Vegetation Reconfigures Barrier Coasts and Affects Tidal Basin Infilling Under Sea Level Rise

Abstract

Worldwide, many tidal basins associated with barrier coasts have infilled over the past millennia due to the combination of sediment supply, wave-tidal sediment transport, and eco-engineering effects of vegetation. However, the biogeomorphological interactions between saltmarsh and the morphodynamics of an entire coastal barrier system are poorly understood, especially under sea level rise (SLR). Here, we study the evolution of a barrier coast for combinations of mud availability, presence of vegetation, and SLR. We developed a novel biogeomorphological model of an idealized barrier coast enclosing a tidal basin with sandy-clayey sediments that was subjected to tides and waves for a century. The morphodynamic Delft3D model was coupled to a vegetation code which accounts for the dynamics of marsh-type vegetation. Initially, vegetation contributed to reducing the tidal prism while sediment was imported. However, with SLR this trend was reversed and the tidal basins started to export sediment for vegetated runs after about 50–60 years while the unvegetated scenarios continued to infill in pace with the SLR. The sediment export was caused by cascading biomorphodynamic feedback effects triggered by vegetation which modified channel and shoal dynamics. Even under higher mud supply, the SLR resulted in vegetation collapse. The hypsometries, similar to natural systems, showed that vegetated systems converge to an alternative stable state condition. We conclude that the long-term resilience of the tidal basin associated with sediment infilling under SLR can be reduced by cascading large-scale effects of vegetation on the morphodynamics of barrier coasts.