Drift current under the action of wind and waves

Accurate estimates of sea surface drift currents are critical to forecasting oil spill transport and fate. The majority of existing spill models employ a drift factor and deflection angle, based on local wind speed, to estimate the sea surface drift vector. The effects of wind induced shear and wave induced transport are lumped together in this formulation. In the present approach the conservation of momentum, water mass, and turbulent energy equations are solved using an implicit finite difference method to predict the vertical distribution of current, turbulent energy, and eddy viscosity at one point. The model includes coupling between the wave and shear induced currents. Input energy from the atmosphere to the turbulent energy and current fields are represented through free surface boundary conditions. The numerical model showed excellent agreement in comparison to an analytic solution of the wind forced shear flow problem. The model was applied to predict surface drift currents for varying wind speeds and predicted results in general agreement with field observations and other numerical and theoretical studies. The model predicted drift factor, F, (%) and deflection angle, θ, (degrees) decrease with increasing wind speed, W, (m/s) and can be approximated by the following curve fits: F= 3.91-.0318W, θ= 23.627-7.97 log W. The model was applied to three intentional oil spills performed on the Norwegian continental shelf in 1991 and predicted the observed trajectories with reasonable accuracy