Analysis on the Characteristics of Channel Scour and Deposition in the Nanjing Reach of the Yangtze River After Impoundment of the Three Gorges Reservoir

AbstractThe construction and operation of the Three Gorges reservoir (TGR) alters the water and sediment conditions in the downstream channels, rendering the sediment transport capacity of water flow in these downstream channels at an unsaturated state, subsequently leading to scour and affecting flood prevention. Therefore, this study statistically analyzed the variations of channel scour and deposition in the Nanjing reach since the operation of the TGR, using the measured hydrologic and channel topographic data. The results showed that, the scour in the Nanjing reach was weak during 2001–2006, but prominently intensified during 2006–2020, with the multi-year average bankfull channel scour amount being 745.5 × 104 m3/a for the entire Nanjing reach. For the period of 2020.11–2021.03 after the basin-scale big flood in 2020, the scour was further intensified, with the total bankfull channel scour amount reaching 2677 × 104 m3. Since 2006, the Nanjing reach demonstrated an overall trend of “scour in both main channel and floodplain”, and bankfull channel scour was dominant, with the scour amount being 85% of that of the flood channel scour amount. Furthermore, scour was more intense on the left bank than on the right bank. The left branch of Xinjizhou reach and its downstream reaches were all erosional, demonstrating a pattern of scour amount being larger in the upper reach and smaller in the lower reach. Also, the multi-year average scour amount was the greatest at the Longtan and Yizheng reach, while the multi-year average scour intensity was the strongest at the main reach upstream the Meizizhou bifurcated reach.