海岸盐沼动力地貌演化的数值模拟研究

Salt marshes are valuable but delicate landforms undergoing degradation worldwide. The morphodynamical evolution of salt marshes is complex and involves the interactions of numerous factors, including the halophytic vegetation, hydrodynamics, sediment transport and topography. Numerical models are powerful tools for understanding morphodynamics of salt marshes. The newly developed DeltaRCM is a rule-based cellular morphodynamic model specifically designed for river-dominated deltas. DeltaRCM will be adapted into a morphodynamic model for salt marshes. The updated DeltaRCM calculates water level using simplified shallow water equations and thus accounts for tides. We intend to couple new components into the existing framework to capture the dynamics of the boundary between salt marshes and tidal flats and three ecological components that calculate the trapping rate due to the effect of the plant cover, the net organic production due to the presence of vegetation, the lateral expansion rate due to plant colonization. The ecological components in our coupled model will be based on empirical relationships extracted from field measurements and remote sensing data using multivariate statistical analysis. The updated model would reflect sedimentary and ecological processes and mimick the self-organizing evolution of tidal network. Using sensitivity analysis, the factors and feedbacks that control morphodynamics of typical salt marshes in China will be quantified, and how these salt marshes respond to natural environmental changes and human activities will be investigated. Thus the present conceptual models mainly grounded on researches of salt marshes in Europe and North America may be improved. Meanwhile, evolutionary differences between Suaeda marshes and Spartina alterniflora will be investigated, and mechanisms of Spartina alterniflora invading coastal habitats will be explored in regional and geomorphological perspectives.

海岸盐沼是重要却脆弱的自然系统，在全球面积缩减，功能退化。盐沼地貌演化是植被、水动力、泥沙和地形等众多因子相互作用、密切耦合的结果，具有复杂性。数值模拟是研究盐沼动力地貌演化的好方法。DeltaRCM是一个基于细胞自动机的河控三角洲演化模型，将其水面高程计算规则改为求解简化的浅水方程，加入盐沼前缘地形变化、有机质积累通量、植物黏附泥沙通量、植被扩张等模块（生态相关模块基于经验关系，利用野外观测数据和遥感影像，应用多元统计方法构建），以体现潮流、波浪和植被的作用，可以将其改进为盐沼地貌演化模型。新模型符合盐沼沉积动力学特征，可再现潮沟体系自组织演化过程。通过模型敏感性分析，探寻中国典型盐沼演化的控制因子、反馈机制及其对各种自然、人为环境变化的响应，完善已有基于欧美盐沼研究的演化模型；探寻碱蓬主导和互花米草主导的盐沼演化过程的差异，从区域特征和地貌演化角度，探索互花米草入侵海岸生态系统的机制。

海岸盐沼具有重要的生态功能。气候变暖、风暴潮加剧和人类活动，导致盐沼面积缩减、功能退化。探究盐沼地貌的演化规律和冲淤机制，可指导海岸湿地的保护和复原。项目开展了三项研究。（1）海岸盐沼沉积物抗侵蚀性的时空差异及其原因，及其在地貌演化模型中的应用。（2）江苏沿海米草盐沼的沉积速率和沉积环境变化的判别标志。（3）改进DeltaRCM三角洲演化模型的水沙模块，并借助水槽实验验证新模型的性能。.主要发现（1）如东研究区潮间带中上部沉积物夏季的侵蚀速率是冬季的1.85~1125倍。该差异由物理、生物和化学因素共同造成：风浪、冰冻、人类挖掘贝类以及底栖动物扰动，导致滩面沉积物的粒度、容重和有机质含量存在显著年际和季节差异。扰动对粒度大、有机质含量小的沉积物作用尤其显著，因此盐沼前部和前缘光滩的抗侵蚀性年际和季节差异最大。后者是近十年来盐城自然保护区的米草盐沼发生剧烈冲淤变化，并在前缘形成显著陡坎地貌的直接原因。因此，盐沼地貌演化模型必须考虑抗侵蚀性随深度变化的规律及其季节差异。.（2）如东和盐城地区的米草盐沼中部的长期沉积速率相近，分别为1.25 cm/a和3.3 cm/a。联合使用稳定碳同位素δ13C和叶绿素a指标，可准确指示米草盐沼沉积环境，而碳氮比是最不可靠的指示，沉积物峰度和砂百分比的联合指标是较为可靠的指示。.（3）应用沉积动力学原理，优化了DeltaRCM元胞自动机模型的水沙规则，大幅提高了模型的性能。新模型计算速度快于传统基于计算力学的模型，适合年代尺度的地貌演化模拟。但在准确地模拟5次水流改道后，随模拟时间增加，计算机模型的预测结果和水槽模拟结果差异加大，提示很多模块和参数有待优化、验证和率定。初步探索显示，基于复杂适应系统的宏观演化理论，整合沉积动力学等不同学科的微观规则，针对具体的科学问题制定简化方案，构建复杂度适中的元胞自动机模型，是有前景的地貌演化模拟手段。这类模型需要科学家群体持续投入，构建模型库，不断优化和创建元胞间相互作用的规则。