Evolution of the shoal group coupling and its response mechanism to waterway engineering in the Daijiazhou reach of the middle reaches of the Yangtze River

The evolution of shoals and the adjustment of channel morphology have a strong correlation. Identifying the relationship between the impact of channel engineering on shoal morphology, riverbed scouring and siltation is valuable for evaluating the effectiveness of channel engineering implementation. This study focuses on the Daijiazhou reach in the middle reaches of the Yangtze River, analyzing the coupled evolution of the shoal group and its response mechanisms to channel engineering using 60 years of measured water, sediment, and topographic data. In the natural water and sediment stage (before 2008): the Bahe shoal and the Chihugang shoal exhibited a strong associations. The deposition of the Chihugang shoal weakened the diversion effect of the Yanji node, accelerating the upstream extension of the low shoal at the head of Daijiazhou. The deposition at the Bahe shoal led to scouring of the low shoal at the head of Daijiazhou, while the reduced sediment supply increased the scouring rate of the low shoal. The distribution of flow between the branches of Daijiazhou was unstable, and the proportion of flow into the right branch showed a decreasing trend under the same Hankou station flow conditions. In high-flow years with low sediment supply, the erosion of the right bank of Daijiazhou was accelerated, providing space for the widening of the Lejiawan shoal. In the channel engineering impact stage (since 2008): the implementation of channel engineering further extended the low shoal at the head of Daijiazhou; however, the scouring of its right bank requires significant attention. The channel engineering at the inlet of the Daijiazhou branches compressed the lateral space of the river and accelerated the disappearance of the Bahe shoal. As the Chihugang shoal eroded, the degree of flow dispersion increased, which was detrimental to enhancing the hydrodynamics at the inlet of the branches. After the right bank of Daijiazhou was protected, its integrity improved. However, the separated portion of the Lejiawan shoal, resulting from the transition from deposition to scouring, entered the channel and obstructed navigation. The implementation of the 6m depth channel engineering strengthened the integrity of the Lejiawan shoal, significantly improving navigation conditions in the middle and lower reaches of the right branch.