Vegetation and Shear Strength in a Delta-splay Mouth Bar

The mechanism by which new deltaic wetlands form is a complex suite of biological and physical processes that can modify one another. Understanding these processes and their interactions is imperative to successful coastal restoration. This study investigated the relationship between belowground plant biomass and sediment cohesion. We hypothesized that greater root densities increase shear strength, variably across plant communities, and that these communities are associated with distinct inundation regimes. A significant relationship was found between belowground biomass and surface shear strength when accounting for sediment grain size, water content, and organic matter content. Sites dominated by native graminoids or woody species had significantly higher shear strengths than unvegetated areas. However, sites dominated by Phragmites australis or forbs did not differ significantly in shear strength from the unvegetated sites. Sites dominated by Phragmites australis were also subject to significantly higher inundation rates during the previous water year than any other vegetation type. These results suggest that vegetation community differences lead to differences in shear strength that result in locally differential erosion rates, in turn modifying future geomorphology, hydrology, sedimentation, and vegetation distribution. This feedback implies that certain vegetation communities in wetland restoration projects could not only impart immediate erosion resistance to the substrate, but affect the long-term potential for land creation.