Study on the relationship between beach trough evolution and navigation obstruction characteristics in Jingjiang reach of the Yangtze River

The Yangtze River is China′s "golden waterway". Through systematic waterway improvement projects and dredging maintenance, the channel depth of the Jingjiang reach downstream of the Three Gorges Dam has increased from 2.9 m in 2002 to 3.5—3.8 m in 2020. However, the water depth is still lower than that of the upstream Three Gorges Dam reservoir area (4.5 m) and the downstream reach (4.5—6.0 m). The water depth of the Jingjiang reach does not connect with the upstream and the downstream, which restricts the comprehensive benefit of the channel of the Yangtze River. In order to adapt to the water depth of upstream and downstream, it is necessary to improve the channel scale of the Jingjiang reach, and it is urgent to clarify the relationship between channel water depth resources, navigation obstruction characteristics and river evolution. Based on the data of discharge, sediment transport and topography during the period of 1960—2020, this study analyzed the relationship between the beach trough evolution and the elevation of channel water depth resources in the Jingjiang reach of the Middle reaches of the Yangtze River. The results indicate that after the operation of the Three Gorges Project, the scouring of the Jingjiang reach mainly occurred in the dry river channel, accounting for 90.97% of the total scour. The central bar area and point bar area was reduced by 18.3%, of which the area of the the central bar and point bars ware reduced by 9.4% and 24.9% respectively. Under the combined action of riverbed scouring and channel improvement projects, channel scale verification was carried out with 4.5 m×200 m (water depth ×width). The total length of navigation obstruction in Jingjiang reach accounted for 5.3% of the whole reach. The navigation obstruction characteristics of 4.5 m water depth were as follows: the decrease of dry water level in sandy pebble riverbed reach was higher than the undercut depth of river channel, resulting in insufficient water depth of channel. The scouring of convex bank and siltation of deep channel in concave bank, in sandy riverbed reach cause the instability of beach trough shape and channel boundary. Low flow route instability and insufficient water depth were caused by the shrinkage of the shoal and uneven scouring between the branching channels during the dry season.