The thresholds of sediment resuspension within emergent vegetation under combined wave-current conditions – A flume experiment

Abstract

The threshold of resuspension can be described by a critical velocity. Salt marsh vegetation modifies the flow conditions such that the critical velocities for resuspension derived on bare beds no longer apply. Within vegetation canopies, the critical velocity for resuspension was previously shown to decrease under pure current and pure wave conditions. In this study, we experimentally investigated how emergent salt marsh vegetation affects resuspension under combined wave-current flows that are typical for intertidal coastlines. Emergent artificial vegetation canopies inspired by Spartina Anglica and fine noncohesive sediment were placed in a wave-current flume. We set a constant current and incrementally increased the wave height until resuspension was measured. We repeated this procedure with a bare bed, three stem densities, and for a range of wave-current combinations. The critical velocity for resuspension was significantly reduced by vegetation-induced turbulence. Turbulent vortices shed by the stems entrained sediment particles from the bed. The critical velocity for resuspension decreased by 35–64% compared to bare bed conditions, depending on a ratio between the current velocity and wave velocity amplitude. The critical velocity decreased the most under pure current flows and the least under pure wave conditions. Finally, with vegetation present, the stem density did not affect the threshold of resuspension for the range of densities tested here. Our results can be implemented into sediment transport models through an adjusted Shields parameter for vegetated beds, which can be used for the management of salt marshes and the design of interventions for nature-based coastal protection.