Abstract
Local effects of flow interaction with seagrass structure modify meadow scale hydrodynamics, resulting in lower current velocities and wave heights within a seagrass meadow. This attenuation promotes the deposition of suspended sediment, increasing the light available locally to benthic organisms. To elucidate the relationship between small scale hydrodynamics that occur at the sea floor and the meadow scale effects of seagrass, high resolution velocity profiles were recorded adjacent to the sediment-water interface within a Zostera marina seagrass meadow in South Bay, Virginia. Additionally, instrumentation was deployed across the meadow to seasonally monitor suspended sediment concentrations and corresponding changes in wave height across the seagrass meadow. Results show that wave height was reduced by 25 – 49% compared to an adjacent bare site, and -19 – 64% compared to an analytical model of wave attenuation over an unvegetated seafloor with the same bathymetry. The lowest attenuation of waves values occurred in the winter and the highest attenuation occurred in the summer. Wave attenuation coefficients, αw, calculated for the expanse of meadow ranged from αw = 0.59 in the spring to 0.19 in the winter and were highly dependent on wave conditions, with greater attenuation for larger wave height, longer period waves.
Near bed flow characteristics showed a reduction in shear velocity and boundary layer thickness within the seagrass meadow, accompanied by lower average bed orbital velocities. Even in high wave conditions, the contribution to the total shear by turbulent motion was more prevalent than by wave motion within the seagrass canopy, however overall within canopy turbulent kinetic energy was reduced compared to the bare site. Comparing nearly identical wave conditions, shear velocities (u*) were the same at both the vegetated and unvegetated sites, suggesting that vertical orbital velocity attenuation through the water column was unaffected by seagrass. Lower wave heights within a seagrass meadow caused by horizontal attenuation of wave motion may be more influential for near bed shear conditions. Accurately predicting the location and thickness of the wave boundary layer was difficult due to the complexities of and non-linear interactions between currents, waves, and vegetation. Measured values of wave boundary layer thickness were an order of magnitude larger than predicted by theory over smooth beds, suggesting that both vegetation and benthic roughness dramatically alter near bed flow conditions.
