Mechanisms of sediment transport around finite patches of submerged aquatic vegetation

Feedback between flow transformation by submerged aquatic vegetation (SAV) and bed morphodynamics influences the long-term survivability of SAV habitats and the engineering services that SAV provides. We conducted a full-scale wave flume experiment to investigate the mechanisms through which finite patches of mimic SAV produce shoreward depositional features. Hydrodynamic observations indicate that a depositional mound is built in two phases, depending on the strength of nearshore seaward mean currents (undertow). During milder wave conditions when undertow is weak, the patches enhance TKE, wave velocity skewness, and within-canopy shoreward currents that together suspend and transport sediment shoreward to form near symmetrical depositional mounds. Although patch-induced transport mechanisms are greater with higher within-patch stem density, the size and volume of the depositional mound are shown to depend more on patch diameter (i.e., form drag) rather than stem density for the range of patch geometries tested; the small dense patch produced a smaller depositional mound than the larger sparser patch. When waves are more energetic and undertow strengthens, the convergence of undertow with canopy-induced shoreward currents and wave velocity skewness enhances deposition and skews the depositional mound's shape. As such, the convergence of canopy-induced shoreward currents and undertow-induced seaward currents causes the volume of the depositional mound to nearly triple shoreward of the largest tested patch of SAV.

Automated summary

The study shows that submerged aquatic vegetation has significant impacts on bed morphology through transformation of flow, affecting the long-term survival of habitats and engineering services. Experimental results indicate that the size and volume of vegetated patches are more influenced by patch diameter rather than stem density.