Impact of Vegetation‐Generated Turbulence on the Critical, Near‐Bed, Wave‐Velocity for Sediment Resuspension

Abstract

Laboratory experiments examined the impact of model vegetation on wave‐driven resuspension. Model canopies were constructed from cylinders with three diameters (d = 0.32, 0.64, and 1.26 cm) and 12 densities (cylinders/m2) up to a solid volume fraction (ϕ) of 10%. The sediment bed consisted of spherical grains with d50 = 85 μm. For each experiment, the wave velocity was gradually adjusted by increasing the amplitude of 2‐s waves in a stepwise fashion. A Nortek Vectrino sampled the velocity at z = 1.3 cm above the bed. The critical wave orbital velocity for resuspension was inferred from records of suspended sediment concentration (measured with optical backscatter) as a function of wave velocity. The critical wave velocity decreased with increasing solid volume fraction. The reduction in critical wave velocity was linked to stem‐generated turbulence, which, for the same wave velocity, increased with increasing solid volume fraction. The measured turbulence was consistent with a wave‐modified version of a stem‐turbulence model. The measurements suggested that a critical value of turbulent kinetic energy was needed to initiate resuspension, and this was used to define the critical wave velocity as a function of solid volume fraction. The model predicted the measured critical wave velocity for stem diameters d = 0.64 to 2 cm. Combining the critical wave velocity with an existing model for wave damping defined the meadow size for which wave damping would be sufficient to suppress wave‐induced sediment suspension within the interior of the meadow.

Impact of Stem Size on Turbulence and Sediment Resuspension Under Unidirectional Flow

“…Specifically, resuspension in wave flows has been linked to a constant turbulence threshold over a range of stem densities, and a smaller k t(c) has been reported for finer sediment. Tinoco and Coco (2018) observed k t(c) ≈ 7 cm 2 /s 2 for d s = 0.28 mm (Figure 9d in their paper), and Tang et al (2019) measured a smaller threshold (k t(c) ≈ 3 cm 2 /s 2 ) for finer sediment (d s = 85 μm). Compared to this study in unidirectional current, the two oscillatory flow studies report relatively smaller k t(c) , considering their larger grain sizes.…”

“…Tinoco and Coco (2018) observed k t ( c ) ≈ 7 cm 2 /s 2 for d s = 0.28 mm (Figure 9d in their paper), and Tang et al. (2019) measured a smaller threshold ( k t ( c ) ≈ 3 cm 2 /s 2 ) for finer sediment ( d s = 85 μm). Compared to this study in unidirectional current, the two oscillatory flow studies report relatively smaller k t ( c ) , considering their larger grain sizes.…”

Turbulence and Particle Deposition Under Steady Flow Along a Submerged Seagrass Meadow

“…In contrast, for low meadow density (small ah ), near‐bed turbulence may be enhanced by two mechanisms. First, velocity is not significantly diminished within a low‐density meadow, such that individual seagrass shoot may generate turbulence (e.g., Tang et al, 2019; Tinoco & Coco, 2016; Zhang et al, 2018). Second, turbulence generated in the shear layer at the top of the meadow can penetrate to bed and enhance resuspension (Luhar et al, 2008).…”

Turbulence and Bed Load Transport in Channels With Randomly Distributed Emergent Patches of Model Vegetation

“…Recent studies have suggested that near‐bed turbulence, k t , may be a better predictor of sediment transport. For example, the initiation of both bed load and suspended load transport in vegetated channels can be better described by a threshold value of k t than of τ (Tang et al, 2019; Tinoco & Coco, 2018; Yang et al, 2016).…”