近40年来长江口沉积物粒度变化及其对底床冲淤的响应

在全球变化和人类活动的双重驱动下，长江口的沉积物来源、输运和河床冲淤正在经历持续性调整。为了探索近40年长江口沉积物输运和海底冲淤时空变化趋势，基于1980—2020年长江口海底沉积物粒度和水下地形数据，采用沉积物粒径趋势模型和多种空间分辨率网格统计分析方法开展研究。结果表明，长江口北支沉积物总体向河道中下部汇聚，南支则在南、北港分流处靠近崇明岛区域形成沉积中心，该结果与基于2012—2020年地形计算的冲淤变化基本相符，沉积中心具有较大的淤积幅度。多年统计结果表明:1980—2003年为高强度输沙时期，整个河口区处于淤积状态，沉积物粒径变细但整体变幅较小；2003—2012年河流输沙量快速降低背景下，2009年前河口区整体依然处于淤积状态，粒径变化不显著；2009—2012年是由淤积转为冲刷的过渡期，口外区域首先出现了小范围侵蚀和沉积物粗化现象；2012—2020年，河口区整体出现大面积侵蚀和粒径粗化现象，粒径与侵蚀变化间呈显著相关。研究发现，高分辨统计网格可以更有效地捕捉到小范围地貌由淤转冲的信号。总体而言，近40年来，长江口已经由显著淤积转为局部侵蚀，未来可能面临持续性的海岸侵蚀灾害。

Analysis of sediment grain size change and its response to erosion and deposition pattern within the Yangtze River Estuary for the past 40 years

Driven by global changes and human activities, the sediment source/transport, and riverbed erosion and deposition pattern of the Yangtze River Basin are undergoing continuous adjustments, which influence both magnitude and property of fluvial sediment to the estuary. In order to explore sediment transport and correlated topographic changes of the Yangtze River Estuary within 40 years in response to variations of fluvial sediment discharge from the upstream basin, this study uses sediment grain size trend models and multi-year zonal analysis methods to predict temporal and spatial variations of morphological changes within the Yangtze Estuary, based on grain size data and topographic data from 1980 to 2020. The results show that sediments converge toward the center of the channel within the northern branch, and a deposition center forms close to the south and north channel diversion area within the southern branch, consistent with the morphological changes between 2012 to 2020 calculated from sea maps, validating the effectiveness of the grain size trend model in predicting morphological changes within the Yangtze River Estuary. The results of the multi-year zonal analysis show that: 1980—2003 was characterized by a high volume of fluvial sediment discharge, and the entire estuary area was dominated by deposition with slightly finer sediment. Fluvial sediment discharge experienced a rapid decrease from 2003 to 2012, while the estuary area was still dominated by deposition before 2009 with similar grain size. 2009—2012, however, was a transitional period when erosion and sediment coarsening started to appear out of the estuary, although limited to small areas. From 2012 to 2020, large-scale erosion and grain size coarsening occurred widely in the estuary, and a significant correlation was found between the particle size and the magnitude of erosion. Our study shows that high-resolution zonal analysis can effectively catch the local morphological change signals in the Yangtze River Estuary. In the past 40 years, the Yangtze River Estuary has changed from deposition-dominant mode to partially erosion mode, and it may face continuous coastal erosion in the future.