Application of One-way-nesting Technique on Tidal Current Simulation in Coastal Waters using a Two-dimensional Shallow Water Model

Because of the rapid growth in population, economy and urbanization, many countries have faced serious problems in aquatic environment of coastal waters. In these coastal waters, numerical simulation techniques based on the governing equations of physical and bio-chemical phenomena are necessary and effective in order to estimate the present situations and to predict feasible measures that can be attempted to prevent water environment degradation and rehabilitate the environment. In utilizing numerical simulation techniques in coastal waters with more complicated geography, more calculation points would be needed to understand the spatial distribution of tidal current in detail, resulting in taking more computation time. Nesting technique allows us to conduct numerical calculations efficiently: speedily in an entire wide area and finely in a specific narrow area. In this study, using a two-dimensional depth-integrated shallow water model, the applicability of so-called “one-way-nesting technique” on tidal current simulation was investigated based on the calculations in a virtual coastal area. The wet-and-dry scheme was also incorporated with the simulation model to express the generation and disappearance of tidal flat. In the application of the one-way-nesting technique, the tidal currents and the water depths were firstly calculated by a two-dimensional shallow water model in the large-mesh area that covered entire calculation area. And then in the small-mesh area that was a portion of the entire calculation area and would be needed to obtain the spatial distribution of tidal current with higher resolution, the residual components were calculated by the induced governing equations of the components and the boundary condition based on the Sommerfeld radiation condition. In this study, the reference calculation, in which the large-mesh area was all covered by the small meshes with the same resolution as that in the small-mesh area of the nesting calculation, has been also executed in order to estimate the accuracy of the nesting calculation. The simulation results indicated that the current velocities and the water levels obtained by the nesting calculations had a good agreement with those by the reference calculations, while the computation times of the nesting calculations were much shorter than those of the reference calculations. On the other hand, the rapidly-varied flows such as the trailing vortex downstream of structural objects were not appeared in the nesting calculations. In order to improve the reproducibility, it is necessary to consider the adoption of the so-called “two-way-nesting” with feedback calculation