On the siltation of the middle Yangtze River

来源 :Science in China(Series D:Earth Sciences) | 被引量 : 0次 | 上传用户:zfx249220414
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About 5.2×108 t/a of silts from the upper reach of the Yangtze River are transported through Yichang into the middle Yangtze River. The middle Yangtze itself and its tributaries yield ca. 0.8×108 t silts annually. Of this total 6×108 t/a input of silts, ca. 1.2×108 t/a are deposited within the river artery and lakes, while ca 4.83×108 t/a are at first transported into the lower reach through Datong, and then either deposited along the river and the Yangtze Delta, or output into the sea. Along the middle Yangtze River silting generally exceeds scouring, but the spatial and temporal distributions of scour-and-fill, as well as its distribution in the river valley transections, are uneven. Along the Yichang-Chenglingji and Jiujiang-Datong Segments of the middle Yangtze, scouring exceeds or equals silting. Silting mainly occurs along the Luoshan-Wuhan-Huangshi Segments with Wuhan as the pivotal point, as well as in the Dongting and Poyang Lakes. Even in these silting-dominated areas, there are times and parts of segments that scouring prevails. As to the transections, generally the river channel of main stream is scouring-dominated, forming V-shaped sections to a depth of 2030 m or more, whereas the floodplains and bar islands are silting-dominated, forming wide and relatively high flats, which are often artificially transformed into protected embankments. Because the Jingjiang Levee separates the Jianghan Plain from Yangtze to the south, little has been deposited in that plain except a few sediments from the Han River—a tributary of the middle Yangtze. This factor plus the tectonic sag of that plain has made the altitude of plain surface several meters lower than that of the levee in most of the areas. Sil-tation along the Luoshan-Wuhan-Huangshi Segments has resulted in the elevation of flood level in this and upstream segments. River channel scouring plus floodplain silting have caused the transections of the middle Yangtze valley typically with deep channel and heightening flat bank, separated from the descending Jianghan Plain only by the Jingjiang Levee. Such a transection explains why the flood level becomes higher and higher with the same water flowage. High water level has to be prevented only by the elevation of the levee, which in turn increases hydraulic pressure to the levee and to the protected low plain, thus making it easier to cause leakage and related disasters. The increasing height difference between the low plain and the heightening floodplain and flood level is a serious and long-term Achilles' heel of the middle Yangtze. About 5.2 × 108 t / a of silts from the upper reach of the Yangtze River are transported through Yichang into the middle Yangtze River. The middle Yangtze itself and its tributaries yield ca. 0.8 × 108 t silts annually. Of this total 6 × 108 t / a input of silts, ca. 1.2 × 108 t / a are deposited within the river artery and lakes, while ca 4.83 × 108 t / a are at first transferred into the lower reach through Datong, and then either deposited along the river and the Yangtze Delta, or output into the sea. Along the middle Yangtze River silting generally exceeds scouring, but the spatial and temporal distributions of scour-and-fill, as well as its distribution in the river valley transections, are uneven. Yichang-Chenglingji and Jiujiang-Datong Segments of the middle Yangtze, scouring exceeds or equals silting. Silting mainly occurs along the Luoshan-Wuhan-Huangshi Segments with Wuhan as the pivotal point, as well as in the Dongting and Poyang Lakes. Even in these silting-dominated areas, t here are times and parts of segments that scouring prevails. As to the transections, generally the river channel of main stream is scouring-dominated, forming V-shaped sections to a depth of 2030 m or more, but the floodplains and bar islands are silting -dominated, forming wide and relatively high flats, which are often artificially transformed into protected embankments. Because the Jingjiang Levee separates the Jianghan Plain from Yangtze to the south, little has been deposited in that plain except a few sediments from the Han River-a This factor plus the tectonic sag of that plain has made the altitude of plain surface several meters lower than that of the levee in the most of the areas. Sil-tation along the Luoshan-Wuhan-Huangshi Segments has resulted in the elevation of flood level in this and upstream segments. River channel scouring plus floodplain silting have caused the transections of the middle Yangtze valley typically with deep channel and heightening flat bank, separated from the descending Jianghan Plain only by the Jingjiang Levee. Such a transection explains why the flood level becomes higher and higher with the same water flowage. which water level has be be prevented only by the elevation of the levee, which in turn increases hydraulic pressure to the levee and to the protected low plain, thus making it easier to cause leakage and related disasters. The increasing height difference between the low plain and the heightening floodplain and flood level is a serious and long-term Achilles' heel of the middle Yangtze.
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