Experimental Study On The Mechanism Of Saltwater Intrusion In Estuary With Runoff And Tide

Estuaries are the confluence of rivers and oceans.Saltwater intrusion is a natural disaster in the estuary area,and there has been an increasing trend of saltwater intrusion in our country in recent years.Therefore,it is of great significance to study the problem of saltwater intrusion in estuary area.Runoff and tide are the main factors affecting saltwater intrusion.In the past,there was a lack of systematic study on the mechanism of saltwater intrusion with runoff and tide.So this paper studied the mechanism and law of saltwater intrusion under runoff and tide with the flume experiment,and the focus of this study was the speed and intrusion of saltwater intrusion.In the experiment,the runoff was simulated by the fresh water flow,and the tide level was simulated by the sinusoidal saltwater level.The salt water intrusion was studied by four conditions,which were no runoff and no tide,runoff and no tide,no runoff and tide,runoff and tide.No runoff and no tide experiment was the most basic experiment in this paper,also called the lock-exchange experiment,which was used to study the long distance saltwater intrusion only under density gradient force.It showed that: Saltwater would continue to intrude,the saltwater velocity increased with the increase of density gradient force caused by the density difference between salt and fresh water.The velocity of saltwater head decreased with the increase of the saltwater displacement,and the relationship between saltwater head displacement and the time was in good agreement with the quadratic function.By using the formula of fluid mechanics and making appropriate assumptions,the theoretical function expressions of saltwater displacement and saltwater head velocity were deduced,which were in good agreement with the experimental results,and the parameters of these formula were determined.Runoff and no tide experiment,based on the results of no runoff and no tide experiment,was used to study the saltwater intrusion only under runoff.It showed that: Saltwater intrusion would stabilize at some position.The increase of runoff would weaken saltwater intrusion.Based on the theoretical analysis and experimental data,the formula for calculating the stable saltwater intrusion distance was obtained,which could be used to explain the experimental phenomena,and could also be explained that the inhibition effect of runoff on saltwater intrusion was enhanced with the increase of saltwater intrusion distance.The process of saltwater intrusion was approximately consistent with the quadratic function,and the calculation method of quadratic function was obtained.No runoff and tide experiment,based on the results of no runoff and no tide experiment,was used to study the saltwater intrusion only under tide.It showed that: Saltwater would continue to intrude.The overall saltwater intrusion velocity was uniform,decreased with the increase of tidal range in general.And the formula for calculating the saltwater intrusion velocity was obtained,which could reflect the experimental phenomena.The density gradient force caused the saltwater flood period to be significantly longer than the ebb tide.Runoff and tide experiment,based on the results of runoff and no tide experiment and no runoff and tide experiment,was used to study the saltwater intrusion under runoff and tide.It showed that:The saltwater intrusion reached steady required a smaller tidal range with the larger runoff,saltwater intruded faster under smaller tidal range,and the saltwater intrusion length decreased with the increase of runoff.The runoff caused the saltwater flood period to be shorter than the ebb tide during the saltwater intrusion stabilization.When the tidal range was very small,the saltwater intrusion length could be longer than the intrusion length under no tide,and this experimental phenomena was explained from the point of view of the average water level in half tide cycle.When the tidal range was larger,the saltwater intrusion length would decrease with the increase of tidal range,we considered that the reason was the tidal range enhancing the mixing of salt and fresh water,which decreased the density gradient force and increased the inhibition of runoff on saltwater.The process of saltwater intrusion was also similar to the quadratic function,and the calculation method of initial velocity of saltwater intrusion was obtained.