Marsh boundary of marsh bordering Plum Island Sound at RefugeNorth, Rowley, MA.

We present high-resolution field measurements of five sites along the United States Atlantic Coast, and cellular automata simulations, to investigate the erosion of marsh boundaries by wave action. According to our analysis, when salt marshes are exposed to high wave energy conditions their boundaries erode uniformly. The resulting erosion events follow a Gaussian distribution, yielding a relatively smooth shoreline. On the contrary, when wind waves are weak and the local marsh resistance is strong, jagged marsh boundaries form. In this case, erosion episodes have a long-tailed frequency magnitude distribution with numerous low-magnitude events, but also high-magnitude episodes. The logarithmic frequency magnitude distribution suggests the emergence of a critical state for marsh boundaries, which would make the prediction of failure events impossible. Internal physical processes allowing salt marshes to reach this critical state are geotechnical and biological, and related to the nonhomogeneity of salt marshes whose material discontinuities act as stress raisers.

本研究针对美国大西洋沿岸5处测点开展高分辨率野外实测，并结合元胞自动机（cellular automata）模拟实验，以探究波浪作用下盐沼（salt marsh）边界的侵蚀过程。经分析可知，当盐沼处于高波浪能环境时，其边界会发生均匀侵蚀；由此产生的侵蚀事件服从高斯分布（Gaussian distribution），最终形成相对平整的岸线。与之相反，当风浪较弱且本地盐沼抗蚀能力较强时，则会形成锯齿状的盐沼边界。在此情形下，侵蚀事件的频幅分布呈长尾特征：既存在大量低强度侵蚀事件，也会出现高强度侵蚀事件。这种对数频幅分布表明盐沼边界进入了临界状态，使得失效事件的预测完全不可行。促使盐沼达到该临界状态的内在物理过程涵盖岩土与生物两类机制，且与盐沼的非均质性密切相关——盐沼内部的物质不连续面会充当应力集中源。