Beach erosion and protection effect analysis on the upstream of the North Branch of the Changjiang (Yangtze) Estuary

Abstracts: Duo to frequent construction activities in the upstream and downstream of Changjiang (Yangtze) River, and water and sediment flows from the South Branch and North Branch in the Changjiang River mouth, the upstream of the North Branch has suffered from severe erosion and recession in recent years. To reveal the evolution mechanism of the upstream of the North Branch, control the river regime change, and protect the ecological background, the underwater terrain data of the whole river course of the North Branch since 2001 were collected, and the fine measurement of underwater terrain in the construction area from 2017 to 2020 was carried out. Based on the hydrodynamic mathematical model, the beach protection measures were studied and demonstrated, and the effect of the protection measures after implementation was evaluated in fractal theory. Results show that: ① the sandbar development at the entrance of upstream of the Northern Branch leads to the increase of the curvature of the upper S-shaped principal axis, which aggravates the instability of shoal, sandbar, and beach; ② the spur-dike and berm combination can effectively prevent the erosion and retreat of the most severely scoured beach; ③ the beach tends to be stablized and the ecological base has been restored as shown by after many-year underwater topography measurement according to fractal theory calculation and evaluation. Finally, the complexity of the evolution was aggravated by the changes of shoal, sandbar and deep channel in the upstream of the North Branch, it is suggested to establish a comprehensive coordination department as soon as possible to put forward a comprehensive improvement plan to meet the multi-objective needs of water conservancy, flood control, water supply, navigation, ecology, etc.