Study On The Evolution Law Of The Newly Opened Pebble Channel In Dongxikou

DongXikou waterway is the typical bending braided pebble rapids in upper reaches of the Yangtze River.The mechanics of gravel movement and obstruction navigation characteristics are complicated.Opening a new channel in the left during low-water level to improve the channel level is an effective engineering method.However,the left channel is located in gravel transport route,suffering from the strong gavel movement,the transportation route may span the entire channel in the flood season,which will affect the stability of the channel and bring a greater difficulty to improve the channel level.Based on theoretical analysis,movable bed model experiment and the numerical simulation of two-dimensional water sediment,this paper studied the method of determining the regulation width of gravel rapids,pebble transportation law and regulation scheme of DongXikou waterway and the stability of new channel.Based on the method of guiding and controlling river to make it become more smooth which is carried out by the correlation calculation of river hydraulic geometry relation and the maximum sediment transporting rate method which is calculated by Einstein correction expression and Ackers' formula,the reasonable width of dredged Channel can be determined.The calculation results about the channel width range by two methods are almost the same.This study also uses mathematical model to estimate the optimal width-depth ratio of dredged Channel.Researching on the pebble bed load transport properties of the new channel and its influence on the evolution of channel under the worst conditions of water and sediment through the small-scale moving bed model experiment with natural sand.The researchi ng result reveals that the new channel presents a trend of weaker silting under the condit ion of the flood and the silting mainly concentrated in the channel entrance while the mi ddle and lower of the channel remain stable Basically.The new channel shows obvious effect of splitting water and sand after dredging and damming,also the pebble bed load t ransport belt presents tongue shaped distribution in the channel entrance.Under the action of secondary circulation,two-dimensional hydrodynamic module and the moving-mass dynamic module are adopted in the curvilinear coordinate system to simulate the long-term effect of the water flow on the pebble bed-load of the new dredged channel,the scouring and deposition characteristics and the evolution regularity of the new channel are obtained through the analysis of the characteristics ofgravel bed-load transport and the relationship with the hydrodynamic conditions.The results show that after the remediation,Cumulative deposition trend is clearly showed in the inlet section and exit section of the newly opened gravel channel,the stability of the inlet section is poor,so the deposition depth should be increase.In this paper,the research method of the stability of the new pebble channel has important guidance and reference significance for the regulation of the pebble rapids in large mountainous river.