Study On The Motion Characteristics Of Reverse Density Currents In Xiangxi Bay Of The Three-gorge Reservoir

After impounding, water environment of the total Three Gorges Reservoir (TGR) has been significantly changed due to dramatically increased water level, lower flow velocity, poorer diffusion ability and longer retention time in the tributaries; these problems have been a matter of social concern. More and more scholars realized that dramatically decreased flow velocity was the key factor driving eutrophication and algal boom problems of the TGR tributaries. Firstly, low flow velocity in tributaries reduced transport ability of the flows and strengthens the biomass H0accumulation directly. Secondly, quite low flow velocity in tributaries prolonged water retention time, enriched nutrients and increased H1light transmittance which indirectly accelerated algal growth. In addition, it is noteworthy that research indicated the tributaries were strongly impacted by reverse intrusion of the TGR arm water with high nitrogen and phosphorus concentration. Therefore, a hydrodynamic characteristic of the tributaries is now a primary goal to study water environment and eutrophication problems of the tributaries.In this paper, Xiangxi Bay (XXB) is chosen as the study area. Based on field data in 2008, the basic hydrodynamic characteristics is analyzed, then a physical model is established to study plunging conditions and motion characteristics of the reverse density currents in XXB. Major works and conclusions are as follows:1. Based on the field data of XXB in 2008, a series of flow velocity, water temperature, density, water level, temperature difference, flow and hydrodynamic parameters were analyzed, the results showed that hydrodynamics of the XXB could be generalized as a bidirectional flow but a one-dimensional flow. The inflow from upstream entered into XXB as a underflow all the time, while the TGR arm water intruded into XXB as reverse density currents most of the time. Underflows occurred in February and March, and then interflows occurred in April to September. Water temperature is the primary contributor to density differences driving reverse density flows, whereas sediment is the secondary in the creation of these flows.2. A series of bottom, mid-depth and surface reserve density current experiments were carried out in laboratory. Initial water level, total inflow and initial temperature difference are the basic variables. Developing processes of the H2correlated hydrodynamic parameters, such as density current front velocity, thickness, height of the front interface, velocity in plunging point are analyzed, the results are as follows: (1) Velocity of density current front during the processes of formation and developing is decreased approximately linearly along H3longitudinal direction; while in the same section it will increase as initial water level, total inflow or initial temperature difference increases;(2) During the propagation process, the height of the front interface of the underflows and overflows will increase linearly and slowly; while in the same section it will increase as the initial water level and total inflow increases; the height of the upper front interface of the interflows are nearly fixed, while in he same section it will increase as initial water level increases.(3) The thickness of density current front decreases linearly along H4longitudinal direction, and more sensitive to the initial water level.3. Comparing the motion of density currents in laboratory with field monitoring results, some conclusions could be obtained:(1) When the reserve density current tends to stable, its interface shape after the highest point is nearly fixed, distribution of interface height of the underflow show opposite tendency with slope gradient of the flume, and the interface slope is lower. Interface height of the overflow is in accord with slope gradient of the flume, similarly, the interface slope is lower.(2) Fr0 2 of the density currents in laboratory are relatively concentrated with an average of about 0.54; the plunging thickness usually occupy half of the entire water depth, and the stable thickness of the density currents decreased to about 0.4 times.(3) To some extent, plunging velocity increases as the initial water level, total inflow and initial temperature difference increases; and density current thickness is positively correlated with the initial water level and total inflow, but negative correlation with the initial temperature difference.(4) Velocity of density current front is negatively related to the density difference and the coefficient of drag, and it is positively related to the initial water level and the total inflow.(5) In general, the hydrodynamic characteristics of the density currents in the experiments are accord with the field monitoring, both show significant positive correlation with the water level and the total flow, and significant negative correlation with the density difference between intruding water and environmental water.