| Grain size is the basic attribution of material, and is also the most important physical parameters and dynamic environment index of sediment. Bed sediment is the result of interaction of factors such as hydrology and sediment in the river, it is product of accumulation in a certain discharge and sediment and dynamic. It records the information of the sediment provenance and the hydrodynamic conditions, and it is the most basic elements for the study of the river landscape environment. There are many researches on the downstream variation of the river bed sediment, but the research on the large sand bed river sediment is very little. The sea is the final home of most rivers, and it is also the sink of the sediment from the river. There are many previous literatures on the sediment of the sea bed, But few of them combined the river sediment (the source) and the marine sediment (the sink) together. The Yangtze River is the longest river of China, and is also one of the longest rivers in the world. After the completion of Three Gorges Dam (TGD), the largest hydropower project of the world, significant erosion was observed in middle and lower Yangtze River and the Yangtze River delta front. But whether the bed sediment downstream the dam has coarsened is still unknown, if it has coarsened, how long the coarsening reach it is, it still need further research. Therefore, the research on the characteristics of the sediment from middle and lower Yangtze River and the offshore area adjacent to the Yangtze Estuary, not only can enrich downstream fining theory of riverbed sediment, and understand the impact of Three Gorges Dam to the sediment of the middle and lower Yangtze river, at the same time, it can research the "river"-"sea" system together, and discusses the variation of the sediment from "source" to "sink"Based on the sediment sampled from the middle and lower Yangtze river and the offshore area adjacent to Yangtze Estuary, and the data collected from the previous study, this paper researched the characteristics of the sediment from the middle and lower Yangtze river to the offshore area adjacent to Yangtze Estuary, analyzed the variation of the sediment from the river (the source) to the sea (the sink); At the same time, this paper also analyzed the changes of the parameters of the sediment from the middle and lower Yangtze River pre-and post-TGD, and discussed the changes of the parameters of sediment in the offshore area adjacent to Yangtze Estuary in recent decades; Based on these works, this paper also discussed the impact of human activities and natural factors to the sediment grain size characteristics. The main conclusions of this paper are listed as below:(1) The fining trend of sediment grain size from the "source" to the "sink". From Yichang to the estuary along the Yangtze main stream, then to the southern of the study area along the main dispersal direction of the sediment, the median size of the sediments show a fining trend overall, although the change is complicated in local. The sediment type in Yichang, the most upper reach, is gravel, but then the median grain size of sediments decline sharply in the reach less than100km downstream from Yichang, and the sediment type changes into sand, and then in the downstream reaches, the sediment grain size fluctuates and declines gradually, at the Yangtze Estuary, sediment grain size declines sharply again, sediment type changes from sand into mud. Because of the existence of the sharply change zone, it is difficult to simulate the downstream variation of the whole area with an exponential formula. But from the Yangtze Estuary to the southern of the study area, the sediment grain size shows an exponential fining trend.(2) Two abrupt transition zone of the grain size in downstream of TGD and the estuary. The sediment size changes sharply in two reaches of the middle and lower Yangtze River. Sediment type changes from gravel into sand in one zone, and another from sand into mud. The change from gravel into sand happens in the reach about100km downstream from TGD. This change mainly reflects the impact of human activity to the riverbed sediment. After the completion of TGD, most of the sediments from upstream have been deposited in the reservoir, the downstream riverbed close to the dam has been serious eroded, and the sand was washed, only the gravel left, so it made the sediment change abrupt from gravel into sand. The change from sand into mud happens in the Yangtze estuary, and this change mainly reflects the impact of nature mechanism——the change of hydrodynamic condition to the sediment. The Yangtze Estuary is influenced by runoff and tidal together, and water velocity is changing all the time, when water velocity is very slow, it is good for the sedimentation of the suspended sediment, and usually, the grain size of suspended sediment is finer than bed sediment, therefore, the sedimentation of the suspended sediment can lead to the decline of the bed sediment grain size, and then it causes the sharply change from sand to mud.(3) The "binary spatial pattern" of the surface sediment in the offshore area adjacent to Yangtze Estuary and its cause. The surface sediments of the offshore area adjacent to Yangtze Estuary in the study area show a spatial pattern that the grain size in east is coarser than in west, and the direction of the isoline of median size4Φ (0.063mm) is northwest-southeast. In the west side of the isoline, the main component of the sediment is fine mud from the modern Yangtze River, but in the east side of the isoline, the coarse residual sand is the main component of the sediment. And several frequency curves of the sediments near the isoline are bimodal, this shows that two kinds of sediment of different grain size mixed in this region, and this region is the transition zone of these two kinds of sediments.(4) The impact of TGD to the grain sizes of the riverbed sediments downstream the TGD. After the completion of TGD, the sediment discharge reduced sharply. Before the closure of TGD, sediment discharge also showed trend of reduction, but the TGD make70%of the upstream sediment deposit in the reservoir, and this is an important reason lead to the sediment discharge reduce. The sharp decrease of sediment discharge results in the serious erosion of the riverbed downstream the TGD and the sediment grain size show obvious coarsening. On the whole, the sediment grain size of Yangtze River in the two period post-TGD is coarser than pre-TGD, and the most obvious area happens in the reach about200km downstream the TGD, the sediment type of this reach changed from sand before TGD into gravel after TGD. And the downstream moving of the transition zone in two periods after the TGD shows that the erosion of the riverbed is still happening and the sediment grain size will become coarser in the further.(5) Analysis of the grain size change of surface sediment in the offshore area adjacent to the Yangtze Estuary in recent decades. According to the comparison with the study in the1980s, this paper finds that in the offshore area adjacent to the Yangtze Estuary, the border between sand and mud moved west in northern30°10’N, it shows that sediment in this area is coarsening. And the vertical distribution of sediment grain size of several core sediments (No.15,16,21,30) also show a trend of coarsening. By analyzing sediment component, this paper finds that the10%isoline of clay content have moved westward obviously, it shows that content of clay reduced in this area; But the change of30%isoline of clay content is relatively complicated, some region have moved eastward, but other region have moved westward. And the change of silt and sand content is also complicated. Therefore, it is hard to judge whether the sediment of the whole study sea area are coarsening.In a word, this paper reveals the change of the sediment in Yangtze River from the "source" to the "sink", and it not only reflects the control of nature (for example, the exponential fining trend of bed sediment in middle and lower Yangtze River before the completion of TGD, the sharply change zone of the grain size in the Yangtze Estuary, and the exponential fining trend of bed sediment from Yangtze Estuary to the south of the study sea area), and also reflects the impact of human activity (such as the appearance of the coarsening and the sharply change zone downstream the TGD; the border of "mud-sand" moved backward in the offshore area; the disappear of the broken clay zone in the area adjacent to the south of Hangzhou bay). Therefore, it is necessary to identify and filter the impact of human activity when using the sedimentology rule. |
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