Prediction of morphological evolution trends in the South Branch of the Changjiang Estuary based on numerical and physical models

The Changjiang Estuary presents a geomorphic pattern of "three-level branching and four-outlet entering the sea", which is influenced by various factors such as runoff, tides, storm surges, and human activities. Its flow and sediment dynamics and channel-shoal patterns are very complex and varied. Accurately predicting the future trend of erosion and sedimentation in the Changjiang Estuary is of great significance for the protection and rational utilization of estuarine resources in the new era. This study selected the South Branch of the Changjiang Estuary, which is the main channel into the sea, as the research object. A physical model of the South Branch was established, and combined with the established numerical model of decadal erosion and sedimentation evolution in the Changjiang Estuary, a study was conducted to predict the evolution trend of the South Branch of the Changjiang Estuary. The physical model scenarios were selected for a series of annual cycles from 2016 to 2020, with four cycles of water and sediment years stacked once every 100 years and once every 300 years. The results showed that the flood maximum velocity in the South Branch was 2.26m/s, and the ebb maximum velocity was 2.69m/s. The deepest point of the main channel in the Liuhekou section had deepened ~7.8m and 8.0m before and after the implementation of the current and planned renovation projects, respectively. The deepest points of the main channel and Xinqiao waterway were brushed to -34.5m and -18.8m, respectively. The results of the numerical model indicate that under the current sediment discharge (125 million t/yr), the South Branch will maintain strong erosion until 2030, and the erosion will weaken by 2050; However, under extremely low sediment discharge (100 million t/yr), this section of the river maintains strong erosion and there is a trend of shifting from "shoal siltation and channel scouring" to "shoal and channel scouring". The predicted trends of erosion and sedimentation in the Changjiang Estuary based on the above two research methods are mutually corroborating, which can provide reference basis for the comprehensive improvement of the Changjiang Estuary in the next stage.