Evolution in river regime in the South Passage of the Yangtze Estuary under new situation

With the sharp decrease of sediment flow in the Yangtze River basin, the construction of estuary foraging projects, the strengthening of ecological environmental protection, and the efficient utilization of navigation environment, the Yangtze River estuary is facing a new round of adjustment. Taking the evolution of the South Passage of the Yangtze River Estuary as the research object, the recent river regime changes of the South Passage were studied based on the underwater topography data and the channel water depth data of the South Passage in the past 10 years, and the effects of the first phase of the South Passage regulation project were discussed. Results show that the main channel of the South Passage presented a scouring trend as a whole from 2010 to 2018, and the tidal creek at the head of Jiangyanan Shoal developed and the tail of Jiangyanan Shoal were scoured down. The tidal creek at the head of Jiangyanan Shoal has changed from scour to siltation environment from 2018 to the present, while the scour and siltation situation in other areas remains unchanged. The shape of the main channel in the South Passage section is generally stable. Under the background of the overall erosion of the main channel, the "barrier gate sand" was gradually flattened. Coupled with the role of the first phase of the South Passage regulation project, the volume of the area in the depth greater than 6 m has been significantly expanded and maintained good stability. The recent change of the South Passage regime is the comprehensive outcome of the influences of water and sediment inflows, and estuarine natural and human activities. The first phase of South Passage regulation project has played an important role in the recent topographic changes in Jiangyanan Shoal. The project effectively controlled the river regime of Jiangyanan Shoal and restrayed the local scour development trend of Jiangyanan Shoal, which is conducive to stabilizing the north boundary of the South Passage channel.