Sediment transport and wave attenuation behind patches of flexible aquatic vegetation: Findings from a full-scale laboratory experiment

Natural and nature-based features (NNBFs) have been shown to reduce wave loads on landward infrastructure and limit shoreline erosion, key services that may help buffer against local and global oceanic climate change. When present in aggregate form, a patch of aquatic vegetation, such as seagrass, dissipates wave energy by impeding flow, generating a drag force across the region and low-velocity wake downstream. Recent studies have shown that although denser patches of seagrass increase wave dissipation, a more distributed patch leads to larger regions of low-velocity flow downstream, resulting in enhanced sediment accumulation.

To investigate the impact of patch distribution on the concurrent processes of wave attenuation and downstream sediment deposition, common goals of NNBF restoration, a laboratory experiment was conducted at prototype scale in the large wave flume at the Hindsdale Wave Research Laboratory. An equal number of seagrass (Zostera marina) mimics were planted into a sediment-filled bed (d₅₀ ~ 200 μm) in circular patches of varying size. Patch diameters of 0.75-1.50 m corresponded to within-patch densities of 419-105 shoots/m². Vegetation patches were subjected to regular wave forcing with increasing intensity, taken from observed conditions in Z. marina habitats across the US West Coast.

Spatial patterns of sediment transport were observed to be consistent across patch trials, with scour occurring at the leading exposed patch edge, accretion at the trailing edge, and hummocks of downstream deposition. The length scale of these regions of deposition L_dep was shown to increase with increasing patch diameter D (L_dep ~ 1.0-1.4D). Preliminary analysis shows that wave transformation was less sensitive to patch diameter and density, as attenuation rates were relatively constant over the range of tested patch distributions. This work builds upon fundamental understanding of wave-sediment interactions with submerged flexible vegetation, a critical need in NNBF design guidance.