Estimations and changes of the dominant discharge in Three Gorges Reservoir channel

Dominant discharge is an important parameter reflecting the water inflow process and river channel shape. Since the impoundment and implementation of the Three Gorges Reservoir (TGR), the bed-forming effect of the river channel in the reservoir area has changed significantly. Based on the observed water and sediment data from four hydrological stations in the upper Yangtze River from 1980 to 2017, the dominant discharge of the Zhutuo, Cuntan, Qingxichang, and Wanxian stations before and after the TGR became operational is calculated using the Makayev method, Han Qiwei empirical formula, and flow guarantee rate method. The results show the following: (1) The Makayev method takes both the sediment transport capacity and bed formation duration into account. The physical meaning is clear and the calculation results are relatively reasonable. (2) After the impoundment of the Three Gorges Project (TGP), the dominant discharge of the river decreased in the variable backwater area, while increasing in the perennial backwater area. With the water level rise in front of the dam, the dominant discharge in the variable backwater area remained basically unchanged, while increasing in the perennial backwater area. (3) The dominant discharge in the reservoir area is affected by incoming water processes and the reservoir operation mode, and shows a strong correlation with annual average discharge, flood season average discharge, and flood peak discharge. Before impoundment, the correlation between the dominant discharge and these three parameters was high. After impoundment, the correlation between the dominant discharge and the average flow during flood season was more apparent, further confirming that the bed-forming period of the reservoir is the flood season.