Study On The Evaluation Of Rain-source River Treatment Schemes In High-density Built-up Areas Based On The Hydrodynamic-water Quality Coupling Model

The process of urbanization and industrialization has destroyed the original river system structure,and the continuous effects of industrial pollution,urban garbage and domestic sewage have led to the emergence of a large number of inferior Type V rain source rivers.The Panxi River is an important first-level tributary of the Jialing River in Chongqing.It is a typical high-density built-up area rain-source type river.Previously,it was also a typical black and smelly river in the urban area of Chongqing,posing a serious threat to the water quality of the Jialing River.In view of the importance of Panxi River’s geographical location,its water environment management work has also attracted the attention of the Chongqing Municipal Government,and a large amount of funds has been invested in the construction of water body restoration and sewage interception pipe projects to eliminate black odor.However,there is still a lack of long-term monitoring and empirical evaluation of the effectiveness of Panxi River’s water environment management,but to grasp the pollution characteristics of such rivers and the law of water environment changes,verify and analyze the theoretical effects of related governance measures,and explore its water environment restoration potential,Has important reference value for the effective management of this type of river.This study takes the Yubei section of the Panxi River as the research object and explores the characteristics of water environment changes driven by internal and external sources based on the monitoring and analysis of water environment indicators.Using the MIKE 21 model as a tool,a hydrodynamic-convection-diffusion model of the studied river section was constructed;on this basis,the constructed model was used to evaluate and simulate the water environment restoration effectiveness of the relevant treatment plan of Panxi River,and The relative contribution of the measures was quantified.The main research conclusions are as follows:（1）The rainfall runoff of the Panxi River Basin has a weak replenishment effect on the river section,and the river bottom and river banks are hardened severely,causing the river to suffer from problems such as lack of ecological base flow,insufficient hydrodynamic force,and lack of river bank ecological barriers,causing the river to face problems The multiple pressures of the input of,surface and endogenous pollutants have caused the deterioration of the overall water environment quality.Through 6-month sampling,detection and analysis,it was found that the water quality of the studied river section was mainly inferior V,and through the CCME WQI method and correlation analysis,it was found that nitrogen and phosphorus nutrients were the main pollutants in the study area;from the time distribution In the rainy season（July to October）,the water quality of the studied river section is worse than that in the non-rainy season（May and June）due to the overflow pollution（CSO）of the confluence pipeline;The water quality in the middle section of the study area is better than that of the beginning and end of the section.Based on the target Class IV water quality standards,the overall water quality indicators are at a"poor"level,and greater improvement and improvement are still needed before the water quality reaches the standards.Under the premise of limited rainfall runoff replenishment,poor hydrodynamic conditions,weak sediment resuspension capacity,and stable water p H,the seasonal algae outbreak in Panxi River may regulate the occurrence of nitrogen and phosphorus pollutants in the sediments and overlying water bodies.The main factors of interface interaction,organic matter,nitrogen and overlying water have strong interaction,while phosphorus tends to deposit stably.（2）Using MIKE 21 software,based on the actual topography of the Panxi River,after extracting the river boundary by the LSV software,a two-dimensional hydrodynamic model（FM）of the Panxi River was constructed,and the model parameters were calibrated and verified by the measured water level data.The hydrodynamic model is coupled with the advection diffusion model（AD）to construct a two-dimensional hydrodynamic-water quality coupling model of the Panxi River research section,and the parameters are completed using the measured TN,NH₄⁺-N,COD,TP and other water quality data Calibration and model validation.The verification results show that the error between the simulated value and the measured value of the FM module is less than 10%,while the AD module is less than 15%,which can more truly reflect the hydrodynamic and water quality change laws of the Yubei section of the Panxi River and meet the requirements of model application.（3）To achieve the water quality of Category IV as the water body restoration goal,use the constructed hydrodynamic-water quality coupling model to simulate the single treatment measures and combined treatment measures such as"source control and interception","ecological water replenishment"and"micro-topography creation".Governance effectiveness,model simulation and water quality evaluation results show that after adopting different single treatment measures,ecological water replenishment measures have the greatest contribution to the improvement of the water environment capacity of water bodies,followed by source control and pollution interception measures,while micro-topography only improves the water of the water body.Dynamic conditions have the least contribution to the improvement of water environment capacity.After combining the three treatment measures,the compliance rate of COD,NH₄⁺-N and TP indicators can reach above 79%,53%～64%and 64%～75%respectively,while the compliance rate of TN is relatively small,only 31%～40%,although the water quality has been better improved,there is still a certain gap from the water quality target,and the larger background concentration of pollutants is still the main factor limiting the further improvement of water quality.（4）In view of the changing characteristics of the water environment of the Panxi River,combined with the simulation conclusions of the MIKE 21 model and the current situation of the surrounding environment of the Panxi River,it is recommended that subsequent treatment measures should be based on the deployment of water purification stations along the route,and strengthen the monitoring of the surrounding overflow sewage.Collect,treat and replenish river channels,adopt low-impact development measures to control initial rainwater runoff pollution,increase ex-situ purification and in-situ ecological treatment of the original river water,strengthen water circulation,and improve hydrodynamic conditions;in deep-water river sections,the original In-place coverage or passivation technology reduces the interaction between sediment pollutants and overlying water pollutants,while in the shallow water hard reach,micro-topography is used to optimize the hydrodynamic conditions of the river and create the river waterscape to realize the water environment management of the Panxi River Long-term governance of goals and water quality.