Evolutional trend of the coastal wetland in Shanghai in response to decrease of sediment supply from the Changjiang River

About 15% of the land needed for Shanghai's urban development came from the reclamation of the tidal flat of the Changjiang River delta over the last 50 years.Further reclamation is needed for Shanghai's urban expansion.However,due to damming,sand extraction and afforestation in the Changjiang River basin,there has been a decreasing trend for the riverine sediment into the East China Sea since 1960 s,which presents a threat to the source of the tidal flats for Shanghai that needs to be further examined.Based on the analysis of the series monitoring data since 1950,the amount of the riverine sediment delivery into the East China Sea and the general scour and silting trend at the outlet(shallower than 5 m isobath) of the Changjiang River Estuary in the next 20 years is estimated.The results show that:(1) The riverine sediment has greatly decreased by two thirds due to anthropogenic activities.The sediment load at the Datong Gauging Station has shown a decreasing trend since the late 1960s and it has been accelerated since the late 1980 s.The sediment discharge in the past 10 years(averaging 2.8 ×108 t/a) is only about 44% of that between 1956 and 1965(averaging 5.04 * 108 t/a).(2) The progradation rate of the tidal flats has been decreasing evidently.The combined progradation rate of East Chongming Shoal,East Hengsha Shoal,Jiuduansha,and East Nanhui Shoal was 19.1 km2/a between 1958 and 1977,5.1 km2/a between 1977 and 1986,and 4.9 km2/a between 1996 to 2004.(3) The area of the tidal wetlands above 5 m isobath at the outlet accounts for 70% of the total,in which East Chongming Shoal is 701 km2,East Hengsha Shoal 464 km2,Jiuduansha,410 km2 and East Nanhui Shoal 462 km2.(4) There is a linear correlation between the sediment load(108 t/a) at the Datong Gauging Station after taking the impact of the dredging activity in the North Passage into consideration and the progradation rate of the tidal wetlands(km2/a) at the outlet,that is,Y=9.5136X-29.05,R=0.925.Based on these results,it can be predicated that the sediment load at Datong will decrease to 1.5 ×108 t/a between 2006 and 2015 and 1.2× 108 t/a between 2016 and 2025.As a result,the tidal wetlands near the Changjiang River mouth may be eroded by 147.8 km2 between 2006 and 2015 and by 176.3 km2 between 2016 and 2025,which is about 6.3% and 7.5% of the present tidal wetlands respectively.The current Deep Waterway Project in the North Passage and the reclamation project at the East Hensha Shoal may hinder erosion in the future.If there are no further engineering protection measures to be taken,there will be a 10 % loss of total tidal wetland area in the next 20 years.The conclusion is as follows: due to the decrease in sediment load from the Changjiang River,the progradation rate of the tidal wetlands in the Changjiang River mouth has been decreasing and the tidal wetlands may be eroded in the future if there are no engineering protection measures to be taken.But the submerged delta may provide a source for sediment via resuspension resulted from flood tide erosion.If suitable measures are adopted to utilize the high suspended sediment concentration,Shanghai is expected to have new tidal wetlands for urban development.Further protection of the tidal wetlands and reclamation under high suspended sediment condition will secure the existing tidal wetlands resource and provide land for Shanghai's urban expansion.