Research On Key Issues Of Water Quantity Security And Water Quality Security In The Middle Route Of South-to-North Water Diversion Project

The Middle Route of the South-to-North Water Diversion Project(MR-SNWTP)is a great long-distance water diversion project that solves the problem of uneven spatial distribution of water resources in China.It is currently the world's largest and most complex trans-basin and trans-regional diversion project.Since December 2014,the project was completed and operated,bringing tremendous economic,social,ecological benefits.Due to factors such as engineering structure,geographical environment,climatic conditions,and social activities,MR-SNWTP has been challenged by water security issues with respect to water quantity and quality.First,the total length of MR-SNWTP is 1433km with 1000 bridges.Each of these bridges shows risk of water pollution.However,there are only 11 water quality monitoring stations along the project.It is difficult to accurately and timely detect sudden water pollution incidents.Second,MR-SNWTP traverses many natural rivers,and more than 600 crossing structures have been constructed for flood drainage.However,due to insufficient consideration of actual operation conditions in the design stage,the actual flow capacity of the crossing structures is lower than the designed value.In this case,the flood below the design standard may overflow into the channel,which not only affects water quality but also causes sudden changes in water level influencing the scheduling of water diversion gates.Third,MR-SNWTP crosses three climatic zones.The ice conditions are complex and changeable during the winter,which significantly increases the difficulty of a dispatching operation.Once the operation method is improper,it is easy to cause ice damage such as ice jams and ice dams,endangering the security of water delivery.According to the actual needs of MR-SNWTP,this study focuses on addressing three risk issues by carrying out a systematic in-depth research with the water supply safe abnormal pattern index system and recognition process constructed,and the abnormal pattern database designed,based on the existing monitoring system of MR-SNWTP.The water supply security information platform is designed and implemented to provide the scientific basis and technical support for ensuring the water quantity security and water quality security of MR-SNWTP.The main research content and results are described as follows:(1)The water supply security problems existing in MR-SNWTP are systematically sorted out then focused on the three key issues of water quantity security and water quality security.It is found that the core scientific and technical problems involves:1)how to balance the deployment cost and monitoring efficiency to formulate the placement scheme of the water quality monitoring station for sudden water pollution incidents.2)how to quantify the impacts of multiple factors on flood overtopping risk and evaluate the flood overtopping risk.3)how to quantify the change characteristics of ice jam and regulates corresponding operation method.(2)The cost-benefit analysis method and a multi-objective optimization model for site layout was constructed to address the problems of too few water quality monitoring stations to detect sudden water pollution incidents.The cost-benefit analysis method was adopted to determine the indicators indicating the monitoring efficiency and deployment cost,misreporting rate of detection(MRD)and the shortest time of detection(TD),the number of stations(NS)and the monitoring instrument accuracy(MIA).Based on these,the multi-objective optimization model has revealed a competitive relationship between NS and MRD,NS and TD,enabling to analyze the impact of MIA on the competitive relationship.Results show that MIA has limited effect on the competition between NS and TD.The influence of the competitive relationship between NS and MRD is greater than that.In addition,changes in the costs of water quality monitoring station placement under different control indicators have been analyzed.The optimal placement scheme is determined based on the maximum allowable MRD(1.00%)and the maximum allowable TD(120min).The key position of the placement is analyzed to further verify the rationality of the optimal placement scheme.On this basis,the impacts of uncertainty of pollutant degradation coefficient on accuracy and timeliness are discussed.These results provide theoretical support for the placement of water quality monitoring stations in MR-SNWTP.(3)The flood flow model has been constructed by coupling a flood calculation model,a sediment transport model,and a pipeline overflow model,aiming to cope with the flood overtopping risk caused by the coupling effect of multiple factors on the crossing structures(especially the left bank drainage structures).Based on physical causes and RSA and Sobol global sensitivity analysis methods,the influencing degree of different factors on flood overtopping risk is qualitatively and quantitatively analyzed,as well as the flood overtopping risk under the single factor and multiple factors.Based on these,the early warning threshold has been determined.The results show that sedimentation and blockage have the greatest impact on flood overtopping risk,followed by engineering aging and rainfall changes.If only sedimentation and blockage factor is considered,the return period of flood is quantified as 10 years,20 years,50 years,200 years,and the critical values of sedimentation and blockage depth are 2.00m,1.60m,1.00m,and 0.20m,respectively.Considering the integrated effect of multiple factors such as sedimentation and blockage,engineering aging,and rainfall variations,the threshold range of the critical value of sedimentation and blockage depth is quantified as[1.51m,2.12m],[0.93m,1.69m],[0.19m,1.21m],and[0.00m,0.75m].These results provide references for setting the early warning threshold of flood overtopping risk for MR-SNWTP.(4)For the evolution of ice conditions and the control method of water transportation operation during the ice period,the channel between FenZhuang River Regulator and NanJuMa River Regulator that is most prone to ice jam has been taken as a case study.The ice evolution model is constructed by coupling hydraulic model,thermodynamic model,and ice model.Historical water level and water temperature and ice thickness data are used to verify the model.After that,based on historical operating data to set the typical scenario with different water level,flow velocity,and temperature combination,the variation characteristics of ice jam under different water depth,flow velocity and temperature conditions are examined.The results show that high flow velocity presents when water depth<4.0m,low flow velocity occurs when water depth?4.0m,and ice jams are prone to occur as negative accumulated temperature?-100?.According to the variation characteristics of ice jam,the negative accumulated temperature<-100? is used as the criteria to start the water delivery during the ice period,and the operation method of the ice period water delivery is formulated:water depth<4.0m,maintaining flow velocity 0.2m/s;water depth?4.0m,maintaining the flow velocity of 0.6m/s.These results provide a theoretical basis for reducing the risk of ice jam in MR-SNWTP and serve as a reference for the water delivery scheduling during ice periods.(5)For practical implementation of the findings,the research results for key issues are further expanded,and typical abnormal conditions are summarized as abnormal patterns,including 3 types:water quality(artificial,groundwater infiltration,flood overtopping,and eutrophication),water volume(flood overtopping,gate failure,and water leakage)and glacial periods(ice flowers accumulation,unsteady-ice block accumulation,and nature-ice block accumulation).The normal and abnormal critical value of each indicator and the recognition process are determined,and an abnormal pattern database was designed to provide a unified data standard for further expansion of abnormal patterns.Finally,a water supply security information platform was designed and implemented,supporting the identification of abnormal conditions in actual operation.The information platform of abnormal conditions serves as a powerful tool for effectively guaranteeing the water quantity security and water quality security of MR-SNWTP.