Wind-Induced Hydrodynamic Interactions With Aquatic Vegetation in a Fetch-Limited Setting: Implications for Coastal Sedimentation and Protection

Abstract

Interactions between a patchy degraded Zostera noltei seagrass meadow and waves, currents, and sedimentary processes were analyzed from data obtained from a strongly wind-influenced micro-tidal brackish water lagoon in southeastern France. Measurements were conducted on offshore and foreshore morphology (topography, bathymetry), on hydrodynamics (waves, water levels, and currents) under different wind conditions within and outside the meadow, and on meadow biometry (shoot density, leaf length). The main impact of this patchy meadow on wind-wave transformations seems to be attenuation of waves further offshore than in the absence of vegetation. This attenuation is particularly notable above the meadow front edge, and is related to wave heights, water levels, and wave periods that are, in turn, dependent on wind intensity and fetch length. The data show that the patchy meadow does not attenuate small and short waves, especially when water levels are high, but is capable, like salt marshes and artificial seagrass, of attenuating relatively high and long waves. Notwithstanding its patchy and degraded character, the meadow also strongly influences the vertical distribution of currents. Whereas currents are strong and significantly influenced by wind and wind waves above the meadow, both waves and currents are dissipated in a transitional canopy-water layer. These wave and current modifications are reflected in the evolution of the seabed. Erosion and sedimentation are mainly controlled by the hydrodynamics but the seasonal state of the meadow plays a role by modulating the hydrodynamics. These substrate changes are, important, in turn, in influencing protection of the shoreline.