Computation of Tidal Current in Osaka Bay and Investigation on Physical Models

Abstract

Numerical experiments on the tidal current of M_2 tide in Osaka Bay are carried out using 2D and quasi-3D models. The computed results such as the instantaneous velocity field, tidal residual current, and hodographs are compared with those calculated from the observed data. The 2D model gives important features of actual tidal current in Osaka Bay such as the formation of stagnant region in the east part, twin vortices in the vicinity of the Akashi Straits, Okinose Circulation and Suma Offshore Counter Current, but it would compute much higher flow speed at the Straits. The Quasi-3D model which includes the submarine topograph could improve the above defect and realize Tomogashima Counter Current and East Coast Southward Current as well, but it would compute too high flow speed on the shallow region in the east part even though the bottom friction is taken into account. At spring tide the twin vortices formed at the Straits when the flow through the Akashi Straits is eastward maximum travel to the middle part of the Bay, and get the energy to continue rotating from the northward current along Awaji Island when the flow through the Akashi Straits is westward. At neap tide the vortices stay near the Straits and disappear at the turn of tides. When the twin vortices are shed from the Straits, they lose the symmetrical feature and eventually remains as Okinose Circulation and Suma Seashore Counter Current. Fractal dimensions for flow phenomena in Osaka Bay are estimated using the observation data. The result suggests the chaotic characteristics of actual tidal current makes its long-term prediction very difficult.