Behavior of a Small Pulsed River Plume in a Strong Tidal Cross-Flow in the Akashi Strait

We carried out a field study of a small river plume emptying into Osaka Bay near the Akashi Strait in western Japan, to understand the fate of its urban-runoff-laden waters. As the river is weak compared to tidal currents, we find that the behavior of the plume is strongly influenced by phasing between tidal stage and currents (a westward-traveling progressive tidal wave). When the tidal stage rises, sea water floods up the river, and concentrated river effluent cannot be seen in Osaka Bay. During most of the river’s ebb, a topographic eddy transports river effluent towards the energetic Akashi Strait, while strong vertical shear in the tidal flow mixes the effluent with seawater. However, there is a short interval of time during which the direction of tidal flow in the Strait changes direction and the magnitude of the current is weak. At this time, shear is weak enough to allow a stratified plume of concentrated river water to form, and this plume is driven offshore, and into Osaka Bay, by baroclinic circulation. A sewage outfall, which is located very close to the river mouth, is subjected to these same conditions and therefore exhibits similar behavior. Understanding the relation between tidal phase and plume behavior will be useful to Kobe City, as well as other cities in a similar environment, for minimizing the environmental effects of their wastewater and non-point-source runoff.