Current Field, Residence Time And Sources Of Saltwater Intrusion At The Water Intake Of Qingcaosha Reservoir

This thesis studied the current field, residence time and sources of saltwater intrusion at the water intake of Qingcaosha reservoir in the Changjiang River Estuary by field observation, numerical model and dynamic analysis. The main results are as follows.1. Using a base observation system, water level elevation, current profiles, salinity, water temperature, turbidity and waves in the Qingcaosha reservoir was examined. The observation was performed for a month in winter and summer respectively, and abundant first-hand hydrological data were obtained. The observations shown that the current speed is quite small on the whole. The velocity in the waterway on the north side of Qingcaosha island is more sensitive to wind stress. The velocity on the south side of Qingcaosha island is stronger than that on the north, and has a rapid increase when the upstream water intake was operated. Water from the upstream intake flows mainly through the waterway on the south of Qingcaosha island to the downstream water outlet. The current speed on the east side of the reservoir is smaller than that on the west side and there is no obvious influence of the operation of upstream water gate.2. The three dimension, high resolution numerical model of Qingcaosha reservoir and Changjiang River Estuary were built up, respectively. Compared with the observation data of water elevation, current speed and direction, salinity, the two models were validated. The validation results are fairly good.3. Using the numerical model of Qingcaosha reservoir, the structure of current field and its formation mechanism was studied. The inlet and outlet flow under constant water intake and water output, wind-driven current under prevailing wind in the summer and winter, and the total current field under their coexistence were simulated.The results of model calculation indicate that the inlet and outlet flow in steady state is driven by the inertia of water intake and output flux. In the western part of the reservoir the velocity and water flux on the south side of Qingcaosha island is larger than that on the north side. The current flow mainly through the south part in the middle and eastern part of the reservoir, and is quite small in the middle and north part.Driven only by the wind, wind stress drives surface water flow, water accumulation in downwind area, results in water level dropped in upwind area and rise in downwind area, forming a reverse water level gradient force, driving bottom water flow from the downwind area to upwind area. Thus, wind-driven current at the surface and the bottom is reverse, and actually, the bottom flow is a compensation current. Current field structure meets the conservation and continuity of fluid motion.Under the coexistence of inlet and outlet flow and wind-driven current, the total current field is almost same as the one driven only by wind stress, except in the two small areas in northwest part of the reservoir and near the water outlet. This indicates that the total current in the reservoir is controlled mainly by the wind-driven current, the inlet and outlet flow is relatively small.4. Using3-D numerical model of Qingcaosha reservoir, the residence time was simulated and analyzed. Due to the large area of the reservoir, residence times are differences in space. We divided the reservoir into6regions to investigate the residence time of the water body, and design5different numerical experiments to calculate and analysis the residence time.The numerical calculation results shown that, under summer normal circumstances (southeast wind5m/s, two tidal water intake and output, water supply5million tons/day, freedom water level), there are bigger differences of water residence time among different regions. The residence time in the south central part is the longest up to more than30days, while in southwest part is only about6.25days. The average residence time of the surface, middle and bottom of the reservoir are25.21,25.21and25.33days, respectively. In the no wind case, it is in favor of water replace. Compared to southeast wind (prevailing wind in summer), the northeast wind is unfavor of water exchange. The increase of water supply shortens significantly the residence time under the conditions of summer prevailing wind. Low water level operation reduces the upstream water intake flux and downstream water output flux, weakens the hydrodynamic, results in the residence time becomes longer, and is negative to reservoir’s water exchange.5. Applied the modified numerical3-D model of saltwater intrusion in the Changjiang River Estuary, source of saltwater intrusion at the water intake of Qingcaosha reservoir under normal dynamic condition was analyzed. The calculated results shown that under normal dynamic condition the saltwater intrusion resource at the water intake of Qingcaosha reservoir from the saltwater-spill-over from the North Branch into the South Branch (SSO) accounts for69.5%,89.3%,98.5%and99.5%during moderate tide following neap tide, spring tide, moderate tide following neap tide and neap tide at surface, respectively, while accounts for34.9%,88.9%,98.5%,99.5%at bottom. The source of the saltwater intrusion at surface and bottom layer is mainly from the SSO, and is nearly all from the SSO especially during moderate tide following spring and neap tide, besides the bottom one is mainly from the downstream open sea (accounts for65.1%) during moderate tide following neap tide.