conditions, reflecting its role as an indirect indicator of flow energy. Based on this relationship, a statistical scaling trend between scour metrics ( S D and V D ) and median grain size ( d 50 ) is established. This relationship reflects coupled sediment sorting and hydrodynamic forcing rather than a direct sedimentological control. Integration of sediment and hydrodynamic analyses suggests that scour in energetic tidal environments cannot be described solely as a local equilibrium process. Instead, it reflects the combined effects of local erosion processes and broader morphodynamic seabed adjustment under spatially varying tidal forcing.