Study On The Changes Of Suspended Sediment Grain-size Distribution In The Middle And Lower Yangtze Mainstem

The main stream of the middle and lower Yangtze reaches has served as a link between the huge Yangtze drainage basin and the East China Sea. More than 90% of sediment transported by it is suspended sediment. The grain-size distribution of suspended sediment is one of the important properties of fluvial sediment It can reveal sediment source, reflect the composition of land surface materials in the drainage basin,vand 'tell" sedimentary processes of erosion, transportation and deposition in both drainage basin and river channel. The changes of suspended sediment grain-size distribution in the area are studied in this paper.The historical hydrological records and the recently observed data were collected to examine the properties of suspended sediment grain-size distribution in the study area.Methods of hydrology, sediment transportation and statistics are used to analyze the temporal and spatial variability of suspended sediment grain-size distribution and the relations among suspended sediment grain-size, discharge and suspended sediment concentration(SSC). Multiple regression analysis is applied to calculate quantitative relationships between grain-size distribution of suspended sediment and suspended sediment flux to the Sea. The results show:suspended sediment grain-size distribution in the middle and lower Yangtze stream fluctuates dramatically in different months, because %<0.025 and %>.05 in suspended sediment change strongly but reversely. The maximum of suspended sediment grain-size appears in spring, while the minimum occurs in summer.Mean yearly grain-size distribution of suspended sediment is close to the average of wet season, but far from that of dry season, primarily due to the main transportation of discharge and sediment occurring in flood-period. The relatively clear flow may scour the coarse bed materials in dry season, so the suspended sediment is much coarser than in wet season in Jingjiang section, which is the reverse of mat at Datong hydrological station. Before the construction of Danjiangkou dam, the suspended sediment grain-size distribution in wet season nearly equals to that in dry season in Hankou, while coarse sediment sufficiently increases in most dry seasons after finishing the dam.suspended sediment is coarser in the middle river than near bank, which indicates a more strong flow in the middle of river. With water depth increasing, suspended sediment grain-sizeincreases, whichis the general law for all the transects in the study area.The spatial variations in suspended sediment grain-size from Yichang downstream in wet season markedly differs from that of dry season: in the wet season, The suspended sediment grain-size fluctuates slightly along the trunk channel, with the maximum occurring at Yichang and the minimum appearing at Luoshan, which shows the same sand source from the upper Yangtze, siltation between Yichang and Luoshan, and probably scouring bed materials between Hukou and Datong in the very season. In the dry season, The suspended sediment grain-size changes strongly, increasing from Yichang to Xinchang, while decreasing from Xinchang to Datong, which indicates the strong suspending bed sand in Jingjiang section, and the siltation from Luoshan to Datong. The fact that the surficial suspended sediment in middle and lower Yangtze is far coarser than that in the upper Yangtze (2002.5-6.) indicates that only in the flood-period that the upper Yangtze becomes the main sand source for the middle and lower Yangtze.A strong correlation exists among grain-size, discharge, and SSC in the interior channel such as Xinchang station, a poor correlation occurs at the very place where a main tributary debouches, like Hankou Station. The trend of suspended sediment grain-size with discharge and SSC increasing is different spatially: the grain-size decreases with discharge and SSC increasing at Yichang, Xinchang, and Luoshan hydrological stations, which is a trend opposite to that displayed at Datong station. At Hankou station, generally speaking, there is not perceptible...