Sediment exchange and transport processes in the Yangtze River Estuary: Concurrent discussion on the effects of sediment sink in the muddy area

The suspended & surface sediments exchange processes under the high-turbidity and mixing estuarine environments were analyzed based on the grain size spectral calculation, to explore the rate and transport of suspended sediments in the Yangtze estuary-shelf systems. The results showed that: (1) approximately 47% of the sediments from the Yangtze River was accumulated in the mouth bar area and subaqueous delta, while the rest was transported to the Hangzhou Bay and southern Zhejiang and Fujian coasts. 122.5°-123°E was the important limits of the Yangtze suspended sediment diffusing eastward to the East China Sea. This method of grain size spectral calculation provided by this paper can be used for the study of quantitative calculation of sediment transport and deposition in other tidal environments. (2) Higher sediment exchange ratio (p > 0.6) occurred in the muddy area and Hangzhou Bay, and the ratio of the muddy areas outside the South Passage was higher than 0.9, which indicates that most suspended sediment contributed to the seabed evolution and sediment supply. On the decadal to centennial time scale associated with our results, a clearly defined depocenter of Yangtze sediment with the exchange rates greater than 2-3 cm/a is present in the muddy area and Hangzhou Bay. (3) Due to the impact of longshore currents and the Taiwan warm current, the suspended sediment from the Yangtze River deposited firstly in the muddy area, and then was transported to the inner Yangtze River Estuary, Hangzhou Bay and the south coasts by tidal current. Muddy area was the "sediment sink" of sediment processes in the estuary and moved southward. (4) Under the background of decrease in the Yangtze River sediment supply, the delta toward tidal flats was still deposited slowly, indicating that the sediments in the sediment sink made great contribution to the estuarine geomorphologic evolution.