Characteristics of Cross Sections and Dynamic Factors of Bed Fomrming in the Lower Tidal Reach of the Yangtze River

Based on the measured hydrological and topographic data of the reach below Nanjing of the Yangtze river, and the result of the one-dimensional tidal current numerical model, some issues about hydrodynamic factors of riverbed-forming in the tidal river were discussed. It is shown that the factors in tidal reach of the Yangtze River formed by the interweaving of runoff and tidal current, and the hydrodynamic force along downstream increases continuously due to superposition of runoff and tide. Because of riverbed storage and tidal wave deformation, in addition to the general bend and branching characteristics, the overall trend of channel plane morphology is to gradually broaden to form bell-shaped mouth with an inflection point appearing near the flood tidal current limit, but the average depth of cross-sections is maintained in 10~30m. Specifically, in the upper reach of the dry tidal current limit, runoff plays a major role in riverbed-forming, and the main channel width is 1500~2000m and the dominate discharge is 44000~47000m3/s.In the reach between the dry tidal current limit and the flood tidal current limit, both runoff and tidal current act together to riverbed-forming, and the main channel width is 2000-4000m and the dominate discharge is 50000~55000m3/s. In the lower reach of the flood tidal current limit, tidal current plays a major role in riverbed-forming, and the main channel width is 4000-13000m and the dominate discharge is greater than 75000 m3/s.