The contrasting effects of vegetation height and density on flow and turbulence in a patch of vegetation

Coastal vegetation often grows in spatially distributed patches. However, the influence of individual vegetation patches on small-scale hydrodynamics has not been well characterized under natural conditions with tidally varying water depths. We present measurements from manipulative field experiments by creating artificial patches of vegetation mimics with different patch heights and densities. We found that increased vegetation density substantially reduces flow speeds and turbulent kinetic energy (TKE) within the patch. However, for submerged patches, an increase in vegetation height induced a contrasting effect, causing faster flows and greater TKE within the patch, despite an increase in the total vegetation frontal area exerting drag. Denser patches showed the same, although more pronounced, trend. Existing analytical models failed to accurately reproduce both the observed patterns and magnitudes in velocity and TKE as functions of vegetation height and density. In contrast, the corresponding numerical hydrodynamic simulations captured both patterns and magnitudes well. These results underscore the need to correctly account for such opposing effects of vegetation geometries on flow and turbulence, and hence on sediment transport when predicting geomorphic evolution in coastal vegetation habitats.