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

在高度分层的水流中，例如在河口处发现的水流中，盐的扩散问题对农业和渔业的考虑是至关重要的。控制从河流中抽取淡水的闸门的位置和控制已经成为社会问题。以前的研究假设这些流动是稳定的，并且集中在界面附近的剪切不稳定和自然湍流的问题上。下面的研究从实际的角度考察了这些流动，如大气压、潮汐、河流流量和风等非定常因素是否以及在多大程度上影响了河口周围的盐扩散。这些结果被用来试图建立一种表达式，以改进对任何给定时间点的河流表面盐度的预测。这些研究主要是在北海道石笠河的下游流域进行的，这是典型的高度分层流。在1992年、1993年和1994年的夏季进行了大约两个月的观测。观测方法逐年改进，获得了足够的定性和定量数据，实现了上述研究目标。研究结果表明：1)风是影响河流表面盐度的主要因素。2)盐楔的平均夹带系数为：118m^3/sec时E = 1.92 *10^<-6> E ^<0.405^ W>， 146m^3/sec时E =2.24*10^<-6> E ^<0.281^ W> 3)盐楔向河面扩散的过程如下：风吹，产生波浪，上层湍流增加，上层盐分布均匀，界面周围密度梯度增加，盐在界面上的扩散增加，表面盐度增加。4)流动系统的时间平均分布几乎在整个分层流动系统中都是稳定的。(5)河水流量的增加加速了扩散过程，但河水本身的盐度降低。(6)桥墩和河底隆起对界面造成很大的破坏，但这种破坏对盐扩散的影响是局部的。7)为了预测任何地点目前的地表盐度，有沿河道的风速和流量的每小时记录就足够了。少

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