Research On The Formation And Evolution Characteristics Of Alluvial Braided Channels

With the development of society and the increasing human efforts in river development,the geomorphological features of many alluvial braided rivers have undergone significant changes,such as frequent channel erosion and sedimentation,and transformations in river patterns.The migration and changes in river channels and floodplains have brought about numerous issues related to flood control,navigation,and river ecosystem.Understanding the processes of water and sediment transport,riverbed evolution,and the formation and evolution of typical units in braided river channels,as well as their interactions with the overall river system,is crucial for better predicting the evolution process and trends of complex rivers and addressing the aforementioned problems.Due to the complexity and variability of the factors that affect actual river evolution,the dynamic mechanism of the overall evolution of alluvial braided rivers is not yet fully understood.Investigating the formation and evolution process of unit bar,which are the basic geomorphic units of braided river morphology,can help further understand the evolution mechanism of braided rivers.This thesis studies the formation and evolution process of typical bar units on a laboratory-scale channel using numerical simulation,and analyzes the evolution characteristics of alluvial braided rivers combined with the actual case of the Qingtongxia to Rencundu section in the upstream of the Yellow River.Firstly,a mathematical model of water and sediment transport is established to simulate the formation and evolution process of unit bar under the widening of the channel planform,and analyze the flow field changes and the interaction between the water flow and the bed during the evolution process of unit bar.Then,a mathematical model of the Qingtongxia to Rencundu section is established,and the river evolution during 2007-2011 is simulated based on hydrological data.The changes in flow field,bed erosion and sedimentation,and bed shear stress are analyzed,and the interaction process between the overall evolution trend of the river,local morphological features,and bed morphology changes and water flow are studied.The results show that:(1)Under the disturbance of reduced upstream inflow,a relatively stable unit bar can form in the widening section.This process mainly includes three stages: the sudden expansion of the mouth,sediment deposition,and the formation of an incipient midchannel bar;the adjustment of unstable water flow and vertical accretion on the bar surface;the channel bifurcation and erosion of the bar.There exists a dynamic equilibrium mechanism between the morphological changes of local units within alluvial braided river channels and the transporting sediment-laden flow.The distribution of the sediment-laden flow field affects the morphological characteristics of the sediment deposition stage and its growth rate in space.The vertical accretion of the sediment body accelerates the flow at the top while its morphological changes,in turn,affect the vertical distribution of flow velocity near the bed.(2)The evolution process of a braided river can be divided into four stages:1)During the low flow period,sediment deposits on the protruding parts of the bed forming water-submerged bodies of varying sizes,while erosion occurs in the concave parts forming channel bifurcations;2)During the flood period,the submerged bodies continue to grow and migrate,forming mid-channel bars that split the flow,and the bifurcated flow continues to erode the bed on both sides of the mid-channel bar forming channels;3)During the flood period,the flow erodes the low points on top of the bar,forming small channels and cutting the bar.The change in bed morphology leads to a change in flow movement direction and the migration of the main channel;4)During the non-flood period,some small channels dry up,causing independent bar units to merge into compound bar,and the flow is concentrated in a few deeper channels.(3)Under the continuous variation of inflow,the movement trend of water flow changes with the alteration of water and sediment conditions,thus affecting the plane morphology of the river channel.This is the main reason for the frequent migration and changes in the river channel.The bifurcation point within the river channel is generally located at the junction between the tail of the bar upstream of the bifurcation point and the head of the bar downstream.The water flow converges and then diverges at this location,creating highly complex hydraulic conditions with high bed shear stress.The channels near the bifurcation point are prone to erosion and scouring,causing changes in the morphology of the tail of the upstream mid-channel bar and the head of the downstream bar,thus resulting in changes in the overall morphology of the river channel.