Research On The Yangtze Estuary Changes Under The Impacts Of Human Activities Based On Remote Sensing

In recent forty years, the development of the Yangtze Estuary has been impacted greatly by the human activities. This study investigated how the estuary responds to the human activities in the river basin and local estuarine area. In the river basin, because of hydraulic projects as well as soil and water conservation, the sediment load from river to estuary decreased greatly. The distribution and transportation of suspended sediment in estuarine area also varied as a response. The field measured suspended sediment concentration (SSC) values were spatially and temporally restricted. It is difficult to reflect the distribution of SSC in the whole estuary just rely on the sparse in-situ measurements (Jin Liu et al,2006; He Chao,2007). The maximum turbidity zone (MTZ) in estuary is dominated by the suspended sediment; it covers a large area and could be monitored by remote sensing images extensively. This study collected29Landsat images from1979-2008to extract the MTZ under diverse seasonal and tidal conditions. The average areas in different periods were compared to study the variations of SSC in the Yangtze Estuary. With the extracted MTZ area, its spatial distribution and movement features were also analyzed. The results show that with decrease of sediment discharge from river to the estuary, the average area of MTZ also declined, which shows the estuarine SSC also decreased on the whole. The re-suspension of sediment particles in the estuary plays an important role to the formation of MTZ; also because of the sediment load from river maybe keep on decreasing, the sediment re-suspension probably will be more critical to the MTZ. The MTZ in the Yangtze Estuary has large seasonal and tidal variations. In different water channels, with the influence of various hydrodynamic conditions, the MTZ also showed different features.The evolution of the Yangtze Estuary was impacted by the human activities not only in the river basin, but also in the estuarine area, such as reclamation on the tidal flat and local engineering. This study takes the reclamation on the Chongming Eastern Beach as a case. Waterlines with different tidal heights were extracted from Landsat images; the+2m contours in different years were interpolated and then compared to examine the extension rate of the flat. It was found the construction of banks on the beach had accelerated the extension of flat, and it was more obvious in the first three years. The extension rate of beach also had some relation with the sediment load and the area outside the bank. The extension direction of the Chongming Eastern Beach was influenced by the different hydrodynamic conditions in different areas, and it towards east and northeast under natural. But with the impact of embankments, it showed new characteristics. Recently, because of bank construction in1998, the Eastern Beach extended towards southeast. With comparison the Landsat images acquired in summer times to study the changes of vegetation cover on the flat. It was found that there was repeated phenomenon of vegetation degenerated into bare flat outside the bank. A large area in the south of beach became bare flat recently and also caused the coast to retreat. How to protect the coastline without disturbing the environment need further research. On the remote sensing images, it is clear to study the progress of Tuanjiesha Sandbank approached and merged into the Chongming Island. This progress is meaningful for the evolution of Sandbank in the North Channel of the estuary.With the continuously human activities in the river basin, such as run of Three Gorge Dam, the construction of cascade reservoirs in Jinshajiang River, further implement of Water Diversion Project and the concede land to forestry, the sediment load from the river basin to the estuary would probably keep on declining. Since the Shanghai City has limited supply of land resources, the reclamation would keep being constructed on lower tidal flat. Also, large engineering projects, like artificial island, were on plan. Studying the response of estuarine water and flat to the human activities as well as the new pattern of whole estuary is meaningful to direct new activities in the future.