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.
Marsh contours have been tracked using a real time kinematic global positioning system and an electronic total station. Data were collected with an average resolution of 1 m. When marsh contours were characterized by significant variations in boundary geometry, measurements were taken as much as 20 cm apart. Marsh boundaries have been monitored every September from year 2008 to 2013 for three sites at the PIE LTER (Stackyard Rd, Parker River National Wildlife Refuge (North and South) in Plum Island Sound. For the two sites in the Virginia Coast Reserve (Hog Island and Chimney Pole), measurements were taken in March 2008 and August 2010. We define as magnitude of an erosion event, for a given year and for a certain point along the marsh boundary, the shortest distance of that point from the marsh boundary of the subsequent year.
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