Abstract: Local scour often occurs in hydraulic and ocean engineering. Wave-current coupling and pile diameter effect make this problem very complicated. In order to study the local scour characteristics of large diameter monopile under combined waves and currents, a numerical model was established in the CFD software FLOW 3D, and a physical model test was designed. By comparing the scour depth in the physical model test and the results in numerical simulation, the accuracy of numerical simulation was verified. Based on an offshore wind farm project, a field-scale numerical model of seabed and monopile was established. The waves were described by random theory and the turbulence was calculated by large eddy simulation method, and the local scour characteristics of a large diameter monopile under combined waves and currents were analyzed. The results show that the spatial development of local scour can be divided into three stages: longitudinal cutting, lateral spreading and stabilization. The maximum scour depth was 4.9 m in front of the pile, which was about 0.9 times of the pile diameter. Correspondingly, the time development of the local scour can also be divided into three stages: rapid growth, moderate growth and stabilization. After 20 hours, the maximum scour depth remained unchanged, and the local scour reached a stable state.

Key words: local scour, large diameter monopile, wave-current coupling, numerical simulation, maximum scour depth