UNSTEADY FACTORS AFFECTING SALT DIFFUSION IN THE STRATIFIED FLOW SYSTEM AT A RIVER MOUTH

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The problem of salt diffusion in highly stratified flows such as those found at the mouth of a river is vital to considerations of agriculture and fisheries. Placement and control of the gates governing extraction of fresh water from a river have become social issues. Previous research has assumed that these flows are steady, and has focussed on problems of shear instability in the vicinity of the interface and natural turbulence. The research described below has examined these flows from the practical standpoint of whether and to what extent unsteady factors such as atmospheric pressure, tide, river flowrate, and wind affect salt diffusion around the river mouth. Those results have been used to attempt to develop an expression for improving predictions of the salinity of the river surface at any given point and time.These studies have been carried out mainly in the lower basin of the Ishikari River, Hokkaido, which is typical of a highly stratified flow. Observations were carried out … More in the summers of 1992,1993 and 1994 for about two months each. The observation methods were improved year by year and enough qualitative and quantitative data have been taken to fulfilll the research goals described above. The results of our research are as follows :1) Wind is the dominant factor in the surface salinity of a river. 2) The average entrainment coefficient of the salt wedge is given by the following formula : E = 1.92 * 10^<-6>e^<0.405^^W> for 118m^3/sec, E =2.24*10^<-6>e^<0.281^^W> for 146m^3/sec 3) The process of diffusion of salt from the salt wedge to the surface of a river has been shown to be the following : Wind blows>waves are generated>the turbulence in the upper layr increases>the distribution of salt in the upper layr is rendered uniform>the density gradient around the interface increases>the salt diffusion across the interface increases>the surface salinity increases. 4) The time average profiles of the flow system are stable throughout almost the entirity of the stratified flow system. 5) The process of diffusion is accelerated by an increase in river flowrate, but the salinity of the river water flow itself decreases. 6) Bridge piers and protuberences in the river bottom cause great disruptions in the interface, but the effect of these disruptions on salt diffusion is localized. 7) In order to predict the present surface salinity at any location it is sufficient to have an hourly record of wind speed along the river course and the discharge rate. Less