Numerical Simulation Of Two-dimensional Hydrodynamics And Water Quality Coupling In Lujiang Rive

In recent years,the rapid development of urbanization and industrialization in China has damaged the original water pattern of rivers,and the continuous convergence of agricultural fertilization and irrigation,industrial pollution and urban domestic sewage has caused eutrophication problems of different degrees in many rivers.The Lujiang River in Honghe Prefecture originates from Shiping Baoxiu,as an important tributary of the upper reaches of the Nanpanjiang River,the nitrogen and phosphorus content in the water body seriously exceeds the standard and has reached the standard of poor V water in surface water,which is a typical eutrophic river.This paper establishes a two-dimensional hydrodynamicwater quality coupling model of Lujiang River with the help of MIKE21,and on the basis of reasonable model verification,the response relationship between environmental factors(temperature,riverbed roughness,wind speed,dissolved oxygen concentration)and TN and TP concentrations in the water body of Lujiang River is studied,and the correlation between each environmental factor and TN and TP concentrations in the water body of Lujiang River is analyzed with the help of SPSS software.Finally,the water quality improvement plan was proposed for the water quality condition of Lujiang River,and the effect of each water quality improvement plan was evaluated by fuzzy integrated index method and integrated nutrient status index method,and the main conclusions are as follows:(1)Response relationship between environmental factors and TN and TP concentrations in water bodies of Lujiang River: temperature in the range of 20℃-30℃ is conducive to the degradation of nitrogen and phosphorus in water bodies,and there is a significant response relationship between temperature and TN and TP concentrations in water bodies of Lujiang River;there is a significant response relationship between riverbed roughness and TN and TP concentrations in water bodies of Lujiang River;there is no significant response relationship between wind speed and TN and TP concentrations in water bodies There is no significant relationship between wind speed and TN and TP concentrations in water bodies;there is a significant relationship between DO concentration and TN and TP concentrations in water bodies;(2)Correlation analysis between environmental factors and TN and TP concentrations in the water column of Lujiang River: temperature has a significant correlation with TN and TP concentrations in the water column of Lujiang River at the level of 0.01(two-tailed);riverbed roughness has a significant correlation with TN concentrations in the water column of Lujiang River at the level of 0.01(two-tailed),and a significant correlation with TP concentrations in the water column of Lujiang River at the level of 0.05(two-tailed).There is no significant correlation between wind speed and TN and TP concentrations in Lujiang River water bodies;there is a significant correlation between dissolved oxygen and TN and TP concentrations in Lujiang River water bodies at the 0.01 level(two-tailed);the correlation between environmental factors and TN and TP concentrations in Lujiang River water bodies is from the largest to the smallest: temperature > dissolved oxygen > riverbed roughness >wind speed.(3)Based on the two-dimensional hydrodynamic-water quality coupling model constructed for the Lujiang River,source control and pollution interception treatments were carried out at several major outfalls along the river,and the results showed that the water quality at all cross-sections was greatly improved,with NH3-N concentration reduced by28.55% on average,improving the water quality level by one compared with that before water replenishment,and reaching Class II water standard;COD concentration reduced by17.84% on average,improving the water quality level by one compared with that before water replenishment,and reaching Class III water standard.The average reduction of TN concentration is 52.74%,and the concentration of TN at each section is still poor V water;the average reduction of TP concentration is 27.28%,and the concentration of TP at S3 and S4 sections reaches V water standard,and the concentration of TP at S1 and S2 sections reaches poor V water standard.(4)Based on the two-dimensional hydrodynamic-water quality coupling model constructed for the Lujiang River,seven different ecological water replenishment schemes were developed for numerical simulation analysis based on the source control and pollution interception,with the replenishment amount and replenishment method as the control factors.The average reduction of NH3-N concentration in S1,S2,S3 and S4 reaches the standard of Class II water;the average reduction of COD concentration in S1 reaches the standard of Class II water,the average reduction of COD concentration in S2,S3 and S4 reaches the standard of Class III water;the average reduction of TN concentration in S1 reaches the standard of Class IV water,the average reduction of TN in S2,S3 and S4 reaches the standard of Class IV water;the average reduction of COD concentration in S2,S3 and S4 reaches the standard of Class III water,S3,S4 section TN concentration is still in poor V water standard;S4 section TP concentration reaches V water standard,S1,S2,S3 section TP concentration reaches IV water standard.The improvement effect of each water quality index is:TP>TN>NH3-N>COD.(5)This paper uses fuzzy comprehensive evaluation method and comprehensive nutrient state index method to evaluate the effect of water quality after recharge,the results show that: different recharge schemes have different effects on the water quality of Lujiang River,the closest S1 section to the recharge outlet has the most obvious water quality improvement effect,while the farthest S4 section has the worst water quality improvement effect;under the regulation of water quantity,the scheme of large recharge is better than small recharge;under the regulation of recharge Under the regulation of water quantity,the uniform water replenishment scheme is better than the centralized water replenishment scheme;the S1 section reaches the regional water function IV water standard under scheme2,scheme 3,scheme 4 and scheme 5;all the schemes in this study cannot make the S2,S3 and S4 sections reach the regional water function IV water standard;the S1 section reaches the moderate eutrophication standard under scheme 2,scheme 3,scheme 4 and scheme 5,and the S2,S3 and S4 sections still belong to the heavy eutrophication standard.S2,S3,S4 sections are still heavily eutrophic.