沉积物入海通量减少背景下长江水下三角洲活动性研究

Sediment activity refers to the percentage of time that seabed sediments are active within a certain period of time. The Changjing River is a typical example in that the construction of dams to store water and sand has led to a sharp decrease in sediment flux to the sea. In this context, the sediment activity of the Yangtze River estuary and its adjacent waters is not only related to the geomorphological evolution of the Changjiang River submerged delta, but also affects the coastal stability of Shanghai and the surrounding areas. Based on the surface samples and box samples collected in the Changjiang River estuary in March and October 2019 and hydrodynamic characteristics collected in the front edge of the Changjiang River Delta, this paper analyses the sediment activity by using CSM (Critical Shear-stress Metre) to test critical sheer stress, and analyzing grain size data as well as hydrodynamic characteristics. The result shows that: (1) In terms of the critical shear stress, the measured value given by CSM is close to the theoretical calculated ones, and CSM can accurately describe the characteristics of sediment threshold. (2) Comparing samples collected in different seasons, the average value of the critical shear stress measured by CSM is lower in summer than in winter, suggesting that the surficial sediments have a looser structure during the flood season and are prone to be eroded. (3) In summer, the shear stress of coupling waves with current decreases in flood tide and increases in ebb tide, and the calculated sediments activity is more than 60% within a 25-hour tiddal cycle. Assuming that the effective wave height in winter is 0.8 m higher than that in summer, while maintaining the same intensity of other hydrodynamic forces (currents), the calculated sediment activity of surficial sediment in winter is similar to those in summer. (4) Compared with the situation before the completion of the Three Gorges Dam, the average particle size of the sediments in the Changjiang River subaqueous delta is coarser, the activity of the sediments is reduced, and their resistance to erosion is slightly increased.