Impacts Of The Three Gorges Reservoir Operation On Stratification Structures Of Mainstream And Tributaries

The Three Gorges Project has entered the normal operation period.While exerting enormous social and economic benefits such as flood prevention,power generation,navigation,and water supplement,it also brought about some ecological problems such as algal blooms tributaries of the tributary reservoirs.The differences in the stratified structures between the mainstream and tributaries have resulted in two distinct water ecological environments,and the situation of algal blooms is thus quite different.Based on the CE-QUAL-W2 model,a two-dimensional,laterally averaged coupling model of the mainstream and tributary of the Three Gorges Reservoir?TGR?was developed by combining numerical simulation with field monitoring.The relevant parameters of the model were calibrated by monitoring data,so that the model can accurately reflect the actual situation.The model was used to calculate the water level,flow rate,water temperature,depth of mixing layer and euphotic layer of the mainstream and tributaries of the TGR under different scenarios during different periods?flood period,drawdown period,and impoundment period?.The flow exchange mode between the mainstream and the Xiangxi Bay?XXB?under different operations and its effect on the algal blooms in the XXB were analyzed.The prevention and control tributary algal blooms scheduling plan and guidelines were proposed,which can provide decision support for the relevant departments of the Three Gorges Reservoir Operation Management.The main work and conclusions are as follows:?1?The longitudinal-vertical 2-dimensional model of the TGR coupling mainstream with its tributary?Xiangxi Bay?can relatively well match the water level of main water level stations alongwith the mainstream and the water temperature distribution of the typical sections.It can also reflect the flow exchange pattern and the vertical distribution of water temperature in different regions of Xiangxi Bay.?2?According to the correlation of daily water level fluctuations,the sensitivity of the TGR region was divided into three or two areas.The results showed that during the flood season,Fengjie to the front of the dam section was the most sensitive area,Qingxichang to Fengjie section was the second sensitive area,and Qingxichang to Fengjie section is the third sensitive area.During the impoundment period,Badong to the front of the dam is the most sensitive area,Qingxichang to the Badong section is the second sensitive area,and Cuntan to Qingxichang section is the third sensitive area.During the drawdown period,the range of sensitive areas increased,Changshou to the front of the dam section is the most sensitive area,and Cuntan to Changshou section is the second sensitive area.?3?Reservoir operation can change the flow exchange mode of the XXB,so that the density currents intrusion mode is continuously switched between the surface,middle and bottom layers.The frequency of the conversion is related to the fluctuation of water level in a short time.The larger the fluctuation,the more frequent the conversion.?4?There is a significant correlation between the Z_mix/Z_eu?mixed layer depth/euphotic layer depth?and the concentration of chlorophyll a in the XXB.Therefore,when the Z_mix/Z_eu equals to 2.8,it can be as the threshold and timing of controling algal blooms.When the Z_mix/Z_eu is less than 2.8,the risk of algal bloom is increased,at this moment it is appropriate to start the operation to prevent and control the risk of algal blooms.?5?With the use of the cascade reservoirs in the upper reaches of the Jinsha River,the increase of hydrological stations and the improvement of the accuracy of meteorological and hydrological forecasting,flood control scheduling of the TGR is becoming more flexible so that it is possible to impound in advance at the end of the flood season.Therefore,the feasibility of preventing and controlling tributaries algal blooms through the operation of the TGR in different periods is greatly increased.?6?Aiming at algal blooms of typical tributaries in different period,the regulation criterion of the TGR is suggested to prevent and control the tributaries algal blooms,which can provide the basis for the department of operation and management of the Three Gorges Reservoir.