|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1992 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1991 : ¥1,300,000 (Direct Cost : ¥1,300,000)
Buoyant surface discharges into stagnant waters such as river plume and thermal plume are characterized by the three-dimensional structure of flow and density fields, which is for the most part controlled by horizontal pressure gradients and the stratification resulting from the density difference between discharged and receiving waters as well as momentum fluxes. Furthermore, their behaviors may change depending on the flow and density fields of ambient coastal seas and the geographical features near the river mouth. A three-dimensional, primitive numerical model with a free water surface is developed for examining the river plume spreading. Turbulent shear stresses and turbulent buoyant fluxes are expressed in terms of the eddy viscosities and diffusivities, in which the stratification effects are taken into account with empirical formulae through the comparison of numerical results with published experimental data.
The model predictions are compared with experimental results of river
plumes with small densimetric Froude numbers, field observation results performed in the Akashi River, and the infrared image of the YodoRiver in flood. As a result, the mechanism peculiar to river plume spreading near the river mouth is clarified; that is, the flow acceleration, the divergence of river water and the lift off phenomena. The numerical model has been applied to river plumes discharged into cross flows and into complicated geographical features. In particular, the effects of spreading angle of river mouth topology on the three-dimensional structure of river plumes are evaluated by using a k-epsilon turbulence model from the scientific point of view.
On a larger scale, the Rossby deformation radius, the earth's rotation in addition to the buoyancy are the major forces that cause a strong geostrophic along-shore current. Consequently, the Yodo River plume flows to the right along the coastal line of Kobe and Awaji Island while forming a narrow coastal jet in the case of flood. The computed density field shows good agreement with the infrared image taken by the NOAA satellite during the flood flow of typhoon 8210. If there is a tidal flows, however, its producing current exceeds the density-driven current, enhancing the tidal excursion of the river plume unless the propagation speed of the coastal jet is too high. Instead, the density effect assists in the formation of the tidal residual current system. Less