Impact Of Riverbank Collapse On Riverbed Monorphology In The Braided Reach Of The Lower Yellow River

In the past two decades,under the dual impact of natural changes and human activities,the flow-sediment regime in the Lower Yellow River(LYR)have changed,and led to frequent bank collapses in this reach.At present,the research on the mechanism of river bank collapse is still immature,and most of the research focuses on qualitative summary of experience.There is a lack of generalized hydraulic flume tests for the complete process of scouring,collapse,and transportation of composite riverbank.Considering this,this research selected the braided reach of the LYR as the research area,combined data analysis,prototype data processing,physical model tests,and theoretical research methods.And this research based on the basic conditions of water and sediment conditions in the LYR,the process,characteristics and influencing main factors of bank collapse in typical section of the reach were studied.At the same time,the process of bank collapse,transport and accumulation in flume experiments were explored,and the mechanism of bank collapse was elaborated.The following research results have been achieved:(Ⅰ)The water and sediment characteristics of the braided reach of the LYR show obvious trend and stage characteristics.Four characteristic parameters of water and sediment were analyzed by means of data analysis and wavelet analysis in the braided reach from 1950 to2021.It is concluded that 1950 to 1999 is the first stage and 2000 to 2021 is the second stage of annual runoff and sediment discharge characteristics change.The evolution period of bankfull discharge is 18 years in the first stage and 5.5 years in the second stage.The bankfull discharge has obvious characteristics of periodic evolution.Meanwhile,the median particle size of riverbank sediment is wholly about 0.02 mm,and there is no obvious change along the river in the braided reach since 2000.And the median particle size of river bed sediment is refined gradually with time and distance along the river.(Ⅱ)The river bed shape adjustment characteristics of different sections in the braided reach of the LYR are quite different.The characteristic parameters of four bankfull channel of Mayugou section,Doumen section and Yangxiaozhai section were calculated by establishing the collapse evaluation method.The results show that the deformed river regime of Mayugou section has gradually disappeared since 2012,the shape of the main channel as a whole tends to be wide and shallow,and the collapse phenomenon is gradually improved.The Doumen section has always been in the typical deformed River section,and the river bank collapse is strong,especially since 2018.The average collapse rate is 553.5m/a.The section of Yangxiaozhai is close to the transition reach,the river regime is relatively stable and the river bank collapse is relatively slight.On the scale of the braided reach,scouring downcut is dominant from 2000 to 2017.The river is mainly widened laterally,and the river bank collapse is more obvious from 2017 to 2021.(Ⅲ)During the process of river bank collapse,the flow condition is the external force condition of river bank collapse,and the width of river bank collapse increases with the increase of the intensity of water flow scouring.Material composition of river bank is the internal factor of river bank collapse.When cohesive content in material composition of river bank is low,the scour resistance is poor and the phenomenon of river bank collapse in section is obvious.When the cohesive content in the material composition of the river bank is high,the river bank has strong anti-scour force and no obvious collapse of the section.At the same time,river regulation works are the main manual constraints,which improve the form of river section.The river phase coefficient of Tiexie-Jiahetan reach in the study section is negatively correlated with the density of river regulation works.(Ⅳ)Lower sandy soil of binary river bank structure is scoured laterally by water flow,and soil after collapse is affected by water flow transportation and has certain accumulation capacity.Soil of binary structure is scoured by water flow at different locations.The width of scouring pit is 3～5cm and the lateral scouring rate is 0.074～0.097 m/h,which is slightly larger than and close to the lateral collapse rate of steep gate section in prototype 0.064m/h.During the sand scouring test,the binary structure soil body is affected by turbulence of water flow,and the change of turbulence energy presents the change and trend of “∩” type.The diameter of the soil body falling to the sand bed is 3～5 cm.Water flow scouring has certain starting and transferring effect on the collapsed soil body,and the collapsed soil body itself has certain accumulation capacity.The starting and transferring capacity of water flow to collapsed soil mass gradually increases with the increase of flow rate and decreases with the increase of distance along the way.The accumulation capacity of collapsed soil decreases with the increase of flow rate and along the distance.(Ⅴ)The mechanism of river bank collapse is revealed by combining the collapse characteristics of prototype section and the simulation results of hydraulic flume.Based on the stress analysis of riverbank soil,it is indicated that the impact resistance of sandy soil is less than water shear stress and less than that of cohesive soil.Under the action of water flow scouring,the lower sandy soil layer is washed by water flow and scouring pits gradually appear.When the lower sandy soil layer is scoured out,3～5cm cracks appear in the upper cohesive layer,and the gravity of the cohesive soil itself accelerates the reduction of its stability and finally collapses.