Simulation Study On The Impact Of South-to-North Water Diversion Middle Route Replenishment On The Water Environment In Bai River

Ecological replenishment is a comprehensive approach to water resources allocation,which has the function of restoring the ecosystem function of the receiving region and achieving a dynamic ecological balance in the receiving area.Affected by complex topographical conditions,climate and human interference,the Bai River has been suffering from the effects of water pollution,with some sections of the river not meeting water quality standards for a long time.In recent years,the South-to-North Water Diversion Middle Route Project has carried out ecological replenishment of the Bai River several times,effectively alleviating the problem of water shortage and greatly improving the quality of the water environment of the Bai River.Based on Mike21 simulation software,a coupled hydrodynamic-water quality model of the Bai River was constructed to simulate the hydrodynamics and water quality of different recharge scenarios in order to analyse the evolution characteristics of the Bai River water quality and optimise the recharge scheme.The research is significant for revealing the mechanism of ecological recharge on the river and providing theoretical basis and data support for the improvement of the water environment and ecological restoration of the Bai River,which has essential theoretical value and realistic significance.Based on the systematic analysis of the current situation of water resources and water environment in the Bai River,a coupled hydrodynamic-water quality model of the Baihe River according to Mike21 software was constructed for the hydrology,meteorology,topography and water environment of the Bai River.By using the constructed hydrodynamic model,the temporal and spatial evolution of the flow,velocity and water level of the Bai River under different scenarios are simulated to analyse the characteristics of the impact of ecological recharge on the hydrodynamics of the river.The water quality model is developed to simulate the evolution of dissolved oxygen(DO),permanganate index(CODMn),chemical oxygen demand(COD),ammonia nitrogen(NH3-N)and total phosphorus(TP)in different scenarios to investigate the mechanism of ecological replenishment on the Bai River water environment.The results show that the flood season tends to have inferior water quality due to the influence of pollutant pooling from multiple sources.The hydrodynamic conditions are optimal during the high water period,followed by the flat water year and the worst in the dry water year.The hydrodynamic conditions are favourable for all ecological recharge scenarios.Scenario 4 has the most significant effect on river water quality,with rivers improving from Class IV to Class III water quality standards,COD and TP indicators improving to Class II water standards,and DO having the greatest improvement rate of 94.67%.The DO indicator has the highest improvement rate of94.67%,and the TP indicator has the greatest reduction rate of 66.67%.