Study Of The Water Quality Impact Of Connection Between Jiuli River’s Trunk And Tributary Based On IWIND-LR Model

The plain river network area has developed social economy,but its urban water environment is under great pressure.With the development of the city and the increase of human intervention,many trunks and tributaries are blocked by sluices and pump stations,the connectivity of the river channel becomes poor and the connectivity of the water body is blocked.As the displacement becomes slow,the pollution accumulation of tributaries is easy to occur,and the shock load is brought in the case of rainfall.The study area is located in the Taihu Basin network area.The lower reaches of Jiuli River main stream in Wuxi City are selected.It is about 3km from east to west and it is located in the upper reaches of wanshandang.The terrain is flat and the velocity is slow.There are four tributaries flowing in,and most of them are over standard of nitrogen and phosphorus.Three of them are separated by sluice,pump station and wood block.In this study,aiming at the possible pollution accumulation of tributaries under the regulation of sluice pump station in plain river network area,the water quality model is constructed,and the relevant non-point source pollution input is calculated by SCS runoff curve method.The model which is verified and calibrated is used to simulate the connection between tributaries and main streams under different water conservancy conditions,and the comprehensive analysis is carried out to improve the connection between the main and tributaries of regional river network The feasibility of reducing N,P and other accumulated pollution can provide theoretical support for optimizing water system structure in plain water network area and formulating scientific and reasonable water conservancy project operation plan.The main research results are as follows:(1)The concentration of nitrogen and phosphorus in the main and tributaries are significantly different.The average concentration(mg/L)of total nitrogen in seven sampling of S1,S4,S6 and S8 in the main stream was 2.72,2.46,2.59 and 1.86 respectively,while that in S3,S5 and S7 in the tributary was 3.97,3.47 and 4.46.The average concentration(mg/L)of total phosphorus in seven sampling of S1,S4,S6 and S8 in the main stream is 0.17,0.14,0.14 and 0.16 respectively,while that in S3,S5 and S7 in the tributary is 0.20,0.22 and 0.29 respectively.In the event of rainfall,the concentration of TN and TP in the tributary increased obviously,while the change of the main stream was small,which may be due to the poor flow and small flow velocity of the tributary,and a large number of non-point source pollution was collected under the condition of strong rainfall.(2)The IWIND-LR water quality model was constructed.The average relative errors of total nitrogen and total phosphorus in model parameter calibration were 18.5%and 11.0%respectively,and the average relative errors of model validation were 17.9%and 13.4%respectively.After calibration and verification,the model can accurately reflect the hydrodynamic and water quality process of the study area.(3)When the inflow flow of the tributary is smaller than that of the main stream and the concentration of the tributary is relatively low,the tributary connection has little impact on the water quality of the main stream at ordinary times,but it can alleviate the impact load generated at this time to a certain extent under the rainfall scenario.The maximum concentrations of TN and TP decreased by 10.3%and 5.1%respectively.The maximum concentration of total nitrogen at the point near the tributary can be reduced by 19.0%.(4)The impact of different concentrations of tributaries connecting with the main stream is mainly manifested in that,with the increase of total nitrogen and phosphorus concentration in the tributary,the nutrient concentration of the main stream will be increased.Compared with the disconnected condition,the role of relieving the impact load of rainfall runoff input will be smaller,and sometimes the concentration will even increase on the contrary.Therefore,it is necessary to strictly control the water quality of the tributaries when the connection is considered.(5)The impact of different flow of tributaries connecting with the main stream is mainly manifested in that,when the input concentration of the tributary is lower than that of the main stream,concentration of the main stream is basically unchanged in most of the time because of the change of tributaries’flow.However,when the main flow is small and the rainfall is large,the runoff pollution caused by rainfall can still be effectively released.(6)When the main stream flow is small or during the heavy rain,the tributary with better water quality can effectively reduce the pollution load of the main stream,and with the increase of the tributary flow,the effect of water quality improvement is more obvious.With the increase of the main stream flow,the impact load caused by the pollution of the tributary can be effectively alleviated by the main stream water transfer.While the water transfer increases the main stream,the water quality of water diversion should also be fully considered when flowing.(7)The river channel restoration of the tributary S5 Xuchongqiaogang and the scheme of increasing the flow of the main stream are carried out at the same time.The simulation results show that the maximum concentration of TN and TP can be reduced by 42.4%and 11.1%respectively when the main stream discharge is small,especially during rainfall.On the basis of the treatment and restoration of the tributaries,connecting them in the dry season and increasing the flow of the main stream with appropriate water regulation can better alleviate the impact load of the pollution of the tributaries,which provides a reference for measuring the environmental benefits of the connection of the tributaries and the corresponding management in the actual work.