Characteristics of main channel migration in the braided reach of the Lower Yellow River after the Xiaolangdi Reservoir operation

The response of main channel migration to the altered flow and sediment regime is characterized by complexly temporal and spatial variations in the Lower Yellow River (LYR),and the investigation into the characteristics of main channel migration is crucial to understand the principles of river regime adjustments and fluvial processes in the LYR. In this study,widths and intensities of main channel migration at section-and reach-scales were calculated based on the data of remote sensing images and 28 observed cross-sectional profiles in the braided reach of the LYR during the period from 1999 to 2016,with the characteristics and the key influencing factor to control main channel migration intensity being identified quantitatively. It is found that the main channel processes in the braided reach migrated back and forth towards the left or right direction,and section-scale migration widths in the middle reach were generally larger than the values in the upper and lower reaches. The width and intensity of main channel migration show a gradual decreasing trend,with the average were 188 m/a and 0.16 respectively. It is confirmed that the incoming flow and sediment regime is a dominant factor to influence the main channel migration,although the channel boundary conditions can influence the intensity of main channel migration,covering the influencing factors of the level difference between main channel and floodplain,and composition of bed material. The main channel migration intensity of the braided reach can be expressed as a power function of the previous three-year average fluvial erosion intensity,and the calculated main channel migration intensities in 1999—2016 using the proposed relation generally agree with the observed data.