An Analysis Of Coastal Wetland Evolution In The Yangtze Estuary And Northern Hangzhou Bay In The Recent Decades

In the recent decades, catchment human activities have resulted in great changes in sediment supply from the Yangtze River to the East China Sea, which affected accordingly the delta evolution. Meanwhile, there is a sharp increase in human requirement of coastal resources. Coastal challenge is therefore evident.In this thesis, bathymetric maps in 1958, 1980 and 2004, and ArcGIS software and Digital Elevation Model were employed to examine the spatial distribution and temporal variation of coastal wetlands in the Yangtze River Estuary and Northern Hangzhou Bay. The evolution of coastal wetlands was analyzed in relation to sediment supply from the Yangtze and reclamation. Based on these, coastal evolution was forecasted for the coming 1-2 decades.The major results are as follows:1. The total acreage of coastal wetlands in the Yangtze Estuary and northern Hangzhou Bay was 861.5, 1709.9, and 3111.3 km~2 above the isobaths of 0 m (Lowest Tidal Level), 2 m and 5 m in 2004, and 982.5,1865.9 and 3496.6 km~2 above the same isobaths in 1980. The current intertidal area is mainly distributed in Jiuduansha (20%), Eastern Chongming (northern border: Beibayao; southern border: Xijiagang) (19%), Eastern Hengsha (8.4%), Northern Chongming (7.9%), and Zhongyangsha and Qingcaosha (4.5%). In the southern part of the study area (south of 31°31.5'N), the total acreage of coastal wetlands above 0 m, 2 m and 5 m isobaths was 562.7, 1126.0 and 2099.9 km~2 in 2004, 617.0, 1201.2 and 2341.3 km~2 in 1980, and 629.0, 1096.3 and 2136.7 km in 1958, respectively.2. From 1980 to 2004, the total acreage of coastal wetlands above 0 m, 2 m and 5 m isobaths in the Yangtze Estuary and northern Hangzhou Bay decreased by 12.3%, 8.4% and 11.0%, respectively, because of higher reclamation rate than progradation rate. The drastic decrease in intertidal area in the Chongming Island and the southern mainland coast was attributed to reclamation, whereas the 99.8 km~2 increase (130%) in intertidal area in the Jiuduansha was due to progradation under influences of riverine sediment supply and the Deep-Waterway Project in the North Passage.3. Compared with the period of 1958-1980, the progradation rate of the coastal wetlands above 2 m and 5 m isobaths in the southern Yangtze Estuary and northern Hangzhou Bay during the period of 1980-2004 decreased by 9% and 18%, respectively, probably due to the sharp decrease in sediment supply from the Yangtze River. On the contrary, the progradation rate of intertidal wetland increased by 91% from 1958-1980 to 1980 to 2004, probably because of the deposition-promoting effect of reclamation projects (the reclamation rate in 1980-2004 was twice as much as that in 1958-1980).4. Taking the seaward coast of the Chongming Island as an example, the thesis examined the sensitivity of coastal response to reduction of riverine sediment supply The results show that as a whole the progradation rate of the study area went down with decrease in riverine sediment supply in time scales longer than a decade. However, the response was complicated by the marine hydrodynamics and evolution of local shoal-channel systems, and reverse trends can be found in shorter periods and smaller areas.5. It is predicted that catchment human activities will keep reducing sediment supply from the Yangtze River to the sea. The sediment discharge at Datong will fall from an average of 1.6×10~8t/a since the TGD operation in 2003 to about 1×10~8 t/a in the next decades, which will result in severer environmental problems in coastal wetlands in the Yangtze Delta area.