Occurrence of hypoxic water in the interior parts of the Ariake Sea [Japan] and its preventive measures

In this study, in order to clarify the generation mechanism of hypoxic water that occurs frequently in the interior parts of the Ariake Sea and to investigate its preventive measures, the field observations of tidal current and water quality near the seabed at an observation point and hydraulic experiments were conducted. From the data observed during May 22-June 26 in 2004, the close relationships between temporal variation of dissolved oxygen concentration (DO) near the seabed and that of tidal level were clarified. That is, DO increased at spring tide and decreased at neap tide. At neap tide during the observation period, DO decreased to about 1.0 mg/l and hypoxic water was observed near the seabed. There were positive correlations between DO and velocity of tidal current or water temperature, and a negative correlation between DO and salinity. A relationship between DO and above three factors was obtained by the multiple linear regression analysis. From these results, it was considered that the stirring and mixing of seawater near the seabed had significant effects on the density-stratification of seawater and the occurrence of hypoxic water in this sea areas. Models of three kinds of blocks for increasing the power of stirring and mixing of seawater near the seabed were made and each block was installed on the experimental channel bed. Moreover, the effects of each block on the velocity properties in the oscillating flows were investigated experimentally. As a result, the effects of each block on the velocity profiles and turbulent properties were revealed. Moreover, the diffusion coefficients and typical time scale of vortex around each block were calculated by the Taylor's turbulent diffusion theory, and their profiles were clarified. From the visualization of flow pattern around each block using a dye-tracer method, the concave block was considered to be better than the other blocks in order to stir and mix efficiently the water near the experimental channel bed.