Evolution process of different abnormal river bends in the braided reach of the Lower Yellow River

Since the mid-1980s, the braided channel reach of the Lower Yellow River has formed some abnormal river bends. Analysis of the evolution processes and characteristics of abnormal river bends are of great significance for the regulation of the braided reach. Based on the remote sensing imagery and the cross-sectional topography, this study describes the evolution processes of abnormal river bends and variation of morphological characteristic parameters. The result shows that the abnormal river bends in braided channel reach of the Lower Yellow River have the characteristics of short evolution duration, high distortion level and prone to natural cutoff and the bends in different location have different evolution characteristics. The first bend in the upper braided reach evolves slowly and the cut-off period is long. The second bend in the middle braided reach evolves rapidly and highly distorted. The third bend in the lower braided reach evolves rapidly and is prone to generate natural cutoff. Variation of river bend morphology reflects the evolution process of three abnormal river bends, which are corresponding to the bend formation and cutoff processes. The formation of the abnormal river bend is a process in which the curvature radius and river bend spacing decreases, and the bend curvature radius, angle of water flow and bend amplitude increase. The maximum curvature radius of the three bends in the braided reach are 1.20, 2.10 and 1.61, respectively, reflecting the distortion degree of different bends. After cut-off, the curvature radius, angle of water flow, bend amplitude and river bend spacing are about 605%, 59%, 27%, and 133% of the extreme value during the evolution of the "Ω" shape river bend. For "M" shape river bend, angle of water flow, bend amplitude and river bend spacing after cutting are about 37%, 83%, and 152% of their extreme values. The cross-sectional profile at the Yiluo River outlet reflected that the beach on the concave bank of the river bend continuously collapsed at a rate of about 94 m/a during the formation period of abnormal river banks. The cut-off has a great influence on the cross-sectional profile. After cut-off, the beaches on both bank collapsed, and the river channel widen at a rate of 148 m/a. The erosion at the riverbed developed from the deep point to its two sides. © 2020 by Journal of Lake Sciences.