Research On The Algae Bloom And Mass Transport Of Typical Tributary In The Three Gorges Reservoir Under The Interactions Of Mainstream And Tributary

Three Gorges Reservoir(TGR),as an important hub,linking the upstream and middle-lower reaches of the Yangtze River,has an important strategic position and plays a critical role in ecological function,which is the main part to keep and regulate and control the ecological health of the Yangtze River basin.Due to the superposition of various factors such as dam regulation,high intensity basin development activities and climate change,the TGR is a newborn giant "manmade lake" ecological system.The main process of water ecological evolution is not stable,and ecological problems such as eutrophication have not yet been fully manifested,showing high degree of complexity,uncertainty and sensitivity.In this work,a research on the evolution of water environment in the TGR tributary bay is carried out by numerical simulation,to explore two technical problems.Firstly,considering the water parcel exchange between the mainstream and tributary caused by water level fluctuation regulated by the dam is similar to tidal process in the bay,water exchange between the mainstream and tributary is domiant by barocline(density current),the Marine Environment Research and Forecasting Model(MERF)is applied to construct the hydrodynamic Model of the mainstream-tributary interaction and the Empirical Orthogonal Function is used to identify the medium and long term mass transport problems under the oscillating velocity pattern in the tributary bay.Secondly,the key nutrient driving mechanism of phytoplankton in the tributary bay is focused,and the water quality model of the TGR is established based on the detailed observation,the dynamic characteristics of the high randomness and uncertainty of the algal bloom in the tributary bay are presented.As the traditional local sensitivity analysis only focuses on the effects of uncertain inputs around a point,and does not represent the full impact of the uncertainty of a parameter on model output,the quantitative influence of different parameter values of the water quality model on the model output is analyzed by using the Sobol global sensitivity analysis method,in order to adjust model parameters.Firstly based on detailed observation of multi-section and multi-layer,combing with water mass physical process tracer by stable istopes,the water parcels exchange process under the interaction of the mainstream and tributary in the TGR and the velocity distribution in the tributary bay are carefully measured,and the hydrological mechanism of the eutrophication of the tributary is depicted,the regime shift after the impoundment is described.Secondly,MERF model is used to study the hydrodynamic characteristics of a typical tributary of the TGR,and provide the physical parameters,i.e.,elevation,velocity and diffusivity for the the study of water exchange and mass transport.The results show that velocity pattern in the tributary bay has obvious oscillating characteristics,and varies significantly in the spatially and temporally.In view of the characteristics of tributary bay,the Empirical Orthogonal Function analysis method can be adopted to extract the velocity characteristic at reservoir different regulation period,and identify the medium and long term mass transport characteristics.By using numerical diagnosis the method,the influence of different dynamic factors on the tributary bay velocity pattern was quantitatively analyzed,and the barocline(density flow)has an average changes of 6.2 cm/s,which was the primary control factor of the dynamic process,and it was pointed out for the first time that the second dynamic characteristic of the tributary bay would alternately present the characteristics of rivers or lakes.It is proved that the result of the model calculation is compared with the isotope tracer,and under the barocline process,the key problem about the source and composition of the water body in the tributary bay was identified.It turns out that the water parcel of Yangtze River occupies more than 90%of the tributary bay,which is a major part of the water body in the tributary bay.Due to the barocline process under the interaction of mainstream and tributary,water water exchange flux is in the range of 100 m3/s to 500 m3/s,the dry season and impoundment season are greater than discharge season and flood season.The water residence time of the tributary reservoir bay ranges 3 to 5 days,which is much lower than that in the same depth of lake in the world.In addition,under the density current,the tributary bay heat budget has also changed dramatically.the heat budget reveals that tributary bay net heat flux is dominated by the mainstream advective heat,and the average contribute rate is 87%during the whole year,while the tributary advective heat and atmosphere contribute minor to the tributary bay heat budget,which is obviously different from the deep lakes.Finally,the TGR water quality model was established.By using the Sobol global sensitivity analysis method,the total effect of different parameters on the model output and its process changed with time was analyzed.On the basis of the one-way coupling with MERF model,the tributary bay of phytoplankton,eutrophication process,and the phosphorus transport and cycle was be analyzed.The results show that the distribution of phytoplankton in the tributary bay is characterized by its spatio-temporal variation,and it is mainly concentrated in the upstream of the reservoir bay during the discharege season and the concentration of chlorophyll-a is higher.During the flood season,it is concentrated in the midstream and downstream,and the concentration of chlorophyll-a in the reservoir bay is reduced,while during the impoundment season,it is concentrated in the upstream of the bay again.High concentration of chlorophyll-a is always concentrated in the specific velocity range of 0 to 3 cm/s,the position of high chlorophyll-a is not fixed in a specific position in the tributary bay,but moving between upstream and middle stream.In the sensitive season of algae bloom,the reproducing rate of phosphorus caused by the organic detritus is about 34%,which is an important part of the nutrient supply to the algae growth.In the discharge season,the dissolved inorganic phosphorus(DIP)which need by the phytoplankton growth is mainly derived by the Yangtze River intrusion from the surface layer,and the average transport flux is 332 kg/d.While in the flood season and the impoundment season,the source of DIP for the phytoplankton growth is from the Yangtze River water parcel,which flows along the bottom of the bay and enters the upstream of the reservoir bay,then flows out from surface layer again.The average transport flux is 169 kg/d and 193 kg/d respectively.The DIP and organic phosphorus(OC),represented by phytoplankton and detritus,which is transported by tributary into the bay is 1.491 t and 0.069 t respectively,the DIP and OC transported by Yangtze River into the bay is 479.722 t and 14.149 t respectively,while DIP and OC transported by tributary bay into the Yangtze River is 469.2 t and 26.894 t respectively.