Morphodynamic evolution of the Biandan Shoal in the Changjiang River Estuary

Estuary shoal not only provides valuable wetland resources for humans, and also plays an important role in adjusting evolution of river regime. It is of vital significance to understand the morphodynamic evolution of estuary shoal for channel regulation, wetland development and dike protection. Therefore, based on historical chart for 150 years, bathymetric data, hydrology and sediment data, the morphodynamic processes and associated evolution mechanism of the Biandan Shoal, the biggest shoal in the South Branch were analyzed. The main results show that: (1) During 1860−2016, the Biandan Shoal has experienced the process of “deposition-erosion-deposition” repetitively; and its morphodynamic status was formed from original submerged shoal to spindle-shaped configuration above water surface; subsequently, the appearance was evolved to be relatively slim shoulder pole morphology with tail which was broken into a claw-like status; the lower part was gradually accreted due to the filled tidal creek among the claw-like configuration. (2) There is upward trend in both area and volume of the Biandan Shoal above −2 m, −5 m, respectively. The average annual increased rate of area above −2 m and −5 m is 0.88 km²/a and 0.81 km²/a, and corresponding increased volume rate is 1.3×10⁶ m³/a and 5×10⁶ m³/a, respectively. Meanwhile, extremely erosion occurred in 1998 with average thickness of 1.4 m. (3) While yearly sediment discharge from upstream has insignificant impact on the Biandan Shoal, water discharge could be responsible for the erosion/accretion of the Biandan Shoal; (4) The hydrology condition of strong south and weak north of the Baimao Shoal, the project of distributary reach of south and north channels, and the Dongfengxisha Reservoir directly caused the Biandan Shoal to move northward.