A Virtual Blade Model is coupled with a CFD model to simulate impacts from a Horizontal Axis Tidal Turbine under combined surface waves and a steady current. A two-equation model is used to represent the turbulence generation and dissipation due to turbine rotation and background wave-current flows. The model is validated against experimental measurements, showing good agreement in both surface elevation and fluid hydrodynamics. It is then scaled up to investigate a steady current with large stream-wise surface waves in the presence of a turbine. A strong interaction is found between surface wave-induced flows and that around the turbine, which clearly impacts on both hydrodynamics within the wake and wave propagation, and produces large fluctuations in power production. Model results show that the wave-period-averaged velocities are similar to those in the steady-current-only condition. However, the wave enhances the turbulence immediately behind the turbine and reduces the length of the flow transition. The wave height reduces by about 10% and the wavelength extends by 12% when propagating over the turbine region in comparison with the no-turbine condition. The wave shape also becomes asymmetric. Compared with the current-alone situation, the model results suggest that the power production is similar. However, wave oscillation produces noticeably larger fluctuations.