A 3-D model has been developed to simulate the undertow current and sediment transport in surf zone. The flow model uses the 3-D phase-averaged shallow water flow equations with depth-varying wave radiation and roller stresses. The flow model is coupled with a horizontal 2-D wave spectral transformation model and a wave surface roller model. Sediment transport is simulated using a non-equilibrium total-load transport model. The governing equations are solved using an implicit finite-volume method based on a multiplelevel quadtree rectangular mesh on the horizontal plane and the sigma coordinate in the vertical direction. The viscosity is determined using a modified mixing length model. The developed model has been tested in two laboratory experiments, and the calculated wave heights, mean water levels, undertow current velocities and sediment concentrations are in reasonably good agreement with measured data.
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