The Influence Of Outflow Positions On The Characteristics Of Density Currents And Sediment Distributions

The lack of groundwater resources has led to the reservoirs being main source water for urban city gradually.Storm runoff containing high sediment concentration and heavy pollutant load after the storm in summer or the heavy rainfall in winter will enter the reservoir,which can form density currents under certain conditions.Deep reservoirs will be thermally stratified in summer.Using a self-built stratified model reservoir,the pilot experiments on density currents were carried out under different conditions of inflow sediment diameter,inflow sediment concentration and the outlet position,and the effects of outlet position on the sediment distribution,characteristic parameters and the sedimentation flush ratio were mainly studied.In the weakly-stratified waters,fixing the inflow rate of 0.324m3/h,under inflow sediment concentrations of 1.25g/L、3.75g/L、7.5g/L and outlet positions 38.5cm、18cm 、 4cm above the reservoir bottom,the characteristics of density currents were similar for inflow sediment diameters of 0.01 mm and 0.05 mm,and the differences of sediment distribution,sediment flush ratio and characteristic parameters of density currents were minor.In the weakly-stratified and strongly-stratified waters,the density currents were characterized as interflow.Under fixed inflow sediment concentration,when the outlets were located at the upper,middle and lower positions,the depths of density currents,depths of separation point,heights of hydraulic jump and flow times were of little difference.When the outlet was located at the upper position,the density currents flowed out of the reservoir horizontally as inferflow,and the depth of interflow wasroughly constant.When the outlets were located at the middle and lower positions,the density currents would suck the clear water around the outlet,which made the depth of interflow increased.The distributions of sediment concentration and temperature were compared under weakly-stratified and strongly-stratified waters.Fixing the inflow sediment concentration,when the outlet was located at the upper position,the thermocline did not dive downwards.When the outlets were located at the middle and lower positions,the thermocline dived downward the reservoir bottom,especially for the lower outlet case.When other conditions were fixed,the downward movement of thermocline was more remarkable under weakly-stratified waters.In the weakly-stratified water environments,the sedimentation flush ratio was influenced by both the outlet position and inflow sediment concentration.For the upper outlet,the sediment flush ratio of inflow sediment concentration 1.25 g/L was larger than that of 3.75 g/L.For the middle outlet,there was no difference of sedimentation flush ratio of inflow sediment concentrations of 1.25 g/L and 3.75 g/L.The feature of sediment flush ratio for the lower outlet was opposite to that for the upper outlet.When the inflow sediment concentration was low,the sediment flush ratio for the upper outlet was higher than that for the middle or lower outlet.When the inflow sediment concentration was high,the sediment flush ratio for the middle outlet was higher than that for the upper or lower outlet.In the strongly-stratified water environments,when the outlet position was fixed,the sediment flush ratio was little influenced by the inflow sediment concentration.When the inflow sediment concentration was fixed,the sedimentation flush ratio for the upper outlet was much larger than that for the middle or lower outlet.With the flow of density currents,the sedimentation flush ratio was increased,but the mixing in the reservoir was strong,which was not conducive to sediment flush.