Simulation Study On Sudden Sewage Pipe Damaged Water Pollution Accident In Chongqing Main Section Of Yangtze River Considering Fuzziness

In recent years,sudden water pollution accidents have frequently occurred,which seriously threatened social production and urban residents’domestic water consumption.Due to its special terrain,the“mountain city”Chongqing has various forms of drainage trunk pipes along the Yangtze River and Jialing River in the main urban area.Once the sewage main pipe ruptures,it will cause a sudden water pollution accident of the municipal sewage directly discharged into the Yangtze River and Jialing River,thereby endangering the safe of the water quality at the water intake of the downstream water supply plant.This study builds a mathematical model of the hydrodynamic and water quality in the Chongqing section of the Yangtze River based on fuzzy parameters,and applies the constructed model to take BOD₅、COD_Mn,、NH₃-H、TP as the characteristic pollutants to simulate the water pollution process caused by the fault and analyze the impact of the sudden accident on the water quality of the water intake of the downstream water plant.The research results can provide a reference for the optimization of the emergency plan for the fracture of the sewage main pipe in the main urban area of Chongqing,which is of great significance.The main content and results of this research are:（1）Establish a hydrodynamic and water quality model in the Chongqing section of the Yangtze River,perform sensitivity analysis on model parameters,use sensitive parameters as fuzzy variables,and consider the fuzzy uncertainty of model parameters by combining triangular fuzzy theory andα-cut method.The results show that Changes in parameter values under different memberships will cause varying degrees of uncertainty in the simulation results.Whenα≥0.9,the evaluation indexes NSE and R² of the hydrodynamic simulation in the research river section during the high and low water periods are all greater than 0.85.The water quality indicators BOD₅、COD_Mn,、NH₃-H、TP’s NSE and R²are all greater than 0.90,and the simulation effect is ideal.（2）The best parameters determined by the model parameter sensitivity analysis and uncertainty analysis were used to simulate the hydrodynamics in the Chongqing section of the Yangtze River.The simulation results of river flow velocity and water level under the influence of seasonal flow input,175m experimental impoundment and tributary inflow in the Three Gorges Reservoir area are analyzed,and the daily measured hydrological data of Cuntan Hydrological Station are compared with the simulation results.The consequences show that the NSE and R² of the hydrodynamic simulation in the high and low water periods are greater than 0.95,and the average relative errors are smaller than 1%,announcing that the model and simulation results are reasonable.（3）Applying the established hydrodynamic and water quality coupling models to simulate sudden water pollution accidents in the Chongqing section of the Yangtze River in different hydrological periods,different accident locations,and different working conditions（continuous leakage of pollutants 24h,48h,72h）.Analysing its impact on the water body at the water intake of downstream water plants from the standard time of pollutants at the water intake of the downstream water plant exceeds the standard time for surface water classⅡ、duration exceeding the standard、time for restoration of water quality at contaminated intakes after repair of broken sewage mains and peak pollutant concentration.（4）The water quality simulation consequences show that due to the large amount of upstream water flow and fast flow rate during the high water period,the polluted mass reaches the water intake of the downstream water plant in a short time after the sewage main pipe fracture accident occurs,and it can quickly recover ClassⅡwater after the broken main pipe is repaired;in the low water period,due to the small amount of upstream water and the 175m impoundment of the Three Gorges Reservoir,the flow velocity is slow,resulting in the slow migration of polluting masses.The time of the concentration of pollutants at the water intake of each water supply plant exceeded the standard was later than during the high water period,and it lasted a long time and the water quality recovery time was longer after complete restoration.Comprehensive analysis shows that the impact of sudden water pollution accidents on downstream water bodies is even more serious in the low water period.It is recommended that relevant departments formulate appropriate emergency rescue plans for different hydrological characteristics during high and low water periods.