Ecogeomorphic Heterogeneity Sculpts Salt Marshes

We present cellular automata simulations and high-resolution field measurements of five sites along the United States Atlantic Coast, to investigate the erosion of marsh boundaries by wave action. For several years, we tracked marsh contours of three sites in Plum Island Sound and two sites in the Virginia Coastal Reserve using a Real-Time-Kinematic GPS, and measurements were collected up to 20 cm apart. The cellular automata model consists of a 2D square lattice, whose elements have randomly distributed resistance. Randomly distributed resistance values are meant to take into account the variety of biological and geomorphologic processes affecting each portion of the marsh. Among others, seepage erosion, crab burrowing, vegetation and sediment cohesion make difficult to predict which portion of the marsh will collapse first. In case of high wave power, erosion proceeds uniformly because each marsh portion has similar resistance if compared to the main external driver. On the contrary, when wind waves are weak and the local marsh resistance is strong, jagged marsh boundaries form. From a statistical viewpoint, the system behaves differently for the two extreme conditions of very low and very high wave power. The frequency magnitude distribution of erosion events approaches a Gaussian distribution in case of high wave power. In case of low wave power, the frequency magnitude distribution is characterized by a long-tailed power-law distribution. For the low wave power case, a long time is required to erode very resistant cells. However, once the most resistant cells are eliminated, several weak sites remain exposed and can be rapidly removed, with consequent generation of large-scale failures. Field data confirm model results, and show the passage from a logarithmic frequency magnitude distribution of erosion events to a Gaussian distribution for increasing wave power exposure. 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 self-organized criticality are geotechnical, biological, and related to the non-homogeneity of salt marshes whose material discontinuities act as stress raisers.