长江河口边滩冲淤机制及其泥沙捕集效应研究

The erosion and accretion of the estuarine frontier tidal flat (EFTF) directly reflect the coupling actions between land and ocean, which is an important representation of estuary environment. Meanwhile, the stability of the EFTF is closely related with safety of both alongshore land resources of the large cities on the delta and epicontinental coastal engineerings. Here, the Changjiang EFTF with intensive anthropogenic activities was selected as research area The massive data were collected by large-scale field observations, including hydrology, sediments, and topographical elevations. Based on the field survey, auxiliary multi-remote images and history data, these material were processed by analysis and calculation with mutual validity by the statistical technique, analytic and mathematical models, and radionuclide tracer method. Subsequently, the sediment transport patterns and possible sources were discussed. The sediment trapped mechanisms of the EFTF were diagnosed with clearly knowledge. Thereafter, the spatial and temporal principles of the erosion and accretion over EFTF were brought out with determination of the threshold value in shoreline changes that EFTF had occurred accretion seaward limitation. It can be expected that this study can deepen understanding of morphodynamic processes in the Changjiang Estuarine tidal flat. Meanwhile, this study will be helpful to evaluate sediment fate from river to ocean and safety of coastal engineering. Furtherly, the conclusions in this study will be an important theoretical support of sustainable development for estuarine tidal flat resources.

河口前沿边滩的冲淤敏感反映了陆海相互耦合作用的强弱，是河口环境变化的重要表征。同时，边滩的稳定与否直接关乎三角洲大城市沿海土地资源以及河口陆缘港工建筑的安全。以经受高强度人类活动影响的长江河口边滩为研究区，通过采集逐年、月和日高精度水文、泥沙与地形变化的野外现场观测资料，辅之不同时相的影像和历史相关数据，基于统计技术、解析和数学模型、放射性核素示踪等方法的计算分析与相互验证，探讨边滩的泥沙来源与输运方式，弄清边滩的泥沙捕集机制，进而提出边滩时空冲淤变化规律，确定边滩淤涨极限的岸线变化阈值，由此深化对长江河口边滩动力地貌过程的认识。预期成果既可丰富河口淤泥质潮滩研究的理论体系，为评估流域-海洋的泥沙宿命与港工建筑安全打下基础，又能为河口边滩土地资源的可持续发展提供理论支撑。

河口前沿边滩的冲淤敏感反映了陆海相互耦合作用的强弱，是河口环境变化的重要表征。同时，边滩的稳定与否直接关乎三角洲大城市沿海土地资源以及河口陆缘港工建筑的安全。以经受高强度人类活动影响的长江河口边滩为研究区，探讨边滩的泥沙来源与输运方式，弄清边滩的泥沙捕集机制，进而提出边滩时空冲淤变化规律，确定边滩淤涨极限的岸线变化阈值，由此深化对长江河口边滩动力地貌过程的认识。主要成果包括：三峡大坝运行引起河流泥沙急剧减少，中下游河床提供约50%的泥沙进入河口，河口浅滩和来自海域的泥沙也是三角洲边滩淤积的另一物源。当泥沙急剧下降，河口边滩具有自我调整能力。其中，南汇边滩等深线形态在1958—2013年间等深线结构出现相似性特征，不同年份的分维值差异不大，说明即使存在极端天气—台风和流域来水来沙减少等控制因素的影响，南汇边滩的动力地貌性质并没有发生“相转移”层面的改变，存在某种稳定机制。南汇边滩的淤积在长时间尺度上表现为-5m等深线的向海淤进、边滩等深线的标度保持稳定且没有表现出长期侵蚀趋势，指示三峡大坝所导致的入海泥沙锐减并未对其造成显著的侵蚀效应，南汇边滩并不受控于流域来沙的减少，而海洋作用对其演化有主导作用。低能环境下河口边滩冲淤的频率分布呈现高斯分布的特征，而人类活动的增强导致河口边滩冲淤的频率分布从高斯分布向幂律分布转化，表明人类活动对河口边滩不再是一个次要的动力，其正在深刻地改变河口边滩的动力地貌结构。此外，边滩在过去半世纪持续向海淤进33平方千米，因累积围垦202平方千米，使滩上实际滩涂资源储量锐减29%；河口工程导致大治河以北滩上发育Z型或L型剖面，致使滩面变陡限定可围垦或淤涨土地约80平方千米。成果既可丰富河口淤泥质潮滩研究的理论体系，为评估流域-海洋的泥沙宿命与港工建筑安全打下基础，又能为河口边滩土地资源的可持续发展提供理论支撑。