Modeling the Hydrodynamic Effects of Seagrass Meadows in a Shallow, Back-barrier Estuary

Seagrasses are often referred to as eco-engineers because they stabilize sediment beds and create favorable habitat for other organisms. Simulating the influence of seagrasses on estuarine habitat requires integration of their physical characteristics into hydrodynamic and sediment transport models. A flexible vegetation module has been implemented in the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system and tested first in idealized conditions. Under typical shallow water hydrodynamic conditions, the three-dimensional vegetation drag significantly dampens the current velocity (by about 80 % locally) and balances an enhanced pressure gradient. In addition, vegetation increases water column mixing (up to a factor of 2 at mid-depth) mostly due to the strong shear at the top of the submerged canopy. Flow accelerates on the sides of the patch and vortex circulation forms in the wake of the patch. Vegetation has a direct effect on wave damping and an indirect effect on wave propagation through its effects on surface current. We then applied the modeling system to Chincoteague Bay, MD/VA, to assess the physical role of seagrass meadows in a typical mid-Atlantic shallow back-barrier lagoon under a shifting influence of wave, wind, tidal and freshwater forcing. Different scenarios of vegetation distribution based on detailed historical maps provide a series of testable hypotheses to evaluate the buffering capabilities of submerged vegetation against storms and the sediment transport regime. Preliminary simulations accounting for the presence of seagrass show that storm surge is reduced by approximately 5 % and wind waves by around 40 % at the coast. Meanwhile, fine sediment deposition after storms is enhanced on the eastern part of the bay initially covered by sand over-wash from the barrier island. These results can be used to inform ecosystem-based management of coastal change hazards.