The purpose of the present paper is to establish the theoretical predictions for the vertical distribution of the undertow inside surf zone. The basic equation of these analytical predictions is based on an eddy-viscosity model and on an assumption that the average shear stress over a wave motion could be described in terms of the different linear function of the vertical distance from the sloping bottom (z′) at the two separate regions, which are the upper side and the lower one of the level of the wave trough. In order to solve this basic equation for these analytical predictions of the vertical profile of the undertow, the surface and bottom boundary conditions are given by evaluating the surface vorticity and the bottom mass transport velocity, respectively. According to the analysis, the present theoretical results are in much better agreement with experimental ones rather than those of the other researchers'. The surface vorticity has stronger influences on characteristics of the vertical distribution of the undertow in the surf zone rather than the bottom mass transport velocity, and plays an important role in the present phenomenon.
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