Aimed at providing a simple yet stable method of dealing with a moving boundary problem, a simplified treatment is proposed in the Marine Environmental Committee (MEC) ocean model, which combines 3-dimensional hydrodynamic multi-layers with a shallow water layer. Tidal current around general geometries such as the long shore and the sinking island can be simulated effectively.


In tidal simulations, most researchers make great efforts to deal with complicated geometries in order to get precise results. It is even more difficult when there exist quite a tidal flat (the long shore effect) whose length could be several kilometers in only a few meter's tidal height, and/or very shallow water that exposes its bottom during low tide to form an island that will be submerged during flood tide (the island effect); these are situations where the topologies of meshes will be changed. It requires a numerical model to be precise and stable enough for the simulation of this kind of moving boundary problem. The above problem usually involves cells being flooded or dried during the calculation, which arises in a wide range of free-surface hydraulics problems, such as tidal floods, dam breaks and overland flow of precipitation. Techniques to handle these problems include deformable computational meshes, modified equations in very shallow regions (e.g. Meselhe and Holly, 1993) and shock capturing schemes (e.g. Tchamen and Kawahita, 1994). Akan-abi and Katopodes (1987) gave a brief summary of the problems encountered in the numerical simulation of flood waves propagating on a dry bed. Khan (2000) developed a finite element model for the flow over a frictionless horizontal surface, and reported the smooth and rough surface of the horizontal and sloping laboratory channel and numerical oscillations at the front of the surge. Beffa and Connel (2001) reported numerical oscillations when cells switch from dry to wet and vice versa.

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