Analysis of Evolution Characteristics of Central Bars in the Lubao Section of the Beijiang River under Extreme Flood

The Beijiang River Basin experienced a once-in-a-century catastrophic flood in June 2022, which had a significant impact on the morphology and vegetation distribution of the downstream central bars. Based on the 2022 multi-phase Sentinel-2 remote sensing images and measured river terrain data, combined with normalized difference water index (NDWI) and normalized difference vegetation index (NDVI), this study analyzed the annual evolution characteristics of the planar morphology, cross-sectional terrain, NDVI mean values, and vegetation coverage of five central bars in the Lubao section of the lower reaches of the Beijiang River. The results show that the planar morphology of central bars is closely related to the water level of the river channel, and the area and circumference of each central bar significantly decrease during the flood period. Among them, the areas of the Zhongjian Bar, Lubao Bar, and Taijian Bar without soil embankments decrease even more, reaching 55.4%, 69.0%, and 66.4%, respectively. The reduction in the area of Dengtang Bar and Sixing Bar with earthen embankments is 40.8% and 30.2%, respectively. Except for the period of the "June 2022" catastrophic flood, the circumference and area of each central bar in the Lubao section show relatively small changes in other months of 2022, only fluctuating up and down with changes in the water level of the Beijiang River. The statistical results of the area of each central bar show that it is the smallest in June, the second lowest in July, and the largest value occurred in April or May. The terrain of the river section shows that the main channel is mainly slightly silted up, and the elevation of the central bank does not change significantly after the flood, indicating that the terrain of the river channels near each central bar remains basically stable even under the largest historical flood such as the "June 2022" catastrophic flood. The vegetation inside the embankment of central bars (Dengtang Bar and Sixing Bar) is less affected by the catastrophic flood, and the average NDVI remains at a high level after the flood. However, the vegetation area outside the embankment and on central bars without embankment generally has two peaks in April and October, as well as a valley in June. The vegetation in these areas is significantly affected by the flood, and the vegetation area generally decreases by 32.5%~74.4% after the flood. The study analyzes the differential impact of the catastrophic flood on the morphology and vegetation coverage of central bars in the Lubao section of the Beijiang River, providing a scientific basis for regional river management and ecological restoration.