Flow and Wake Length Downstream of Live Submerged Vegetation Patches: How Do Different Species and Patch Configurations Create Sheltering in Stressful Habitats?

Vegetated canopies modify water conveyance and bed stabilization in aquatic ecosystems, providing shelter for other organisms. The sheltering effect downstream of a canopy is, however, rarely quantified for live vegetation, and it is unknown how this effect is influenced by vegetation patch configuration and characteristics. We measured the sheltering effect of different species and patch configurations by comparing the length of the steady wake region, defined in this study as (a) the zone where streamwise velocity is diminished to more than half of the incoming velocity (L-w1), and (b) the distance downstream of the patch where velocity begins to increase (L-w2). Then, we measured the drag experienced by a plant at different downstream locations as a measure of sheltering for plants growing in the steady wake. We found that L-w1 for channel-spanning cases was longer than for finite-width cases and extended to >= 2.5 patch lengths from the patch leading edge, while L-w2 for finite-width patches was related to the smallest dimension between canopy half-width and height. Species effects were consistent across definitions: the species with the longest wake lengths was G. densa, followed by C. platycarpa and V. spiralis, which showed similar wake lengths for finite-width cases. Drag reduction was related to downstream flow modification for finite-width patches, but not for channel-spanning ones. The results of this study indicate that patterns of hydrodynamic sheltering behind live patches can be complex and difficult to predict based only on patch dimensions, especially due to covariation between patch characteristics in real vegetation.