Frazil Jam Risk Assessment Research For Main Channels Of The Middle Route Of The South-to-North Water Diversion Project

The Middle Route of the South-to-North Water Diversion Project(MRSNWDP)is the world’s largest long-distance water diversion project,has the characteristics of long distance and crossing the river basin and temperature zone.Channels in the north of Xinxiang,located in the warm temperature zone,have varying degrees of ice conditions every year.And frazil jams are common ice phenomena in rivers in winter may causing frazil jam flood and resulting serious consequences.Therefore,the frazil jam prevention is an important task for the diversion operation of MRSNWDP in winter.The frazil jam risk assessment for water diversion projects has obvious advantages in overall controlling the risk degree,risk spatial distribution and key points of risk prevention and control in canal system,which can provide support for the scientific and complete operation management,and improve the water diversion efficiency and safety in winter.Based on the project characteristics of the main channels of MRSNWDP and frazil jam formation mechanism,this paper firstly identifies the hierarchy structure of frazil jam risk factors,and establishes the risk assessment calculations based on analytic hierarchy process(AHP)and fault tree principle(FTP).Then,taking the canal pool as an assessment unit,the probability and consequence severity of frazil jams induced by each risk factor are scored by experts in design institution,research institution and universities,and the basic risk level of the typical pool is proposed.Further,considering the temperature characteristics along the route caused by the north-south span of the project,a method to correct the basic frazil jam risk value of the pool is proposed by taking the long-distance cold winter space synchronization index,and the comprehensive risk level of the typical pool is obtained.After that,risk factors scaling principle between the typical canal pool and other canal pools is established to realize the comprehensive frazil jam risk assessment of the project contained multiple pools.Finally,based on the analysis results,the controllable risk factors and corresponding suggestions on risk prevention and control are put forward.The results show that:(1)in the past 50 years,the winter temperature along the MRSNWDP has a significant warming trend,which will continue for a period of time in the future.The synchronization probability of strong cold year and strong warm year for eight meteorological stations is about 12%,and about 43% for cold and warm winter synchronization,indicating there is a high probability of winter temperature synchronization along the project.(2)There are 20 risk factors for canal frazil jams;and excessive ice production,high-velocity flow and insufficient ice transport capacity are the indirect factors causing hazards.The possibility of risk factors is positively correlated with the consequence severity;and the scoring results are highly depend on expert work positions.The weight difference of each risk factor is small and there is no obvious main risk factor and all factors need to be considered in the prevention and control.(3)It is recommended that the comprehensive risk value of the typical pool is 16.09,which increases by about 20% compared with the basic risk value,and the comprehensive risk level is at IV level.(4)It is recommended that the comprehensive risk levels of the pools in Shijiazhuang in the main channel of the project are all at III level,and the pools in Baoding are all at IV level,which should be taken as a key risk control area.(5)By comprehensively controlling the controllable factors of the risk factors,and analyzing by AHP,the comprehensive risk level of the typical pool can be reduced to level II in an ideal state.Therefore,it is of great significance to establish a favourable,targeted and scientific water diversion operation management system in freezing period for risk prevention and control.In summary,this study based on the major water diversion project,combining frazil jam formation mechanism and diversion project characteristics,the frazil jam risk factors are indentified,and the FTP risk assessment method is recommended.Considering the influence of the cold winter space synchronization on improving the probability and consequences of frazil jam risk values,a comprehensive risk assessment method is proposed,and the spatial risk distribution characteristics is obtained,and the effective risk control suggestions based on controllable risk factors are put forward.The results theoretically enrich the frazil jam risk assessment system and method of water diversion project,which has direct application value to the winter frazil jam risk prevention and control of the MRSNWDP,and provided popularization and application value for other similar projects.