| Reservoirs have recently become the source water supplies of many cities in our country,but the source water pollution is becoming increasingly serious,which affects the safety of urban water supply and is not conducive to the urban development.The vertical thermal structure in a deep reservoir changes with seasons,which causes the vertical structure of water density.During spring and summer,strong stratification is formed by the high surface water temperature of reservoir.During autumn and winter,weak stratification is formed by the low surface water temperature of reservoir.The change of the inflow condition will make the sediment flows with various patterns in the reservoir,and will in turen lead to different characteristics of sediment deposition and water pollution in the reservoir.In order to understand the characteristics of density currents and sediment transport under different compositions of sediment size,systematic pilot experiments were conducted in a self-designed stratified reservoir model,and the effects of composition of sediment size and stratification level on the density currents were mainly investigated.Firstly,sediments with diameters of 0.01 mm,0.02 mm,0.05 mm were selected,the inflow sediment concentration of 3.75g/L and the inflow velocity of 0.005m/s were fixed,and the characteristics of density currents and sediment transport were studied under 8 conditions of sediment size.composition.Then,under varied inflow sediment concentrations of 1.25,3.75,7.5g/L and inflow velocities of 0.00125,0.005,0.01m/s,a series of experiments of density currents were performed with the method of single-factor test.The characteristics of density current and sediment transport were explored under different compositions of sediment size in both strong-stratified and weakly-stratified waters.The water jump and separation point of density currents,the thickness of interflow and the flow speed were analyzed qualitatively.the suspended and deposited sediment volumes at different times were also analyzed quantitatively.The rules of sediment movement in the reservoir were studied under different particle size compositions.Both the sediment size composition and stratification level can affect the characteristics of density currents in the reservoir.The density currents were mainly characterized as interflows under strongly-stratified conditions,but they were jointly characterized as interflows and underflows under weakly-stratified conditions.The inflow sediment size composition may influence the depth of the separation point and the thickness of the interflow The larger the inflow sediment size is,the deeper the separation point is,and the thicker the interflow flow is.The sediment distribution and sediment transport by the density currents were also affected by the the sediment size composition and stratification level.In weakly-stratified conditions,when the inflow sediment concentration of 3.75g/L and inflow velocity of 0.005m/s were maintained constant,the volumes ratios of deposited sediment to the total sediment in the reservoir were within the range of 29% ~ 38%,but the volumes ratios of suspended sediment to the total sediment in the reservoir were within the range of 45%~60%.In strongly-stratified conditions,the volumes ratios of deposited sediment to the total sediment in the reservoir were within the range of 20% ~ 28%,but the volumes ratios of suspended sediment to the total sediment in the reservoir were within the range of 60% ~70%.Under similar conditions,the suspended sediment volumes in strongly-stratified waters were larger than those in weakly-stratificated waters;but the deposited sediment volumes in weakly-stratified waters were larger than those in strongly-stratified waters.The larger the inflow sediment size is,the weaker the sediment transport capacity by density currents is,the more serious the reservoir’s deposition is.The smaller the inflow sediment size is,the higher the suspended sediment concentration is,which indicates that the proper water intake point can be set in the deep water and the water treatment cost will be reduced. |
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