This paper presents a new three dimensional (3D) tidal current model that can compute wetting and drying bed (WAD) in tidal flats due tidal motion. The governing equations are derived from 3D Navier Stokes equations and solved using the fractional step method (FSM), combining the finite difference method in the horizontal plane and the finite element method (FEM) in the vertical plane. First, a model test for Saigo fishery port was carried out in order to investigate the performance of the model. Secondly, the tidal currents with the WAD in the north bay of Ariake Sea were simulated. Finally, the applicability of the model was discussed.


Inter-tidal zone is an important place for ecosystem, circulation, transport of physical materials, and in filling-up problems due to siltation and sedimentation in the Ariake Sea, which has a larger tidal range in Japan. A prediction model for simulating a tidal motion, which is consider WAD in the inter-tidal zone, is needed in order to exactly predict changes of hydro- and morpho-dynamic in a region with tidal flats. Conventionally, 2D models (e.g. Balzano, A., 1998) or 3D multilevel models (e.g. Yu et al., 2004; Ozawa et al., 2003; and Zhang et al., 2000), which are layer integrated models, are mainly used to predict the tidal motion in the tidal flats. Although the multilevel model can express vertical structure at deep water in the inter-tidal zone like the tidal flat, the number of layers of vertical direction decreases, and it is difficult to predict the vertical distribution of currents. Therefore, a new 3D tidal current model based on FSM, used by Kuroiwa et al. (1998), is proposed in this study. By using this approach, it is possible to divide the water depth into equal layers and near sea-bottom layer can be divided finely.

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