Adjustment patterns and causes of the morphology of sandy riverbed downstream of the Three Gorges Dam

The operation of the Three Gorges Reservoir has altered the water and sediment conditions downstream of the dam, resulting in adaptive adjustments in riverbed by scouring and silting, as well as shoal and beach morphology. In particular, the sandy riverbed in the near-dam segment demonstrates the most sensitive response. In this work, the first sandy river segment of the Yangtze River was studied, and water and sediment data from 1955 to 2018 as well as topographic data from 1975 to 2018 were used to quantitatively study the characteristics and causes of riverbed erosion and deposition as well as the shoal and beach morphology adjustment. The results demonstrated that the riverbed underwent cumulative erosion as the sediment inflow from the river basin decreased. The amount of erosion in the low-flow channel accounted for 93.07% of the total amount of erosion in the river channel, and the area of shoals. After the 175 m impoundment of the Three Gorges Project, the scouring intensity downstream of dam on the riverbed increased, the near-dam segment with the greatest scouring intensity moved down to the sandy river segment, and sand excavation accounted for 15.9% of the scouring amount of the beach-level channel. The law of scouring and silting gradually changed from "scouring on low beaches and silting on high beaches" to "scouring on low and high beaches" since the impoundment. At the same time, the channel showed thalweg downcutting, the fluviatile facies coefficient decreased. Before the implementation of the waterway project, the evolution of beach groups demonstrated strong correlation. The channel bar development and downward movement of the head of the Taipingkou waterway caused the area of the upper section of the Lalinzhou point bar to decrease and recede. The tail area of the Lalinzhou point bar increased and the siltation width increased correspondingly, causing the area of the Sanba beach to decrease, the right edge to be eroded and recede, and the position of the beach body gradually move left. After the implementation of the waterway project, the correlation of beach group evolution was gradually weakened and the correlation between the channel bar of the Taipingkou waterway and the upper section of the Lalinzhou point bar was weakened. Moreover, the lower section of the Lalinzhou point bar was widened due to the influence of the waterway project as well as dredging and mud dumping, and the area of Sanba beach continued to decrease, with the right edge receding and moving left, causing the Jinchengzhou shoal to shrink and gradually disperse.