Study On Calculation Of Water Environment Capacity In Man-controlled Slow-lagging Plain Rivers

The main stream of Haihe River runs through the central urban area of Tianjin,which has multiple functions such as flood discharge,water storage and drainage.With the decrease of upstream water supply year by year,the main stream of Haihe River has become a typical slow flow type river channel controlled by gate and pump station.The water body has low self purification capacity,and a large amount of rainfall-runoff flows in during the flood season.The water pollution problem is serious,so the pollution control is urgent.The calculation of water environmental capacity(WEC)is of great significance to pollution control,water environment quality assessment and management scheme formulation.In this paper,the main stream of Haihe River is taken as the research object.According to the different characteristics of runoff in flood period,non-diversion period and diversion period,the WEC of different control units in different periods is calculated,and the low-impact development(LID)plan is proposed.The improvement effect is simulated based on the mathematical model,which provides technical support for alleviating the pollution of rainfall-runoff and improving the environmental quality of the main stream of Haihe River.The main research contents and achievements are as follows:(1)A dynamic WEC calculation method based on hydrodynamic water quality model is proposed to calculate the water environmental capacity of the main stream of Haihe River in flood season.During the flood season,the main stream of Haihe River is affected by the rainfall-runoff,the inflow of pollutants carried by it and the control of the gate.The water level,flow,water quality and other factors change very complex with time.Therefore,this paper first establishes SWMM model to simulate the rainfallrunoff pollution in Tianjin,which is coupled to the hydrodynamic water quality model of the study area in the form of side inflow,and calculates the WEC of different control units in flood season by using the calculation method of dynamic WEC.The main stream of Haihe River is seriously polluted,and TN and TP have little available WEC under the current conditions of each control unit in flood season.(2)In the non-flood period of the water transfer period,the water self purification capacity of the main stream of Haihe River is poor.The traditional one-dimensional WEC calculation model ignores the influence of the mixing process section and the calculated WEC value is too large.Therefore,the improved WEC calculation model based on the non-uniform coefficient is adopted.In the non-water transfer period,because there is no external water transfer and the gate is closed,it can be regarded as a reservoir type river course.Therefore,the WEC of the main stream of Haihe River in the non-water transfer period is calculated by the calculation model of the WEC of the reservoir type river course.The dynamic calculation of WEC except flood season is realized by reasonable method,which provides reference for the evaluation and management of water environment quality in different periods.(3)Based on the sponge city planning of Tianjin,the LID transformation scheme is proposed,and the effect of LID on the improvement of the main water environmental capacity of Haihe River in flood season is simulated.In the SWMM model,we simulated the effect of the four measures on the reduction of rainfall and runoff pollution when they were separately arranged and combined,among which the combined LID measures had the best effect.The WEC of different control units of Haihe River main stream in flood season is calculated under the combined LID scheme.The results show that the WEC of each control unit is significantly improved after the LID measures are deployed,which greatly improves the water environment quality of Haihe River main stream.