Numerical and hydraulic simulations of the effect of Density Current Generator in a semi-enclosed tidal bay

A large dyke built in 1997 in Isahaya Bay (the spatial scale of about 10 km) in Ariake Bay (about 80 km) raises huge scientific and social discussions in Japan. It is said that, since the construction of the dyke, physical and ecosystem environments have changed and fishery has been damaged enormously. On the other hand, it is known that the Density Current Generator is successfully enhancing water quality in Hazama Inlet (the spatial scale of 3 km) of Gokasho Bay. In this study, we conducted numerical and physical model simulations on the hydrodynamics to investigate whether the apparatus is also applicable to Isahaya Bay in Ariake Bay. A laboratory-scale rotating hydraulic model and a three-dimensional numerical code called the MEC Ocean Model were adopted for the physical and numerical simulations, respectively. The MEC Ocean Model has a special feature, which is a hybrid of hydrostatic and nonhydrostatic models. Tests in a simple bay elucidated the importance of the nonhydrostatic model nested in the hydrostatic model, i.e. the former can simulate the appropriate intrusion depth of the mid-density water discharged in stratification. The numerical results for Ariake Bay suggested that the water discharged from the apparatus spreads in the head of the bay within a couple of weeks with nontrivial concentration of nutrients. This was also confirmed by the physical model test by using the luminance measurement of dye spread.