Abstract: Understanding the characteristics of erosion and deposition in river bends helps predict bank evolution, and guides flood control safety, efficient water resources utilization, and navigational channel maintenance. Erosion-deposition patterns in a river bend are typically characterized by concave erosion and convex deposition. However, following the Three Gorges Reservoir impoundment, multiple sharp bends in the lower Jingjiang River shifted to the anomalous pattern－convex erosion and concave deposition. This study examines this anomalous phenomenon through numerical simulations of erosion and deposition in the river section from the Qigongling bend to the Guanyinzhou bend in the lower Jingjiang under the conditions of multiple flow regimes. The results indicate that the reservoir operation leads to longer water flow duration and a sharp decrease in sediment inflow, causing the main flow axis to shift towards the convex bank and the river section to shift to the anomalous pattern. Under medium and high flow levels, the erosion zone is highly consistent with the high-velocity main channel. As the flow level rises, the mainstream path deviates towards the convex bank, and the erosion there intensifies accordingly. Sediment concentration and its variation play a regulatory role in erosion-deposition intensity. Under the same flow level, a significant decrease in the concentration strengthens erosion, while an increase promotes deposition. We have obtained the concentration thresholds as a switching condition between erosion and deposition under different flow levels. This study gives a preliminary clarification of the synergistic mechanism of flow level and sediment concentration underlying the spatial distribution and intensity of sedimentation, laying a basis for understanding the evolution of curved river channels downstream of reservoirs.

Key words: middle Yangtze River, sediment transport dynamics, bank evolution, flow rate, sediment load