The structure of the tidal residual current due to vertical viscosity is investigated both theoretically and experimentally. It is found that the interaction between the vertical component of the oscillatory current and vorticity,wT1ξT1, induces a strong residual constituent outside the boundary layer and forms a circulation which is quite similar to gravitational circulation and that the vertical profile of the oscillatory current not only affects the magnitude of the residual constituent but also decides the direction of the circulation. In the hydraulic experiment, the residual constituent is larger than the theoretical prediction and a phase difference in the oscillatory constituent between the upper and lower layer is observed. The amplitude difference is caused by the strong nonlinear effect of the residual constituent and the phase difference is caused by the interaction between the residual current and the basic oscillatory current. The principal generating force of the residual constituent outside the boundary layer,wT1ξT1, is observed in a bay where the tide is nearly a standing wave.
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