| Grain size is an important index of sediment characteristics. Sediment characteristics of surficial sediment in estuary has important significance on geomorphology, ecology and engineering.The estuary of Yangtze river and the adjacent area is not only a multidisciplinary research hotpot, but also a crucial social economic activity. Building dams, water and soil conservation project and other human activities lead to the decrease of sediment flux into the sea, which has important impact on the erosion and deposition of delta in the recent 30 years. On the other hand, a number of huge water conservancy projects have changed the regime pattern and sedimentary dynamic environment in different degree. It is an important scientific issue that whether the grain size and distribution of surficial sediment in the Yangtze estuary and the adjacent area have dramatic changed, which is worth to be discussed. In order to understand the estuary sedimentology under the influence of human activities, surficial sediment samples at large sarcle have been collected, and used the same research methods(Sieve analysis and pipette analysis) to analyse the grain size of sediment. The main results are as following:1) The D50 of surficial sediment in Yangtze Estuary and the adjacent area was gradually downstream decreased. At present, the average (D50) of surficial sediment in upstream of North Branch is 57 μm, and 16μm in downstream. The average (D50) of surficial sediment in South Branch is 66 μm, and in upstream of South Channel and North Channel is 41 μm, in South Passage, North Passage and North Channel (Turbidity Maximum) is 25 μm, in offshore is 20 μm The component of sand in upstream of North Brach is 49%, and 10% in downstream, it is 45% in South Braneh and 29% in up stream of South Channel and North Channel. In South Passage, North Passage and North Channel (Turbidity Maximum), the component of sand is 17%, in offshore is 10%. The component of silt in upstream of North Branch is 37% and 63% in downstream. The South Branch is 40%, in South Channel and upstream of North Channel is 53%, in South Passage, North Passage and North Channel (Turbidity Maximμm), the component of silt is 61%, in offshore is 66%. The component of clay in upstream of North Branch is 15% and 27% in downstream. The South Branch is 15%, in South Channel and upstream of North Channel is 19%, in South Passage, North Passage and North Channel (Turbidity Maximum),the component of silt is 22%, in offshore is 24%.This trend reflects the dynamic pattern under the interaction of runoff and tide and the sedimentary selective transport process which is from the "source" to the "sink". At present, the pattern that the sediment in the mouth of estuary is obvious coarser than that out of the mouth of estuary and offshore is same with the sediment grain size distribution characteristics in the study area thirty years ago, but there is some differences between in and out.2) After 30 years the surficial sediment in North Branch obviously tinner. According to the comparison and analysis of data from the same position of sampling point, the D50 of surficial sediment in the North Branch has decreased from 97μm to 35 μm, the component of sand decreases from 76% to 28%, the component of silt increases from 17% to 51%, the component of clay increases from 8% to 21%.These changes can be attributed to the power weakening which is caused by the shrinking trend in the North Branch and the human activities such as reclamation.3) The surificial sediment in the South Branch system is obviously thinner during 30 years. Recently, the D50 of sediment in the South Branch system (including South Branch, upstream of North Channel and South Channel) decreased from 89|am to 57 μm, the component of sand decreased from 54% to 15%, the component of silt decreased from 32% to 43%, the component of clay decreased from 14% to 15%. The main reason for these changes may be sand mining.4) The sediment out of the Yangtze Estuary obviously coarsen during 30 years. The D50 of surficial sediment increased from10 μm to 16 μm. The component of sand increased from 8% to 9%, the component of silt increased from 52% to 68%, the component of clay decreased from 39% to 24%. The sediment coarsening is mainly attributed to the underwater front delta erosion after sand reducing from basin and the trend that the D50 of sediment increasing.5) The surficial sediment of Yangtze Estuary and adjacent area had changed correspond with sorting coefficient, skewness and kurtosis. The sorting coefficient, skewness and kurtosis have statistical relationship with D50.These relationship reflected that the coarser the sediment is, the greater the sorting, the bigger kurtosis and the greater positive skewness are. Therefore, as the surificial sediment fined, the sorting poored, the positive skewness reduced and the kurtosis decreased. On the contrary, the sorting of sediment in the offshore area became well, positive skewness increased and the kurtosis rised.In a word, the grain size of sediment in Yangtze Estuary and adjacent area significantly changed, the reason was human activities. In different area the grain size of surificial sediment characteristics was different and the reason was different. In North Branch, the sediment fining was due to reclaimation accelerated the channel declined, its water power faded, suspended fine particle deposited. In South Branch system, because of sand mining and other human activities the surficial sediment fined. The sand component of sand was extracted and water section increased, which leaded to water power iaded and fine particle deposited in the channel. The reason for the sediment in offshore area coarsen is that on the background of sediment reduced, the under water delta turned deposition to erosion.High frequence of human activities may lead to the sediment grain size changing. These changes are also significant to benthic and ocean engineering. In view of the river delta high strength human activities, the understanding of the changes in surface sediment particle size of the Yangtze River estuary and adjacent waters may have certain enlightenment function to other estuarine delta. |
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