Generating mechanism of the tidal residual current due to the coastal boundary layer

The tidal residual current is produced by a non-linear effect caused by flow separation of the tidal current due to a complicated coastline. This paper shows that the similar non-linear effect is produced also by the coastal boundary layer with a theoretical model having a simple coastline. It is found that the shear of the tidal oscillatory current produces the non-linear effect and its longitudinal gradient generates the tidal residual current flowing from the bay head to the bay mouth inside the boundary layer or shear region. The velocity of the residual current is proportional to the square of tidal current amplitude. Consideration of the generating mechanism suggests that the tidal residual current is generated by the change in depth of the bay.