A Study About Impacts Of Human Activities In The Mid-lower Yangtze On Fluvial Sediment Supply From River Basin And Sediment Discharge From The Yangtze Into The Sea

The change of sediment discharge from river into the sea is an interesting issue and has received widespread attentions from the relevant academic circle. The changes in sediment discharge from river into the sea will induce the erosion or aggradation in the estuarine-deltaic morphologic system and its vicinity coastal system. It will influence the engineering structures, environment, wetlands, and biodiversities in the estuarine and coastal area. This thesis studies the human impacts on recent change in sediment discharge from the Yangtze River into the sea. The study area is focused on the middle and lower Yangtze basin between Yichang and Datong. The study methods include the sediment-transport balance, empirical formula and other hydrological statistical methods. The main problems touched in this thesis include the sediment discharge change at Datong hydrometric station and its controlling factors, the sediment transport balance between Yichang and Datong, the impacts of human activities in the mid-lower Yangtze on fluvial sediment supply and sediment discharge of the main Yangtze, the sediment deposition in reservoirs and its sediment-trapping effect, and the future trend of change in sediment discharge from the Yangtze into the sea.The yearly mean sediment discharge at Datong from 1953 to 2001 is 4.3 X 10 t, it is about 5X108t before the 1970s. Since the end of 1960s the annual sediment discharge at Datong began to decrease and such trend become remarkable since the end of the 1980s. In 2000, the yearly water discharge at Datong is more than the long-term average water discharge, but the annual sediment discharge is only 3.39X 108t, and further decrease to 2.76 X 108t in 2001. The analytic result indicates that the change in sediment discharge at Datong is related to many factors, such as the sediment discharge from the upper Yangtze (recorded at Yichang hydrometric station), the sediment discharge from the tributaries in the middle and lower Yangtze, the adjustment of relationship between the main Yangtze and the Dongting Lake in the middle Yangtze, the riverbed erosion and aggradation, the sediment discharge carried away by the water abstracted from the Yangtze River, sand mining from the riverbed, and so on. Among which the most important three factors are the sediment discharge from the upper Yangtze, the sediment discharge from the tributaries in themid-lower Yangtze, and the adjustment of relationship between the main Yangtze and the Dongting Lake. The decrease in sediment discharge of "the three outlets" (three outlets from the main Yangtze to the Dongting Lake) induces the increase in sediment discharge of the main middle Yangtze. The decreases in sediment discharge from the upper Yangtze and tributaries in the mid-lower Yangtze directly result in the decrease in sediment discharge at Datong.Based on the above factors, the sediment balance equation between Yichang and Datong was established using the sediment-transport balance method. The sediment-balance status between Yichang and Datong during four periods (1959-1968, 1969-1985, 1986-1998 and 1966-1996) was estimated and analyzed according to the sediment balance equation. The result indicates that the average sediment transportation between Yichang and Datong is at a state of balance as a whole. Based on which the amount of sediment discharge change about above most factors was estimated during three periods (1959-1968, 1969-1985, 1986-1998). The extent to which each factors influence on the sediment discharge at Datong was analyzed. The results show that the decrease in sediment discharge at Datong from the last 1960s to the last 1980s was mostly caused by the decrease in sediment discharge into the main Yangtze from tributaries in the mid-lower Yangtze. The total sediment discharge decrease from tributaries was about 67.44% of that from all factors. Among the tributaries the Hanjiang River is the direct factor, the total sediment discharge decrease for which accounts for about 48.06% of that for all factors. The further decrease in se...