The Coastal Erosion-deposition Evolution And Controlling Factors Of The Abandoned Yellow River Delta In Northern Jiangsu Province

The abandoned Yellow River delta in northern Jiangsu developed from 1128 to 1855 when the Yellow River captured Huaihe River and flew into the Yellow Sea, and the delta suffered serious erosion due to the Yellow River returned to northern channel and flew into the Bohai Sea after 1855. In the last 800 years, the abandoned Yellow River delta shows unique characteristics of large-scale fast development erosion conversion, which can be divided into development stage with about 700 years and erosion stage with nearly half a century. Due to burst of the Yellow River, the flowing route changes frequently, so the delta evolution was divided into stages with obviously different evolution characteristics in different periods. Overall, a huge amount of sediment carried by the Yellow River is the material base of the delta evolution. Because of the continuing erosion of the delta, the sediment transported and redeposit under the hydrodynamic effect. Therefore, based on the history information of the delta evolution, remote sensing image and data collected from field measurement and laboratory, macroscopic qualitative description and microscopic quantitative analysis of the delta are given in the thesis. The methods mainly include Geostatistics, grain size analysis, grain size trend analysis, and environmental magnetism et al.. The erosion-deposition characteristics of delta evolution process and the evolution trend are studied, and sediment response to delta evolution and controlling factors of coastal evolution are also analyzed.Based on the historical information of the abandoned Yellow River delta in northern Jiangsu, the whole evolution process is systematically studied and the periodic evolution characteristics are illuminated in the thesis. As a result, the evolution of the abandoned Yellow River delta is divided into six stages:slow deposition period, rapid deposition period, fluctuation deposition period, fast erosion period, erosion adjustment period, and erosion reducing period. The characteristics analysis shows that a large number of the sediment accreted to the sea under the runoff and tide hydrodynamics in development period, and a large number of sediment was eroded under wave and tide hydrodynamics in erosion period. On the whole, the elvolution process has the characteristics of slowly development in earlier period, rapid development and rapid erosion in middle period, and slowly erosion in later period.Based on satellite remote sensing data, bathymetric data of five sections from 1993 to 2007 and six fixed sections with field measurement from 2008 to 2013, the recent change of coastline and section evolution in littoral area are analyzed. As a result, the erosion of the underwater slope is the most seriously, and the erosion depth of delta beach is about-10m. With the continued erosion and reduction of erosion intensity, the erosion rate of coastline is trend to decrease in future, and the beach trend to a erosion-deposition balance, moreover, the increase magnitude of the distance of coastline change trend to reduce with the reduce of erosion rate. The results also shown that erosion between Zhongshan and Biandan will continue, and sea level rise and storm surge will intensify erosion in the future.Based on the sediment grain analysis of the abandoned Yellow delta, the response of the sediment to the delta evolution is grain size trending to be coarser. Silt is a major sediment component of the delta. Sediment types in the study area mainly including silty sand, sandy silt, silt, sandy mud, mud and sand, as among the rest of their, the distribution of silt is the most widely, following by sandy silt. Based on extracted sensitive grain size and magnetic parameters, the sedimentary dynamic environment of the delta is analyzed. The results show that:the hydrodynamic conditions are more powerful in nearshore than offshore area, and stronger in delta corner area than two side areas of the delta, and sedimentary environment on the vertical shows obvious fluctuation change.Finally, the thesis analysis the controlling factors of the delta evolution, direct controlling factors including hydrodynamics environments and sediment factor, indirect controlling factors including human activities and climate changes. In addition, using grain size trend analysis model, the sediment transport trends in littoral area are studied, and the scouring or silting of coastal area under the modern sedimentary dynamic environments is discussed in the thesis. The results show that: sediment transport in north-northwest direction at northern delta, in south-southeast direction at southern delta, and transport off-sea around delta corner. Therefore, under the influence of hydrodynamic condition and sediment transport trend, coastal area which provides sediment appears erosion, while littoral area that receives sediment appears the balanced state of silting scoring or a little silting in the abandoned Yellow River delta.