Modern Geomorphic Processes Of Eastern Chongming Island, The Yangtze Estuary

During Apr. 2002 and 2003, field observations and laboratory analysis including mark-stakes and sediment-boards monitoring, particles size analysis, magnetic measurements, 210Pb determination, and sea charts comparison, were conducted on tidal flats of eastern Chongming Island, the Yangtze estuary. The aim is to explore the characteristics of erosion-accretion cycle at spring-neap tide, seasonal and inter-annual scales. The evolution tendency of tidal flats in eastern Chongming Island is also discussed. The main conclusions are:Particle size analysis indicates that sediments become coarser from the north to the south along the sea-bank unit. The sediment of high tidal flat in the southern part is coarser than northern part. Sediments in the northern part show a obvious vertical particles size variation than its southern counterpart. Magnetic measurements are stronger than those from the southern part. The results indicate that sediment composition and hydrodynamics exist difference between the southern and northern parts of the study area.Under calm weather period, the quantity of sediments deposited during a complete spring-neap tide cycle is higher at the northern, southern parts than the central parts of the tidal flat in the study area. During Nov., the deposited sediment declined from high tidal flat to low tidal flat. It shows the inversed trend during Apr.. If only the observed results during calm weather condition are considered, the net deposit rate would be 27 cm/a,47 cm/a, 60 cm/a for high, middle and low tidal flat. Respectively, these figures are 8-10 times higher than the annual average value of mark-stakes, 14-62 times higher than sedimentation rates from 210Pb analysis. It suggests that the deposits during a spring-neap cycles are unstable, which are easy to be eroded by subsequent tides.The seasonal erosion and deposition cycle is closely is closely linked to theseasonal change of vegetation. The deposition rate is positively related to suspended matter concentration in the water, and negatively with tidal range, in general, summer and autumn are seasons of deposition, while spring and winter season of erosion, The erosion-accretion cycles are more obvious in middle tidal flat than in high tidal flat, But there exists some exceptions.Based on the sea chart comparison, it is found that the development of eastern Chongming island can be divided into two stages, a quick period from 1950s to 1980s, and a slow period from 1980s to now days. During the latter stage, erosion occurs at some parts of the study area.Tidal flat reclamation and coastal protection engineering promoted the development of the tidal flat. The dam built become Beigang and Tuanjie Sha during 1970, especially accentuate the deposition of the study area. It is revealed that the development of the tidal flat is positively related to the sediment supply of the Yangtze River. Erosion of the study area may become serious under the declining supply of sediments from the Yangtze River.