Preliminary Studies On Morphological Processes Of Meandering Rivers At Different Spatial Scales In The Source Region Of Yellow River

Erosion-deposition changes of meandering channels at the source of the Yellow River are a dynamic equilibrium process.Periodic lateral migration of meandering rivers at long-term scales,from micro-bends and sharp bends to close to natural cuts,and finally cuts occur,and the dynamics of meandering river morphology at different scales have different evolution processes and laws.This thesis uses a combination of field surveys,remote sensing images and numerical simulations,starting from the three scales of curved river group,curved river section and neck-cut bending examples,the spatial distribution,morphological characteristics,and boundary conditions of the meandering river group at the source region of the Yellow River are summarized.Using drone aerial survey,the amount of sediment produced by the erosion and collapse of the concave bank and the amount of sediment deposited at the bank of the convex bank point were calculated.In 2017-2019,three in-situ observations were made on a bend where a neck cut occurred in the lower reaches of the Heihe River.The section-scale bank collapse model BSTEM was used to simulate the process of bank collapse triggered by bank collapse,and the acoustic Doppler ADCP meter measures the flow field of the river channel after cutting and the topography of the new river channel.The main results of this paper are as follows:(1)The meander belts of the tributaries of meandering river groups are greater than 1000 m wide,which belong to the Zoige river group with the highest sinuosity.The widest meander belt is the Black River(3300 m),followed by the White River(2500 m).The largest channel slope is the Maduo-Dari meandering river group(0.022),the smallest one in the Zoige river group(0.009).The average sinuosity is the largest in the Zoige river group(1.67),conversely the smallest one in the Maduo-Dari river group(1.19).Bank boundary condition of the meandering river groups is composed of two-layer composite material,i.e.,the upper layer is soil-root complex and the lower layer is non-cohesive silt-sand or gravel-sand layer.(2)Sediment loss amount generated by bank collapse on concave bank and sediment deposition amount on convex bank are imbalanced,i.e.,a net loss of sediment amount existing.For a single bend,the sediment loss per unit channel length in Lanmucuo River is about 0.191 m3,and about 0.045 m3 on average in Maiqu,Haqu,and Gequ Rivers.Nonetheless,the sediment loss in the reach scale of meandering channel is non-uniformity that indirectly reflects the difference in lateral migration rates among different bends.(3)A neck bending event occurred in the lower reaches of the Heihe River in July 2018.BSTEM was used to simulate the neck collapse process and the 5 hydrological conditions caused by the collapse of the measured neck section under the extreme flood process on July 10-13,2018.Field observations show that in June 2019,the near-shore velocity at the right bank of the neck bend is lower than the left bank,the difference is about v=0.4 m/s,the right bank is still being rapidly eroded,and the new river channel continues to widen.