In the period of strong density stratification from early summer through early fall, the supply of oxygen from the sea surface to the deeper water is cut off. At the same time, organic matter decomposes near the ocean bottom, so that the anoxic water mass forms. In inner Tokyo Bay, when a northeasterly wind(directed from the inner bay toward the mouth of the bay)blows, the anoxic water mass upwells(an “Aoshio” occurs). In some cases fishes and shellfish die along the coast. Based on the report of results of continuous observations of water temperature, salinity and dissolved oxygen content presented by Fukuoka et al, 2005, here we have used an improved fluid flow model to carry out 3-dimensional calculations of the water level, water temperature, salinity and flow distributions. The computational results have reproduced the observational results well. The calculations showed that upwelling of the anoxic water mass that forms during the stratified period is not only affected by the continuously blowing northeasterly wind, but also by a continuous southwesterly wind that blew several days previously. Surface water blown against the coast by this continuous southwesterly wind is pushed downward; the calculations reproduced the process by which the rising force of this previously downwelled surface water also affects the phenomenon of anoxia. Furthermore, we presented the results of time dependent analysis of quantities relevant to water quality, including dissolved oxygen, which is closely related to the Aoshio, using the flow and diffusion model and a primary ecological model during the stratified ocean period, the sinking period and the upwelling period. We have compared the computed results to the results of continuous observations of dissolved oxygen during occurrence of an Aoshio in 1992 at observation point D-2, and confirmed that this model is an appropriate one to describe this phenomenon.
In order to reproduce the physical characteristics in the vicinity of the Nanko thermal power plant located in Osaka Bay, a semi-enclosed coastal bay, in summer, a numerical simulation model was developed. The horizontal and vertical distributions of water temperatures were fitted to the observed and model results, although the calculated salinity values are slightly lower compared to the observed values. This model can reproduce the physical environment in Osaka Bay and the Nanko area. Calculation results showed that the discharge from the power plant may lead to a disturbance of the stratification within the area of 500m away from the power plant discharge.
As an investigation of environment of enclosed coastal seas, we attempted the estimation of the permissible load of COD considering the internal production. COD was separated to pollution by inflow load and pollution by internal production, and on COD pollution by inflow load, the load curve was deduced from the material balance which made the bay to be one box. In addition, it was shown that it was possible to estimate the permissible load on COD, by giving the transformation coefficients from total nitrogen or total phosphorus on the internal production of COD proposed by Nakanishi et al. These results of the study can contribute to the proposition of the water quality improvement plan according to the characteristics of the bay.