Turbulence Dictates Bedload Transport in Vegetated Channels Without Dependence on Stem Diameter and Arrangement

Abstract

Aquatic vegetation is a crucial part of wetland and floodplain ecosystems. It stabilizes river banks (Hackney et al., 2020), protects coasts from waves and storm surges (Barbier et al., 2011), and provides habitat for fisheries (Costanza et al., 1997). Wetlands sustain themselves partially through their ability to retain and accrete sediment. However, the influence of vegetation on water motion and sediment transport is complex. On the one hand, vegetation provides additional drag, which reduces current (Kouwen & Unny, 1973), facilitating sediment deposition (Abt et al., 1994) and increasing bed elevation. On the other hand, vegetation generates turbulence (Tanino & Nepf, 2008;Xu & Nepf, 2020), which can enhance erosion (Niño & Garcia, 1996), alter the vertical distribution of suspended sediment (Tseng & Tinoco, 2021), and decrease bed elevation in the vicinity of vegetation (Norris et al., 2021;Yagci et al., 2016). Recent studies have proposed ways to incorporate the impacts of vegetation on bedload transport by considering the related process of scour hole formation (Wu et al., 2021), or by modifying the parameters of the Einstein (1950) probabilistic model (Armanini & Cavedon, 2019). A deeper understanding of sediment transport within aquatic ecosystems is still critical (Fagherazzi et al., 2017) to facilitate wetland protection and restoration projects (Paola et al., 2011).

Velocity, Turbulence, and Sediment Deposition in a Channel Partially Filled With a Phragmites australis Canopy

“…Similarly, the critical Shields parameter can be defined as $${\theta }_{\mathrm{c}}\left(=\frac{\rho \omega {k}_{t(c)}}{\left({\rho }_{\mathrm{s}}\,-\,\rho \right)g{d}_{\mathrm{s}}}\right)$$, with k t (c) the critical turbulence threshold for resuspension. As shown in Zhao and Nepf (2021), k t (c) can be estimated from the critical bed shear stress for bare beds, τ b(c) : $$${k}_{t(c)}=\frac{{\tau }_{\mathrm{b}(c)}}{\rho \omega }=\frac{{\theta }_{\mathrm{c}}\left({\rho }_{\mathrm{s}}-\rho \right)g{d}_{\mathrm{s}}}{\rho \omega }$$$ …”

“…In vegetated regions, the near-bed TKE, k t(nb) , is the dominant factor initiating resuspension (e.g., Liu et al, 2021;Zhang et al, 2020), so the dimensionless parameter θ has been modified to reflect this contribution (see Shan et al, 2020;Zhao & Nepf, 2021):…”

“…In vegetated regions, the near‐bed TKE, k t (nb) , is the dominant factor initiating resuspension (e.g., Liu et al., 2021; Zhang et al., 2020), so the dimensionless parameter θ has been modified to reflect this contribution (see Shan et al., 2020; Zhao & Nepf, 2021): $$$\theta =\frac{\rho \omega {k}_{t(\text{nb})}}{\left({\rho }_{\mathrm{s}}-\rho \right)g{d}_{\mathrm{s}}}$$$ …”

“…, with k t(c) the critical turbulence threshold for resuspension. As shown in Zhao and Nepf (2021), k t(c) can be estimated from the critical bed shear stress for bare beds, τ b(c) :…”

Simulation of Accelerated Subcritical Flow Profiles in an Open Channel with Emergent Rigid Vegetation

“…At the scale of the hydrographic network, the general velocity reduction influences the travel time of water particles, making easier the peak flow to control [27]. With reference to the hydraulic aspects, the focus of researchers, in addition to the flow resistance, is on the analysis of turbulence characteristics [28][29][30][31][32] and solid transport [15,18,[33][34][35].…”