Changes of channel bars in the wandering reach of the lower Yellow River from 2018 to 2021

Since Xiaolangdi Reservoir began to retained sediment in 1999, the Lower Yellow River (LYR) has deepened and widened continuously. The bankfull discharge has increased obviously, and the average depth has increased 1.3m～3.3m. The incoming water was abundant in recent four years from 2018 to 2021, and the peak discharge in the four year were all greater than 4000m³/s. The maximum discharge of Xiaolangdi station has reached 5500m³/s, which is the largest one since 1996. The evolution of channel bar in wandering reach is always the focus in sediment-laden rivers, especially in erosion period. Therefore, to appraise the changes in wandering reach of LYR in the erosion period, this study presents a detailed investigation of the channel bar changes in recent typical floods form 2018 to 2021. We described the bar pattern formation and sensitivity in wandering reach of LYR. Furthermore, we analyzed the numbers and area of channel bars based on the remote sensing images. We convert the channel bar at the same level from the relationship between the area of channel bar and water level at low water period. The results show that the channel sinuosity has decreased from 1.25 to 1.22, while the radius of curvature has increased from 2.80 to 2.96km. The number and area of channel bar have increased slightly. This phenomenon was affected mainly by the operation of Xiaolangdi Reservoir. The clear water and few bankfull discharge in 21 years since 1999, the channel erosion efficiency has decreased in the first ten years. So the erosion in recent four years floods was fewer. And the changes of channel bar slightly in recent four years. But the channel bar and channel pattern evolution dramatically from 1999 to 2021.