Changes in Sedimentation Rate of Tidal Flat and Environmental Significance at Qidong Foreland of the Yangtze Estuary

The sedimentation rate is an important environmental parameter to understand the evolution of coastal geomorphology. The tidal flat around Qidong foreland is located in the junction between Yangtze Estuary and Jiangsu coast, where the land-ocean interactions are strong and highly sensitive to environmental changes. The QDZ-1 core sediments collected at Qidong foreland was analyzed for 137Cs dating and grain size. The results showed that silt is the main sedimentary type with a trend of gradually fining from the bottom to the top, conforming to the sedimentation characteristic of the silt muddy tidal flat. The sedimentation rate could be divided into three distinct stages: 1963 to 1986 was 2.61 cm/yr, 1963 to 2011 was 1.82 cm/yr, and 1986 to 2011 was 1.10 cm/yr. Based on these, further explanation was made for the significance of environmental changes. According to the estimation of sedimentation rate, the impact of extreme environmental change was reflected by the sudden increase in sand proportion at the depth of 172.5 cm. Since the introduction of Spartina alterniflora in the 1970s, it has played an important role in tidal flat development. The tidal flat has a high sedimentary rate during this time. With continuous accretion of the tidal flat, low tidal flat turns to high tidal flat, the sedimentation rate gradually declines, and tidal flat sedimentation appears.