How does vegetation community shape geomorphological evolution? Tradeoffs among tide, sediment fluxes and vegetation configuration in the Virginia Coast Reserve (VA).

During recent decades coastlines have experienced unprecedented morphological modifications caused by sea level rise, subsidence, extreme events like hurricanes and a reduction of sediment supply. Most previous modeling studies on flow-vegetation-sediment interactions have focused on one specific vegetated community, but we lack a general understanding of the conditions that lead to the emergence of multiple vegetation species feedbacks. Using a modeling approach, this study generates new understanding of how sediment transport and ecogeomorphic interactions involving water flow, sediment, and vegetation influence landscape in coastal wetlands. The broad goal of this project is to distinguish between the influence of saltmarsh and seagrass experiencing different feedback and forcing on sediment deposition. We focus our study on the Virginia Coast Reserve LTER site, where we apply numerical modeling (Delft-3D) and subsequent analyses to determine the sets of environmental conditions under which eco-geomorphological feedbacks drive the wetland system to different landscape structures. Our numerical results show that salt marsh and seagrass reduce the volume of water in a shallow coastal bay up to 15% during each tidal cycle when compared to the case without vegetation. From a morphodynamic point of view, our study reveals the important role that vegetation plays in altering water residence times and increasing sedimentation in the bay. Vegetation also affects bay geomorphology by locally reducing bed shear stress and hence increasing sediment deposition. By evaluating alternative densities, heights and spatial distributions of seagrass and salt marsh, we develop different future projections that should be considered in the design of restoration strategies in shallow coastal bays such as those in the Virginia Coast Reserve.