| Meandering river has a special plane shape,which completes periodic evolutionunder the adjustment of natural cutoff.Neck cutoff is usually a process in which the development of the river bend reaches the limit sinuosity,and the water erosionsuns through the upper and lower watercourse of the neck,resulting in the formation of an Ω-shaped oxbow lake.Neck cutoff can significantly reduce the sinuosity and morphological complexity of local meandering river sections,and play an important role in the morphological dynamics of meandering river.This thesis uses a combination of field surveys,remote sensing imagesand UAV aerial surveys.It starts from the three scales of meandering rivers in different basins around the world,typical meandering rivers at the source region of the Yellow River,and neck cutoff in the two reaches of the Heihe River on the Zoige Plateau.The morphological characteristics of the neckcutoff of 45 meandering rivers in different climatic zones,the neckcutoff events and time series of 4 meandering rivers at the source region of the Yellow River,the process and triggering mechanism of the neckcutoff in the lower reaches of the Heihe River on the Zoige Plateau are summarized.The main results of this thesis are as follows:(1)45meandering rivers in different climatic zones and regions(from the lower reaches of the river to the upper reaches of the river)were identified and counted 243 high-sinuosity river bends and 94 rivers bends with ηt<1.The relative width ηt of the 243 river bends ranges from 0 to 2.8,the neck width of most river bends is similar to the average width of the river section.The Sm of the 243 river bends is mainly distributed between 6 and 16,of which 149 river bends in tropical regions have a large sinuosity.In different climatic zones,the average narrowing rate of the neck of the river bends is between 1.14 meters and 61.3 meters per year.The narrowing of the neck of the upper reaches of the river bends is obvious,whichoccurred obvious erosion and collapse.(2)In the typical meandering rivers of the Zoige Basin(Heihe,Dene River,Haqu and Vache River),there were 105 neckcutoff events in 100 years.Among these 105 neckcutoff events,more than 50%of the neck cutoff events have a residual sinuosity Sr of less than 10,and the oxbow lakes with Sr between 6 and 9 are the most.Most of the cutoff events took place for a long time and the remaining oxbow lake was poorly connected to the main river.The relative time series of the neckcutoff of the Heihe,Dene River,Haqu and Vache River are t in the range of 5 to 90,and the relative time is inversely proportional to Sr.It is predicted that 39 neckcutoff events will occur in the next 80 years in Heihe River,Dene River,Haqu River and Vache River.(3)From 1991 to 2018,the narrowest part of the neck of the bend in Heihe Ranch was narrowed from 43m to 7m.From 1977 to 2018,the narrowest part of the neck of the bend in Nenwa Township was narrowed from 49.4m to 12m.The bank slope of the bend in Heihe Ranch and Nenwa Township were undergoing erosion and collapse in 2018.There were many collapsed blocks along the shoreline,and the bank slope continued to steepen.At this time,the slope of the upstream bank of the bend was obviously greater than the slope of the downstream bank.The curve of Heihe Ranch fractured near sections 1 and 6 to 8.The curve of NenwaTownship was fractured and collapsed near sections 6 and 7.When the neck cutoff was triggered,the collapse rate of the upstream bank slope is greater than that of the downstream bank slope. |
