For planning various kinds of the very large offshore structures in the enclosed sea, it became more important to grasp the change of the tidal current and assessment of water quality due to the diffusion of the pollutant, for example, COD (chemical oxygen demand). The authors previously calculated the tidal current and its seawater exchange around an offshore structure in Osaka bay. In this paper, using the above tidal current data, the diffusion of the primary COD flowing into Osaka bay from Yodo river and so on was calculated numerically by use of the diffusion equations. The obtained results were compared with the several calculated and experimental results already published and the accuracy of calculation was confirmed. This method was also applied to the Kansai international airport (phase 2: under construction) and compared with COD distribution described in the EIA report.
In Japan surrounded with the sea, having the long coastline, their effective application has been strongly expected and the very large offshore structures like mega-float and so on in the enclosed sea are planned now. In this situation, not only the building technology but also the environmental impact assessment technology has become very important. The change of the tidal current and the water quality due to the diffusion of the pollutant, for example, COD(chemica1 oxygen demand) are the important factors of the assessment. The COD concentration flowing from the river is called the primary COD. From the viewpoint of ecosystem, the primary COD is reproduced in the enclosed sea through the biochemical change and the reproduced COD is called the secondary COD (autochtonous COD) which is numerically examined by the equation of so-called ecohydrodynamics which connects the tidal flow with the ecosystem model, and this value is finally important and we aim at this level.
