Analysis And Simulation Of The Formation Of Deformed River Bends In The Wandering Reach Of The Lower Yellow River

In the practice of Yellow River control,the unstable river regime is called unfavorable river regime,and some unfavorable river regime gradually develops into"Deformed river bends"in the evolution process,which brings serious harm to the Yellow River beach area with the dual attributes of flood and sediment transport and production and life.The shape of the deformed river bends distorts out of the river planning control line,and the evolutionary trend of the river bend is difficult to foresee when it meets floods.If not controlled,the flood may even impact the Yellow River levee.In view of the complexity of the evolution process of the deformed band,the existing research lacks systematic continuous observation and in-depth understanding of the deformed band.Based on extensive data analysis,theoretical analysis,model test and other technical means are used to analyze the damage and control of the malformed bays in the wandering reaches of the lower Yellow River,and carries out research work from three aspects:the spatio-temporal distribution of the deformed river bends,the test and cause analysis of the deformed river bends,and the evaluation and treatment of the deformed river bends.The main research contents and conclusions are as following:(1)Through the analysis of the measured water and sediment data of Huayuankou,Jiahetan and Gaocun hydrographic stations from 1951 to 2019,the data of the section topography in typical years and the historical and recent riverbed evolution data of the Yellow River,the results show that:The deformed river bends in the lower reaches of the Yellow River is mainly concentrated in two stages.One is the uninterrupted adverse flow of minute water,which leads to the bending of the river and forms the deformed river regime.The other is the adjustment stage of the flood setback water and the river regime,which does not come back to the river bends and forms the deformed river bar.Based on the sinusoidal derived curve fitting results,the prevalent conception of deformed river bends in the wandering reaches of the lower Yellow River is verified.Remote sensing satellite images from 1986 to2019 were utilized to extract the annual binding coefficient of the deformed river bends and the length of the devious engineered river bends.The process of occurrence-gradual change-extinction of the deformed river bends was determined,and the temporal and spatial characteristics of the distribution of the deformed river bends were analyzed.The results showed that:Deformed river bends occurred frequently in the upper reaches of the Yiluo River Estuary section,in the middle reaches of the Doumen section,and in the lower reaches of the Youfangzhai section(2)Based on the solid model test of blood preview for the lower Yellow River in 2018and 2021,the evolution of deformed river bends was observed and analyzed in encryption.The results show that:the inlet angle and the outlet angle of the river bends are generally no more than 120,and the length of the river beds is generally no more than 30 after the bending is straightened.The span of the river bends continues to grow,and the overall change tendency of all deformed river bends is basically the same.After 100h of flood evolution,the flood with discharge level above 5000m~3/s lasted for 60 hours,and the deformed band was obviously improved,and the bending coefficient decreased from 1.6 to 1.2,showing a general downward tendency.In terms of the transverse migration,the transverse migration rate of the"Ω"shaped bend is more than 2 times that of the"reverse S"shaped bend,and that of the inlet section and outlet section of the gulf is 4-8 times that of the gulf top section.The maximum lateral migration rate of the river bends appear in the gulf top section of the"Ω"shaped bend without engineering boundary constraints,which reaches 33.06m/h;Flood scale water and sediment conditions are the main factors for the formation and development of deformed bend.Other conditions are equal,the disharmonious combination of"low water and high sand"is more likely to cause deformed river bends;In the downstream river bends,a large amount of suspended sediment[0.175,0.346]fell in the Beijing-Guangzhou Tieqiao-Dongbatou reach,and the sediment fall in the riverbed provided unstable boundary conditions for the evolution of the riverbed or the development of the deformed bend.The improvement of regulation work alone cannot completely eliminate the formation of deformed river bends.After 2007,the maximum bending coefficient of the river bends reduced significantly,and the harm of the deformed river bends reduced.(3)An evaluation model for the vulnerability of deformed river bends based on the weight of evidence method was proposed,and 12 influencing factors fell into three categories to construct the evaluation model index system.The results showed that the factors assisting in the vulnerability of deformed river bends were the average discharge during flood,the median grain size of riverbed,the acceleration of river swing,the difference of beach lip height and the sediment transport rate during flood.In the course of flood control and control of the Yellow River in the future,attention should be paid to the Beijing-Guangzhou Railway Qiao,Weitan and Wang’an.The AUC area corresponding to the three evaluation models based on WOE is 84.7%(WOE),91.8%(WOE-BP)and 94.2%(WOE-GA-BP)respectively.The forecasting ability of WOE-GA-BP model is superior to woe model and woe-bp model,and reaches the precise standard.In the future,there will be less water,less dry water and less sand flow combination,so it is necessary to use the limited water,regulate the water-sand relationship through the reservoir.Bending straight engineering is a kind of emergency rescue method,using the technology to implement bending,deformed river bends and the old river bending ratio is generally between 1:3～1:7,the diversion river section area should be the main river area of 1:10～1:30,the diversion river and the mainstream line Angle should not be more than 25°,the diversion river project is most appropriate in flood season water,is conducive to the diversion river scour.