Prediction And Control Strategy Of Aquaticenvironment Qaulity For Yulin River Basin Under Dynamic Operations Using A SWMM-EFDC Coupled Model

“Making all-out efforts to protect Yangtze River and forbidding large-scale development of it”has been put high on the agenda in developing the Yangtze River economic belt as Chinese president Xi stressed the importance of environmental protection.Yulin River,the primary tributary of Yangtze River,is pivotal in the security of water ecology in Yangtze River basin.Currently,the total water quality of Yulin River is in good condition,assessed as class Ⅱ-Ⅲ stably during whole year.Whereas,as the construction of Liangjiang New Area,Chongqing and water conservancy facilities over Yulin River has escalated,so too has the deterioration of water environment due to regional soil loss,tributary water quality aggravation and hydrodynamic condition alteration,demanding researches on the water environment security of Yulin River basin.However,conducting research on water environment in the scale of basin causes much difficulty if solely relies on experiments or in-situ monitoring,on account of broad space,sophistication of material migration and transformation,and entanglement of impact factors.Ergo,applying coupling mathematic models consisted of hydrological,hydrodynamic and water quality functions has been the major method in studying such issue.Based on coupled SWMM and EFDC to build basin water environment model,the main results this study has obtained include:1.Rainfall-runoff-pollutant model using SWMM was constructed.Through analysis on spatial and temporal distribution of runoff and area-source pollution,it was found that the distribution had correlations with precipitation and land uses,resulting in pollution accumulating in the southwest of research area,of which the main land use is construction.Thence quantified the relation between pollution load generation,migration and uptake of receiving water body,among which the annual pollution load of COD,TN,NH₃-N and TP is 3595.95t,207.29t,56.83t and 16.30t respectively.2.Computed results from SWMM were input as partial boundary conditions to develop the hydrodynamic and water quality model in EFDC.The spatial-temporal variation of hydrodynamic and matter fields was studied.According to the results,average velocities of the upstream section（from Meixi to Shujia）are higher than those of the downstream section（from Shuajia to Eco-adjustment dam）by reason of bathymetry and overburden of the riverbed.Besides,the biggest velocity disparity between these two sections occurred in normal period,since,during which the Three Gorges Reservoir schedues a transition period from the beginning to the stationary phase of impoundment or from the beginning to the stationary phase of water release.In terms of water quality,COD_Mn,TN,NH₃-N,TP all reached its highest in rainy season both in two sections,rising to 4.75,3.05,0.668,0.118 mg/L（upstream section）and 4.35,2.95,0.617,0.139 mg/L（downstream section）.Also,the distribution of water quality was signifigantly influenced by the types of land uses through which it flows.Wangjiazui–Zhangjiawan was the section where highest concentrations of water quality were observed,in which COD_Mn,TN,NH₃-N and TP amounted to 6.13,2.75,0.620 and 0.122 mg/L.3.Based on coupling those two models,impacts of the eco-adjustment dam on hydrodynamics and water quality in Yulin River was analyzed The simulation results show that:after operation of the dam,water quality got worse as the flow velocity of Yulin River greatly decreased（14%～49%lowering in upstream and above 35%drop in downstream）and an obvious water quality segmentation was observed in the dry season on account of the sluggish flow.4.Setting operation of eco-adjustment dam as precondition,further simulation research of impacts that the proposed Beikou Reservoir construction in upstream on Yulin River will make was made.The results illustrate that:concentrations of water quality saw an uptrend in normal and wet season,while within only 15%increase.A downtrend displayed in dry period on account of the great amount of ecological basic flow.5.According to the pollution control measures to be implemented,the improvement of water quality in Yulin River was forecasted.The simulation outcome presents that pollution restraints notably ameliorate the water quality,capable of maintaining Class Ⅱ for whole year,but part of reaches declined to class Ⅲ in wet season,demanding dynamic scheduling the upstream and downstream water conservancy facilities to improve its hydrodynamic condition as to better water quality.