Optimization of Numerical Models for Environmental Dynamics in Closed Water Basins

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Closed water basins such as lakes, inner bay, etc., have serious problem on water quality due to the lack of water exchange with outer fresh water. It is therefore important and urgent require to develop a reliable numerical model, which can quantitatively predict behavior of flow and contamination dynamics. Numerical approach seems to be effective for such purpose from practical and economic points of view. However, it is not easy to increase dramatically the accuracy of numerical model because the model have a multiplex structure composed of many modules, i.e., numerical scheme, turbulence models, source, sink and transportation of contamination, etc. and each module requires simplification of real phenomena. So far many models were proposed for every module though the model performances have not been sufficiently elucidated yet.The objective of this research is optimization of the numerical model for environmental dynamics in closed water basins, which can predict flow and contamination not only qualitatively but also quantitatively. The research can be decomposed into two parts, namely, the basic part and the application part. In the basic part, various numerical schemes, turbulence models and contamination transport models were comparatively examined focusing on conservation, numerical viscosity and CPU time. In the application part, practical closed water areas were targeted. We studied various areas with different spatial scales, such as, Lake Biwa (largest lake in Japan), Lake Shinji and Lake Nakaumi (brackish lakes connected with a river), Ago Bay and underground area. Through these application data, we could elucidate advantage and shortcoming of the numerical models