Long-Term Sediment Accretion Rates of Floodplains Using Remote Sensing Waterline Extraction Method: A Case Study of Poyang Lake, China

With a typical floodplain in Poyang Lake selected as the study area, this paper employed the remote sensing Waterline Extraction Method (WEM) to invert its topographic changes based on 264 Landsat images from 1987 to 2024. The research systematically revealed the spatiotemporal variations in sediment accretion rates over the past 40 years and their influencing factors. By comparing different WEMs, the object-based method was identified as the most suitable for this study area. Accuracy validation of the topographic inversion showed that when using no fewer than 13 images, the average elevation error rate remained below 7.0%, indicating good reliability. The period from 1987 to 2024 was divided into 15 sub-periods, and digital elevation models of the floodplain were reconstructed for each. Results indicated that: (1) natural floodplain unaffected by sand mining experienced continuous accretion, with an average rate of approximately 3.1 ± 0.7 cm yr−1 (surface elevation change) between 1987 and 2024; (2) in areas impacted by sand mining, the sediment accretion rate after mining (about 1.7 ± 0.8 cm yr−1) was lower than that before mining (about 2.6 ± 2.7 cm yr−1), likely due to the loss of vegetation cover reducing sediment retention capacity; (3) different vegetation types notably influenced accretion rates, with mixed Carex–T. lutarioriparia communities showing a consistently higher rate (about 3.5 ± 0.9 cm yr−1) than pure Carex communities (about 1.7 ± 0.7 cm yr−1), primarily attributable to differences in plant morphology, root architecture, and inundation tolerance. Further analysis revealed that riverine sediment supply was the fundamental material source for floodplain accretion. The phased decline in sediment discharge from the Ganjiang and Xiushui rivers since 1996 generally corresponds to the decreasing trend in sediment accretion rates observed after 2004.