The Evolution Of The Yangtze River, Jiangsu Section Of The River Since The Last Glacial Maximum And Modern Scour Analysis

The research of river evolution under the influence of nature and human activities is one of the frontier research issues in Earth science. Therefore, accumulating knowledge on channel evolvement and modern river erosion-siltation process is of great importance for not only scientific theory but also production practice.Yangtze River is the biggest river in Asian Monsoon zone. Study on changes of its paleo-channel and paleo-flux since the LGM will provide crucial evidence for deeply understanding variation of hydrological features and processes of big rivers in this region.However, some previous programs involved in studying the Yangtze River paleochannel in Jiangsu reach rarely provided samples of fluvial sediments and reconstructed channel pattern during the LGM, primarily because of lacking enough targeted drilling cores and exploration. For age estimation of the paleochannel, most of them are based on the comparison between stratigraphies of nearby locations, thus lacking of supports from direct numerical age determination.Seven cross-sections of incised-valley depositional sequences were established with references of hundreds of bore datum and documents of Nanjing No.3 Yangtze River Bridge, the Yangtze River Bridge, Nanjing No.4 Yangtze River Bridge, Runyang Yangtze River Bridge, Yangzhong Yangtze River Bridge, JiangyinYangtze River Bridge and Sutong Yangtze River Bridge. Through analysis of 18 age results of ¹⁴C from samples of 10 drilling cores near Yangzhong Yangtze River bridge and Nanjing N0.4 Yangtze River Bridge which is operational, it is discovered that: 1. Nanjing Reach of the Yangtze River existed three obvious rhythms of river sedimentation from coarse sand to fine one since the LGM. The preliminary results confirmed that the formation of some sequences of sedimentation was belonged to the LGM, the Late Glacial Epoch and Holocene respectively. It is also indicated that the time range of incised-valley at -60-90m depth near Nanjing is within the period of LGM. 2. The ancient Yangtze River deep channel expressed locally and deeply incised features near Nanjing Yangtze River bridge and Nanjing N0.4 Yangtze River Bridge. The (?)/H gradually increased along downstream. 3. The paleo-incised valley in Zhenjiang Reach of the Yangtze River showed the typical character of river sediments and estuarine deposits. The sedimentary cycle of the Yangtze River channel below Zhenyang reach since the LGM consisted of the riverbed phase, floodplain phase, estuarine/shallow marine phase and deltaic phase, with sequence from bottom to top.According to the former publications, the mainstream paleochannel of Yangtze River is well studied and clearly constrained. The incipientvelocity of the LGM (18KaBP), the Late Glacial Epoch (12KaBP) and Holocene was calculated with Vc = 4.60d^1/3h^1/6 , Vc = 1.281g(13.15·h/d(95))(?),Vc = 1.49(?), etc., in terms of the river shape, sedimentary grain size and sequences near Nanjing No.3 Yangtze River Bridge and the Yangtze River Bridge section. Moreover, the discharge in Nanjing Reach of the Yangtze River during the three periods has been estimated according to the relationship of discharge, velocity of flow and cross-section. Based on the results of this paper, we provide important evidence not only for further understanding of variation of hydrological features and processes of big rivers in Middle Asia, but also for verifyingthat Yangtze River was a seagoing river during the LGM.The variation and modulation of siltation in the past decades linking to the mainstream riverbed of Yangtze River lower reaches has recently become a focus of research in related domestic academic circles. Nevertheless, studies in this research field within the range of Jiangsu reach of Yangtze River have been restricted to some shorter reaches basically due to scarce of hydrologic detection station and limited research techniques and methods. For the past more than 40 years, there were little research works carried out on the whole Jiangsu section of Yangtze River. To evaluate the dynamic changes of sedimentation and erosion of the Jiangsu reach of the Yangtze River during the last 44 years, database for five durations of Digital Elevation Model (DEM) of underwater topography was designed and established on the basis of the Jiangsu River Relief Map in 1959, 1970, 1985,1992 and 2003, with which the information of the riverbed area and channel-fill volume were directly extracted from the same height of one reach in ArcGIS9 . What the result indicated is listed as follows: 1. the main course of Yangtze River in Jiangsu Province had experienced an obvious switch from sedimentation to erosion around 1985 because of the decreasing amount of sediment yield from the upper Yangtze River. 2. Between 1985 and 2003, the erosion rate of the lower segment was greater than those of the middle and upper segments, demonstrating propulsive phenomena of "down-reach-ward accumulation". 3. The erosion rate of the deep channels was greater than the shallower ones. 4. Around the node, there showed large fluctuation of scour and silting, while the changes in scour and silting in the whole river channel slightly lagged behind than that in the deep channel. The depth of the river channel arrived at its maximum around the node and gradually shallower along upstream and downstream. The river channel displayed some erosion effects above the node while some sedimentation effects below the node.According to the document of the transported sediment amount of Datong Hydrometric Station from 1954 to 2004 and based on analysis of maximum entropy spectrum estimation, it has been concluded that the transported sediment amount of Datong Hydrometric Station shows a oscillation cycle about 8-year timescale. The model of ARIMA(2,1,1) of which has also been constructed.