Predicting Bed Shear Stresses in Vegetated Channels

Abstract

Shear stresses on vegetated beds play an important role in driving a wide range of processes at the sediment‐water interface, including sediment transport. Existing methods for the estimation of bed shear stress are not applicable to vegetated beds due to the significant alteration of the near‐bed velocity profile and turbulence intensities by the vegetation. In addition, bed shear stress distributions are highly spatially variable in the presence of vegetation. In this study, computational fluid dynamics simulations were used to investigate the spatial variability of bed shear stresses in the presence of emergent vegetation (modeled as arrays of circular cylinders) and the variation of bed stress with characteristics of both the bulk flow and the array. A recently proposed model that assumes a linear variation of stress in the viscous layer immediately above the bed is shown to be a reliable tool for estimating the spatially averaged bed shear stress over a wide range of flow conditions and vegetation densities. However, application of this model is found to be restrictive due to the lack of a reliable predictive tool for the thickness of the viscous layer. Based on a balance between turbulent kinetic energy production in the vegetation stem wakes and the viscous dissipation of turbulent kinetic energy at the bed, an enhanced formulation is proposed to predict the thickness of the viscous layer, which significantly improves the accuracy of model predictions. This improved model enhances the predictive capability for important benthic processes (such as sediment transport) in vegetated aquatic systems.

Impact of Vegetation on Bed Load Transport Rate and Bedform Characteristics

“…At high stem Reynolds number Re d = Ud / ν >4/ C f , the viscous layer thickness so that the impact of vegetation on bed shear stress is negligible and τ = ρC f U 2 . Note that is a reasonable approximation when the stem Reynolds number Re d >1,200, which is true for all conditions considered here (based on Figure 14 in Etminan et al, ). For Re d <600 and vegetation volume fraction ϕ >0.1, an improved H v approximation model has been proposed by Etminan et al ().…”

“…is a reasonable approximation when the stem Reynolds number Re d > 1, 200, which is true for all conditions considered here (based on Figure 14 in Etminan et al, 2018). For Re d < 600 and vegetation volume fraction > 0.1, an improved H v approximation model has been proposed by Etminan et al (2018).…”

“…is a reasonable approximation when the stem Reynolds number Re d > 1, 200, which is true for all conditions considered here (based on Figure 14 in Etminan et al, 2018). For Re d < 600 and vegetation volume fraction > 0.1, an improved H v approximation model has been proposed by Etminan et al (2018). For turbulent flow, C f can be estimated from the sediment size d s and water depth h using the semiempirical equation (Whiting & Dietrich, 1990;Julien, 2010):…”

Hydrodynamic Controls on Primary Producer Communities in Spring‐Fed Rivers

“…Notably, many hydraulic-driven controls on primary producer abundance (e.g., uprooting) are inherently site-specific (Edmaier et al, 2011(Edmaier et al, , 2015, indirectly related to flow velocity and may function via reciprocal feedbacks associated with VP-sediment dynamics (Gurnell, 2014) or interactions between VP patches (Folkard, 2005 attributed the decline in VP abundance at high velocities to a lack of suitable benthic substrate for rooting, and Riis and Biggs (2003) suggested the negative relationship between velocity and VP abundance was due to uprooting or biomass loss. Both of these mechanisms are strongly dependent on near-bed hydrodynamics (e.g., shear stress), which is related to bulk flow velocity but also influenced by local morphology and vegetation conditions (Etminan et al, 2018), potentially obscuring the coherence of its effects on primary producer abundance when assessed across multiple sites (as in our study).…”

“…For example, Hoyer et al () attributed the decline in VP abundance at high velocities to a lack of suitable benthic substrate for rooting, and Riis and Biggs () suggested the negative relationship between velocity and VP abundance was due to uprooting or biomass loss. Both of these mechanisms are strongly dependent on near‐bed hydrodynamics (e.g., shear stress), which is related to bulk flow velocity but also influenced by local morphology and vegetation conditions (Etminan et al, ), potentially obscuring the coherence of its effects on primary producer abundance when assessed across multiple sites (as in our study).…”