Fluvial Processes Of Bend Lateral Migration And Hydrodynamic Adjustment After Neck Cutoff Occurred In Meandering Rivers Of The Source Region Of Yellow River

The Yellow River Source is located in the northeast of the Qinghai Tibet Plateau.The terrain is low and flat.Meandering rivers of different river width scales are widely developed in the Source of the Yellow River,and its surface is mainly covered by peat,meadow and grassland.These meandering rivers are characterized by high curvature,lateral migration,periodic evolution and frequent natural cutoff.The meandering rivers can maintain dynamic balance on a multi-year scale.It increases the curvature through lateral migration and reduces the curvature through cutoff..At present,it is important to understand the material balance relationship between concave bank erosion and convex bank deposition of meandering rivers,the relationship between the complex morphological characteristics of meandering rivers and lateral migration,as well as the regulation law of channel flow structure after natural bend cutoff occurred.However,there is a lack of prototype observation and quantitative research.In this study,the combination of UAV aerial survey,remote sensing interpretation and ADCP observation is adopted.Five bends of Maiqu and Lamucuoqu River reach at the Source of Yellow River are selected and divided into 626 sections to quantitatively analyze the erosion and deposition characteristics of bends at the reach-scale.At the same time,the morphological parameters and migration rates of 177 bends of Black River are extracted and analyzed.Finally,the flow and topographic survey were carried out for the bend with neck cutoff event in the lower reaches of Heihe River in 2018,and the changes of three-dimensional flow structure and secondary flow intensity were compared and analyzed after the neck cutoff occurred.The main results of this paper are as follows:(1)Because the width change of "point bar-concave bank" is dominated by the enlargement of point bar,the reach where the change of river width exceeds 10%is always at the apex of the bend.The feature of "concave bank erosion and convex bank deposition" in the meandering reach is distinct,and the peak values of concave bank erosion and convex bank deposition are symmetrically distributed along the channel at the outlet of the bend.However,the material change of subsequent bends of Lanmucuoqu River is-31 m3 and that of subsequent bends of Maiqu River is 2m3.The sediment deposition on the convex bank of the Maiqu reach is greater than the scouring amount,and the overall depositing and developing.There is an empirical relationship between the scouring width of the concave bank line and the bank erosion volume.(2)The bend curvature of the mainstream and tributaries of Black River is 2～3,the amplitude wavelength ratio of 67%of the bends is 0～1.the compound bends of mainstream accounts for 34.0%,and the compound bends of tributary accounts for 21.5%.About 57.5%of the bends in the Black River Basin are deformed in the extension mode.The Black River tributaries have the highest migration rate through the extension,but the migration rates of the mainstreams are the highest in the translation mode.There is a power function relationship between channel migration rates and bend curvature in Black River Basin,and the distribution pattern is similar to that of other rivers.(3)In July 2018,the bend in the lower reaches of Black River moved laterally close to the limit curvature,and the neck cutoff event occurred in advance due to the extreme flood.Before and after the bend cutoff,the secondary flow structure of the relatively wide and shallow channel with the width depth ratio greater than 30 is weaker and more stable in time and space.After neck cutoff of bend occurred,the hydrodynamic axis at the top of the bend shifts from the convex bank to the concave bank by 0.2 times the river width,and the distribution range of underwater secondary flow pointing to the convex bank expands 0.25 times the water depth to the surface.The flow separation zone moves from the bend top to the erosion side of the new channel,and the distribution range in the flow direction is expanded,but it is still distributed within 0.3 times the river width near the bank.