DOWNSTREAM GRAIN-SIZE VARIATION TO INTERPRET RIVER-CHANNEL PROCESS-FORM IN THE MIDDLE-LOWER YANGTZE RIVER, CHINA

摘要

In 2000 and 2003, five-hundred and twenty (520) riverbed sediment samples were taken from the middle-lower Yangtze trunk channel to examine the river hydromorphological nature. Analytical result shows that the riverbeds mostly consist of mostly medium to coarse sands, but silty fine sand near the river coast. The result further indicates 12 zones (I-XII), identified as alternated coarse-fine-grained riverbed, except zone Ⅰ-Ⅲ. Grain-size associated hydrological parameters indicates higher values in the Jingjiang Reaches (Yichang to Chenglingji) (unit stream power: 5-18 N/ms, boundary shear stress: 14 N/m2 and mean flow velocity: 2-3.2 m/s), while the values obtained from Chenglingji downstream are considerable lower (＜5 N/ms, 1-4 N/m2 and ＜0.7-1.5 m/s). These, when comparing with on-site measured ADP flow column velocity, revealed the erosive riverbed sediment transport in the Jingjiang Reaches, and the accumulative riverbed transport downstream, especially from Wuhan to the river coast. Hydrological parameters together with grain size distribution saw a coarsening riverbed in the Jingjiang river course, largely due to damming since the last half-century. This corroborates the weakening sediment zonation in the Jingjiang Reaches, which we believe will further extend downstream to the river coast in the coming decades.