Study On Pollutant Diffusion Of Typical Secondary River Backwater In Three Gorges Reservoir Area

Since the storage of water in the Three Gorges Reservoir,the return zone of the reservoir area is several hundred kilometers long,and the tributaries of the reservoir area have changed from former natural rivers to reservoir-type rivers.Due to the high water level in the reservoir area,the flow of tributaries has slowed down,and pollutants from upstream have collected in the tributaries,resulting in an increasingly serious pollution trend in the water bodies.Therefore,the study of water quality development trends and prediction of future development in the Three Gorges Reservoir return area has certain ecological construction value..In this paper,we predict the pollutant load in the upstream tributary of Longbao River based on the pollutant dispersion law in the reservoir area,and simulate the pollutant load in temperature and humidity with a generalized model experiment.Stratification,flow velocity,concentration,dispersion state of the bottom sediment under different conditions,based on the selected topography and water level data of the Yangtze River junction.The measured data constructed the DHI MIKE3hydrodynamic module and convective dispersion module to simulate the backwater and Yangtze River confluence zones.pollutant dispersion process.The following main conclusions were drawn.（1）By calculating upstream point source,surface source and internal source pollutant loads,the total upstream pollutant load at the Longbow River confluence area is COD1404.59 t/a,TN80.64 t/a,TP13.44 t/a,NH₃-N50.13 t/a.（2）In this paper,we use a generalized model and MIKE software numerical simulation with upstream pollutant concentration size as the control variable to analyze the high concentration Pollutants diffuse significantly faster than low concentrations of pollutants,and in the probabilistic model high concentrations of pollutants in a single day of COD,TN,TP.NH₃-N attenuated by 0.21d^-1,0.22d^-1,0.2d^-1,and 0.2d^-1,respectively.low concentrations.The pollutants decayed by 0.2d^-1,0.19d-in one day for COD,TN,TP,and NH₃-N,respectively.1,0.18d^-1,0.2d^-1.High concentrations of contaminants also decay faster than low concentrations,while at vertical depths The highest concentrations of contaminants are found in the middle layer,while the lowest concentrations are found in the uppermost layer.（3）The MIKE 3-dimensional model controls the magnitude of the flow rate that changes the hydraulic properties of convective diffusion,with 0.012m/s being a small flow rate.0.035 m/s is the large flow rate,the larger the flow rate the faster the pollutant spreads forward and the longer the location of homogeneous mixing.In the case of upstream pollutants uniformly dispersed,after dispersing to the intersection,the downstream Yangtze River water will lead to the pollutants in the intersection due to the top support.The pollutant concentrations pooled at the mouth bend,where they varied most significantly,with upper>middle>lower layers.A flow rate of 0.012 m/s attenuated COD,TN,TP,NH₃-N at 15 min under a generalized model.attenuated by 0.089m^-1,0.088m^-1,0.145m^-1,and 0.083m^-1,respectively;when A flow rate of 0.035m/s attenuated COD,TN,TP,and NH3-N at 15 min,respectively.0.093 m^-1,0.088 m^-1,0.155 m^-1,and0.091 m^-1.Thus.The greater the flow rate,the greater the attenuation of pollutant concentrations,indicating that turbulence has a greater effect on the degree of attenuation of pollutants,and the deeper the depth of the point at greater flow rates.The less time it takes to detect the concentration value.（4）Using the change in temperature stratification as a control variable,the MIKE 3-D numerical simulation results show that after 20 days the upstream contaminants have been Dispersion to the Yangtze River,but high concentrations of pollutants remain concentrated in the confluence zone.On the one hand,the slow flow and the S-shaped bend in the confluence area result in the concentration of pollutants here,and their dispersal to the Yangtze is very slow.On the other hand,due to the very large volume of water in the Yangtze River,the concentration of pollutants decreases rapidly due to dilution when they enter the river.Under high-temperature stratified conditions,the concentration of pollutants in the lower layer is greater than that in the middle layer,and the minimum concentration occurs in the middle layer,while under low-temperature stratified conditions,the minimum concentration occurs in the lower layer.In the lower layer,the higher the temperature in the upper layer,the faster the diffusion.By varying the temperature variable in the generalized model,it can be seen that the higher the upper layer temperature,the faster the longitudinal diffusion,and the higher the temperature,the faster the diffusion of all kinds of water.Increased microbial activity accelerates the degradation of pollutants and the release of various substances,causing rapid changes in pollutant concentrations and thus enhanced dispersion.（5）The bottom mud has the greatest influence on the vertical concentration of pollutants,and the deeper the water,the higher the concentration.（6）Flow rate is the factor that most affects the dispersion of pollutant concentrations in the backwater zone,followed by concentration values,and finally temperature stratification due to seasonal variations.