Experimental investigation of scour evolution around a vertical-axis tidal turbine under combined wave-current action

Scour around vertical-axis tidal turbine (VATT) foundations can threaten the long-term stability of tidal-stream energy systems. This study experimentally investigates scour evolution around a VATT foundation under combined wave-current action, focusing on the effects of flow intensity, wave height, tip clearance, tip speed ratio ( TSR ), and water depth. Time-dependent scour depths and quasi-equilibrium three-dimensional bed topographies are used to quantify both scour magnitude and spatial extent. The contribution lies in systematically evaluating hydrodynamic, installation, and operating parameters using multiple temporal and morphological scour indicators. Under identical forcing, the operating-turbine case exhibits a maximum relative scour depth of 1.163, approximately 18% greater than the monopile-only case, with the critical scour location shifting from the upstream to the lateral region. Increasing flow intensity from 0.6 to 0.9 raises the maximum relative scour depth from 0.898 to 1.692, while increasing tip clearance reduces scour severity. Increasing tip speed ratio from 0.95 to 3.80 increases the maximum relative scour depth from 1.100 to 1.377. These results show that VATT scour assessment should consider not only maximum scour depth but also turbine-specific spatial bed deformation.