Research On Three Gorges Reservoir Operation For Non-flood Season Based On Controlling Algal Blooms In Near-dam Tributaries

Since the first impoundment of Three Gorges Reservoir(TGR), eutrophication aggravated and the obvious deteriorative hydrodynamic conditions caused algal blooms occurring frequently in the near-dam tributaries. As the environment and nutrient in the main factors influencing algal growth can’t be changed in the short time, to improve the hydrodynamic condition through reservoir operation is an important way to reduce the algal blooms. Combined the observed data and numerical simulation, this paper takes the mainstream of TGR and Xiangxi River(XXR) as the study area, studies the exchange characteristics of hydrodynamics and water quality between the mainstream and the tributary. The influence mechanism of thermal stratified flow on the hydrodynamic and algal transport process is revealed, and the reservoir operations aiming at reducing the algal bloom level are pointed out. According to the time-scale of operation, the optimal operation rules of TGR for non-flood season based on restraining algal blooms in near-dam tributaries are proposed in long-term and sudden situation. And the operation possibility for long-term operation and the emergency operation space are also given in different typical years.The major study contents and conclusions are as follows:Firstly, a vertical two-dimensional eutrophication model of the mainstream of TGR and XXR is developed, based on CE-QUAL-W2 with the formulation of water velocity and algal production modified in it. Model calibration is carried out utilizing the observed data, and through the analysis of exchange characteristics between the two branches in detail for the calibration scenario, a conclusion is obtained preliminarily that thermal stratified flow and reservoir operation are the two main factors influencing the process of algal blooms in XXR.Secondly, based on the theories of thermal stratified flow and density current, five characteristic values are proposed to quantify the formation and development processes of the thermal stratified flow under the different initial water temperature difference between the two branches. The complex stratified flow is decomposed into six simple types, the algal transport processes under single-type stratified flow and multi-type stratified flow are studied, and the influence of tributary inflow on algal bloom level combined with the single-type thermal stratified flow is also obtained. The results reflect that:(1) The plunging quantity, plunging distance and plunging depth are positively correlate with the initial relative density of the two branches.(2) An important reason for frequent occurrence of algal blooms in non-flood season for XXR is that several adverse multi-type stratified flows suffering continuously causes the algae transporting along the river back and forth in the back-water zone, so the algae get plenty time for growing.(3) Increasing the tributary inflow is effective for reducing the algal blooms on the whole, but limited for the period when algal blooms occur frequently in non-flood season in XXR, thus to operate the plunging flow for improving the hydrodynamic condition is necessary to restrain the algal blooms.Thirdly, the operation solutions of TGR for non-flood season is generalized into three types based on the actual operation schedules: short-term operation, medium-term operation and long-term operation. The influence mechanism of different daily discharge process, water level process and operation opportunity on the algal bloom process is studies. The results show that, peak-load regulation, increasing peak-load difference under the same valley-load flow, low valley-load flow and high peak-load flow, increasing the valley-load flow under the same peak-load difference, water storage process and increasing the daily water level rising can accelerate the anti-clock transport process of algae and decrease the algal production, and reduce the level of algal blooms effectively. Based on the principle of using the shortest operation time for the best effect, starting the peak-load regulation from the algae recovery and ending it after the medium period of algae increasing, starting the water storage at the end of algae recovery and ending it after the peak period of algae increasing, carrying out water discharge after the peak period of algal bloom are the beneficial operation opportunity.Fourthly, the optimal operation study of TGR for non-flood season based on restraining the algal blooms in near-dam tributaries is carried out from aspects of long-term and emergency according to the time-scale of operations. The results show:(1) Based on the normal electricity operation schedule of TGR, an operation rule of water storage during the period of March to May when the algal blooms occur frequently, according to the up-stream runoff and under the requirements of minimum discharge flow and water level decreasing without water abandoned is proposed.(2) A practical case verifies that the optimal operation could achieve the double goals of improving water quality and increasing electricity generation in non-flood season.(3) The possibility of optimal operation in different typical years is analyzed according to the historical runoff data. The possibility is highest in relative-wet year when March-May is the operation period, and in wet year when April-May is the operation period. So, the year when the runoff is abundant and distributed equably has a large optimal operation possibility.(4) Aiming at restraining the sudden algal blooms in XXR, the rules of early warning of algal bloom and management strategy are obtained. Considering the runoff process, economic operation of hydro-power station, maximum discharge ability of units, and daily fluctuation of water level synthetically, a method of calculating the emergency operation space is proposed and the simulation case indicates that the relative-wet year in which the runoff distributed equably has the largest emergency operation space.