Virtual wave stress and mean drift in spatially damped surface waves

The concept of virtual wave stress (VWS) is applied to spatially attenuated , deepwater surface gravity waves. With particular emphasis on laboratory w ave tank measurements it is pointed out that VWS induces a mean Eulerian dr ift current that increases with time. This can be important for the determi nation of the wave-induced drift current, especially when the surface conta ins thin slicks of contaminating material. A novel formulation is derived t hat relates VWS to the lateral divergence of the mean wave momentum flux. I t is suggested that this formulation can be helpful in determining the mean drift current in the presence of surface slicks as well as the mean volume flux associated with deepwater waves that break in a limited spatial regio n.