Relationship between the characteristics of water-sediment transportation in river-lake system and the channel forming discharge of the middle and lower Yangtze River

The flood detention and sediment trap effects of the river-lake system in the middle Yangtze River is significantly different from that of general river floodplains. These characteristics may affect the channel-forming discharge in the main stream. Aim at this problem, the long term water and sediment discharge data and long distance channel topography data pre the Three Georges Reservoir impoundment was collected, and two kinds of work was performed. Firstly, the channel forming discharges in the middle and lower Yangtze River were estimated using multiple methods. Secondly, the mechanisms of river-lake water and sediment transportation and the channel forming discharge were systematically analyzed in sub-reaches influenced by diversions or confluence. (1) The river was characterized by flow diversion and sediment deposition in the upstream reach of Chenglingji station, and the river was characterized by flow convergence and sediment concentration dilution in the downstream reach of Chenglingji station. In reaches influenced by diversions, flow frequency curve showed trend of decline and channel deposition was likely to happen when discharge was beyond the critical range of 35000~40000m3/s. In reaches influenced by confluence, flow frequency curves were well fitted with broken power law function and the positive correlation between discharge and sediment concentration became negative correlation when discharge exceeded the critical range of about 40, 000m3/s. (2) The channel forming discharge estimated by various methods agreed with each other. It showed streamwise declining trend and ranged 28000~35000m3/s in upstream of Chenglingji, and it showed streamwise increasing trend and ranged 36000~44000m3/s in downstream of Chenglingji. (3) In upstream of Chenglingji, the channel forming discharge was mainly determined by incoming water and sediment, e.g. flow frequency and water-sediment relation of Zhicheng. In contrast, the diversion ratio of water and sediment was less important factors. In downstream of Chenglingji, channel forming discharge was mainly determined by the frequency of medium and flood flow. Thus, flood detention capacity of the river-lake system was an important factor affecting the channel forming discharge, and the influence of confluency along the reach was minor. (4) After operation of the Three Gorges Reservoir, the flow frequency and the water-sand relationship were both adjusted in the dam downstream reach. However, the influencing factors and their relations remain unchanged in the process of channel adjustment. © 2021, China Water Power Press. All right reserved.