Wind-driven bed shear stress in the presence of dune grasses: implications for species-driven controls on coastal foredune development

Abstract:

Dunes are important geomorphic features in coastal settings due to the flood protection benefits and ecosystem services that they provide. Vegetation plays a principal role in the morphological evolution of coastal foredunes by reducing wind-driven bed shear stresses in the lee of plants and by trapping of sediment through collision of sediment particles with stems. Increasing evidence suggests that different coastal dune grasses influence the shape of coastal foredunes through these ecological controls on the transport field (e.g., Hacker et al., 2019). Analysis and modeling of coastal dune morphodynamics relies on parameterizing shear stress reduction in the presence of vegetation and its implications for wind-driven sediment fluxes. Work on coastal dunes has primarily relied on the methods of Raupach et al. (1993), who developed a shear coupling approach where element roughness is parametrized by lateral cover. However, this scheme relies on a homogeneous dispersal of plants (roughness elements) in a given patch. Work by Okin (2008) instead parameterizes shear stress reduction by the size of the average unvegetated gap size between plants. This approach has not been used in models of coastal dune ecomorphodynamics.

Here we present a hybrid field and numerical modeling study to assess the applicability of the Okin (2008) approach for replicating shear stress distributions in vegetated coastal dune environments. Measurements from horizontal and vertical arrays of 2D ultrasonic anemometers located adjacent to plant stands of different densities and species were collected in summer 2020 during moderate wind conditions at the U.S. Army Corps of Engineers’ Field Research Facility in Duck, NC. In conjunction with co-located ecological measurements, these wind data are used to test the applicability of the Okin method in a coastal dune setting. Given the suitability of this shear coupling model as demonstrated against the available field data for numerous plant configurations, the Okin method is subsequently added into the Aeolis aeolian sediment transport model to explore species-dependent sediment trapping rates on coastal foredunes.

Hacker et al. (2019). https://doi.org/10.3390/d11050082

Okin (2008). https://doi.org/10.1029/2007JF000758

Raupach et al. (1993). https://doi.org/10.1029/92JD01922