| The reservoir has become the main source of water supply of most cities with the increasing shortage of quality-based water resource In summer, the reservoir is easy to be thermally stratified over the water depth. After low temperature storm runoff of a high sediment content and high pollution load enters into the reservoir, different types of density currents can be formed. As a result, turbid muddy water exist in different regions of the reservoir, which has brought many difficulties to water plants using water of source reservoirs and has directly influenced water supply security in cities. In this study, a stratified reservoir model designed by ourselves was used to conduct pilot experiments under different conditions of inflow velocity, sediment concentration, and sediment particle size, and the formation and evolution of density currents under different inflow conditions were investigated. The main research results are as follows.Systematic single-factor experiments were conducted under the following conditions: sediment sizes of 0.01 mm and 0.05 mm, sediment concentrations of 1.25、2.5、3.75、5、7.5g/L, inflow velocities of 0.00125、0.0025、0.005、0.0075、0.01m/s, and temperature gradient of 0.30.3℃/m in the thermocline. Under conditions of inflow sediment concentrations being 1.25-2.5g/L and inflow velocity being 0.00125m/s, the density currents were of surface currents. Under conditions of inflow sediment concentrations being 5-7.5g/L and inflow velocities being 0.0075-0.01m/s, the density currents were of undercurrents. Under other inflow conditions, the density currents were of interflow. The types of density currents are affected by the kinetic energy of the inflow and different density between inflow and the background water,which is the result of the joint effects of forces acting on sediment particles, namely,the gravity force(G), buoyancy force(FF), frictional resistance() and viscous shear resistance(). According to the temporal and spatial distributions of sediment concentration and the result of sediment transport captured by a video camera, the thickness of density currents gradually increased to a constant value at a certain moment and then remained unchanged, the velocity of density currents gradually decreased as inflow with high sediment contents entered into the reservoir continuously. When the interflow was formed, the depth of the separation point gradually increased and the height of hydraulic jump gradually decreased with the increase of inflow sediment concentration and inflow velocity.Inflow velocity, sediment concentration and sediment size had a significant impact on the motion characteristics of surface currents, interflow and undercurrents. The effects of sediment concentration on the motion characteristics of surface currents and interflow were similar. With the increase of sediment concentration, the mid-depths and thicknesses of surface currents and interflow were increased, and velocities of surface currents and interflow were decreased. But for undercurrents, with the increase of sediment concentration, the mid-depth of of undercurrents was decreased and thickness of undercurrents was increased. The effects of inflow velocity on the motion characteristics of surface currents and interflow were similar. With the increase of inflow velocity, the mid-depths of surface currents and interflow were increased, and thicknesses of surface currents and interflow were decreased, and velocities of surface currents and some interflow were increased. But for undercurrents, with the increase of inflow velocity, the mid-depth of undercurrents was decreased and thickness of undercurrents was increased. The effects of sediment particle size on the motion characteristics of surface currents and interflow were similar. With the increase of sediment size, the mid-depth of and thickness of surface currents and interflow were increased, and velocity of surface currents were decreased, the mid-depth of undercurrents was decreased and thickness of undercurrents was increased. |
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