In practice, the amount of riverbed erosion and deposition in the same reach or in the same period calculated by different methods differs significantly from each other. At present, the calculation methods by using cross-section morphological variation, DEM grid morphological variation, and sediment balance are widely used. The amount of riverbed erosion and deposition in the reach from Yichang to Jianli of Yangtze River downstream of Three Gorges Project after impoundment was calculated using the above mentioned three methods based on topographic map, cutting cross-section on topographic map and sediment data of hydrologic stations. Furthermore, the calculation results and the factors affecting the calculation accuracy were analyzed, and the nonlinear fractal theory wasused to study the relation between riverbed fluctuation degree and section’s representativeness. Research results suggest that to improve the sediment balance method, suspended load near riverbed needs to be modified, and the alignment error of sediment measurement, water imbalance, sand mining and sediment diversion should be taken into consideration; as for the method of cross-section’s morphological variation, relatively close sections must be laid in river bends, bifurcated reaches or contractive and expansive reaches; while appropriate interpolation method must be employed for the method of grid morphological variation, and the grid space should be no larger than the distance between measuring points.
We analyzed the variation of dominant discharge and sediment-carrying capacity of flow in the downstream of Three Gorges Reservoir before and after the reservoir impoundment. The analysis is mainly based on data observed from Yichang hydrological station, the representative station in the downstream of the dam. We found that the variations of the primary and secondary dominant discharges caused by water storage of the Reservoir as well as the variations of sediment carrying capacity revealed the trend of longitudinal erosion of the riverbed. Moreover, variation of sediment-carrying capacity before and after the impoundment in the presence of equal flow explained the facts of medium and low flow erosion, medium and low water level, as well as the riverbed variation controlled by nodes.
