What controls marsh edge erosion?

Abstract

The survival of salt marshes depends on their ability to maintain vertical elevation and areal extent. In the lateral direction, marsh edges can expand laterally or undergo edge erosion through mass failure or continuous particle erosion through waves and tidal processes. In this study, we evaluate possible relationships between marsh shoreline type within the Great Marsh in Massachusetts and major geotechnical parameters along the marsh edge. We also explore if wave energy, using fetch as a proxy, affects the presence, type, and distribution of shoreline type. We mapped and classified the marsh into four categories: slumping, vertical and abrading, stable/accretionary, and bedrock or gravel, and sampled the marsh edge at 98 sites. Using over 450 measurements, we present typical ranges of values at these marsh edges for bulk density (0.10–1.43 g/cm3), organic content (0.99–55.07%), belowground biomass (0.11–36.76%), and shear strength (4.04–136.49 kPa at 20 cm into the marsh bank, and 4.04–131.03 kPa at 40 cm into the bank). We show that there are no significant differences in fetch or geotechnical properties for the different marsh edge classes. Thus, none of these parameters explain or correlate with edge erosion, even though the majority of previous edge erosion studies focus on these parameters as determinants of edge erosion. We further emphasize the heterogeneity of the marsh, as edge erosion can occur in highly exposed or sheltered areas alike with no trends in geotechnical properties, and that complex interactions between parameters not generally studied may be responsible for edge erosion.