Abstract
Local scour around monopile foundations poses a critical challenge for offshore wind turbines in tidal–dominated coastal seas, where hydrodynamics span multiple scales from kilometre-scale tidal currents to metre-scale horseshoe vortices. Conventional standalone geophysical fluid dynamics (GFD) or computational fluid dynamics (CFD) numerical modelling approaches struggle to resolve these cross-scale dynamics with sufficient accuracy and computational efficiency. To address this gap, this study extended GFD–CFD coupling from hydrodynamic simulations to sediment transport and scour morphodynamics modelling by integrating the unstructured-grid Finite-Volume Community Ocean Model (FVCOM) with the two-phase sediment transport solver SedFoam. This framework enables monopile scour to be simulated under realistic, time-varying tidal forcing, including flow reversal and rotation. The framework was first validated against laboratory flume experiments under bidirectional currents, reproducing scour evolution with RMSE values below 0.1 S/D (ratio of scour depth to monopile diameter). An idealized rotating tide experiment further showed that near-circular scour hole morphology was reproduced under flow with constantly varying direction. In a real-world application to Laizhou Bay, China, the simulated scour and deposition patterns around the monopile closely matched single–beam sonar surveys. The results suggest that tidal current direction, not residual circulation, governs scour orientation, highlighting the importance of realistic tidal forcing in scour modelling. The coupled framework provides a transferable tool for mechanistic and engineering assessments of monopile scour under complex coastal hydrodynamics.