Estimation of the bed shear stress in vegetated and bare channels with smooth beds

Yang, Judy Q.; Kerger, François; Nepf, Heidi

doi:10.1002/2014wr016042

Cited by 77 publications

(142 citation statements)

References 40 publications

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“…In the Widdows study, the relation for open channel flow, τ = 0.19 k t , was used to estimate the mean bed stress from the measured turbulent kinetic energy

k_{t} = ((), \overset{{u^{'}}^{2}}{false} + \overset{{v^{'}}^{2}}{false} + \overset{{w^{'}}^{2}}{false}) / 2

, with

u^{'}

v^{'}

, and

w^{'}

denoting the velocity fluctuation [ Stapleton and Huntley , ]. However, as discussed in Nepf [], Ricardo et al [], and Yang et al [], this relation assumes that turbulence production is linked to bed stress, which is not true in vegetated systems, for which turbulence production is primarily associated with the vegetation. Therefore, Widdows' conclusion that the critical τ was unchanged between bare and vegetated beds [ Widdows et al , , Figure 6] was incorrect, and, in fact, their data actually show that the threshold for erosion was defined by a critical value of k t .…”

Section: Theorymentioning

confidence: 99%

The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

Yang

Chung

Nepf

2016

Geophysical Research Letters

111

181

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This laboratory study advances our understanding of sediment transport in vegetated regions, by describing the impact of stem density on the critical velocity, Ucrit, at which sediment motion is initiated. Sparse emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different stem densities. The sediment transport rate, Qs, was measured over a range of current speeds using digital imaging, and the critical velocity was selected as the condition at which the magnitude of Qs crossed the noise threshold. For both grain sizes considered here (0.6–0.85 mm and 1.7–2 mm), Ucrit decreased with increasing stem density. This dependence can be explained by a threshold condition based on turbulent kinetic energy, kt, suggesting that near‐bed turbulence intensity may be a more important control than bed shear stress on the initiation of sediment motion. The turbulent kinetic energy model unified the bare bed and vegetated channel measurements.

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“…In the Widdows study, the relation for open channel flow, τ = 0.19 k t , was used to estimate the mean bed stress from the measured turbulent kinetic energy

k_{t} = ((), \overset{{u^{'}}^{2}}{false} + \overset{{v^{'}}^{2}}{false} + \overset{{w^{'}}^{2}}{false}) / 2

, with

u^{'}

v^{'}

, and

w^{'}

Section: Theorymentioning

confidence: 99%

The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

Yang

Chung

Nepf

2016

Geophysical Research Letters

111

181

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“…However, the effect of the spanwise velocity is second order and has been neglected for simplicity (see section for more detail). In addition, the spatially averaged LSM friction velocity ⟨ u * ⟩ LSM and the thickness of the viscous layer H v were determined by a least squares fitting of the temporally and spatially averaged streamwise velocity vertical profile

(⟨⟩, \overset{true¯}{u})

to the analytical profiles obtained through assumption of a linear stress profile:

(⟨⟩, \overset{true¯}{u}) = ({, \begin{matrix} \frac{{(⟨⟩, u_{*})}_{LSM}^{2}}{ν} ((), z - \frac{z^{2}}{2 H_{v}}), z \leq H_{v} \\ \frac{{⟨u_{*}⟩}_{LSM}^{2} H_{v}}{2 ν}, z \geq H_{v} \end{matrix})

(Yang et al, ). Consequently, ⟨ u * ⟩ LSM can be expressed as

{⟨u_{*}⟩}_{LSM} = \sqrt{\frac{2 ν U_{o}}{H_{v}}}

where U o is the uniform streamwise velocity in the upper layer ( z ≥ H v ).…”

Section: Numerical Modelingmentioning

confidence: 99%

“…While it is not yet clear if sediment transport within vegetation canopies can be predicted based on the bed shear stress alone, it is reasonable to expect that bed shear stresses play a contributing role (Nepf, ). In search of a predictive tool for vegetated bed shear stress, Yang et al () proposed a Linear Stress Model (LSM) that defines a viscous layer with a thickness of H v immediately above the bed, within which the turbulent stress is negligible and the viscous stress decreases linearly with distance from the bed, resulting in a parabolic velocity profile. This assumption, which indicates that the bed shear stress is governed by the thickness of the viscous layer H v , holds over most of the viscous layer except very close to the bed ( z + = zu * / ν < 5 with u * the friction velocity and ν the kinematic viscosity) where the viscous stress is constant (Kundu et al, ; Figure ).…”

Section: Introductionmentioning

confidence: 99%

“…Above this transition Reynolds number, H v is assumed to be the same in both bare and vegetated channels. Therefore, Yang et al () proposed that the spatially averaged thickness of the viscous layer could be estimated as H v = min ( d /2, 22 ν / u * ), where the latter term in the parentheses denotes the value for a bare bed. However, the physical mechanisms governing H v need to be further investigated over a wider range of Reynolds number and canopy density to ensure this relationship is broadly applicable.…”

Section: Introductionmentioning

confidence: 99%

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Predicting Bed Shear Stresses in Vegetated Channels

Farooji

Ghisalberti

Lowe

2018

Water Resources Research

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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.

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“…The effects of the stems on the flow are dependent on stem diameter and density, as well as flow conditions (Nepf, , , ; White and Nepf, ; Luhar et al, ; Tanino and Nepf, ; Larsen et al ., 2009; Zong and Nepf, ; Chen et al, ; Follett and Nepf, ; Siniscalchi and Nikora, ; Yager and Schmeeckle, ; Yang et al, ). Very dense vegetation has been shown to sufficiently damp turbulence to promote deposition (Nepf, , ; Zong and Nepf, , ), but turbulence and deposition are also functions of the local spatial distribution of vegetation, which tends to be non‐uniform in nature (Jansen and Nanson, ; Neary et al, ; Kleinhans et al, ).…”

Section: Introductionmentioning

confidence: 99%

Delta size and plant patchiness as controls on channel network organization in experimental deltas

Piliouras

Kim

2018

Earth Surf Processes Landf

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Understanding the feedbacks between water, sediment, and vegetation in deltas is an important part of understanding deltas as ecomorphodynamic systems. We conducted a set of laboratory experiments using alfalfa (Medicago sativa) as a proxy for delta vegetation to investigate: (1) the effects of plants on delta growth and channel network formation; and (2) the timescales controlling delta evolution in the presence of plants. Experiments were conducted with fluctuating discharge (i.e. flood and base flow periods) and variable seeding densities. We found that when deltas were small, channels had no memory across flood cycles, as floods could completely fill the incised channel network. When deltas were large, the larger channel volume could remain underfilled to keep channel memory. Plant patches also helped to increase the number of channels and make a more distributive network. Patchiness increased over time to continually aid in bifurcation, but as vegetation cover and patch sizes increased, patches began to merge. Larger patches blocked the flow to enhance topset deposition and channel filling, even for the case of large deltas with a high channel volume. We conclude that both plant patchiness and delta size affect the development of the channel network, and we hypothesize that their influences are manifested through two competing timescales. The first timescale, Tv, defines the time when the delta is large enough for channels to have memory (i.e. remain underfilled), and the second, Tp, defines the time when vegetation patches merge, amplifying deposition and blocking channels. When run time is between these two timescales, the delta can develop a persistent distributary network of channels aided by bifurcation around plant patches, but once Tp is reached, the channel network can again be destroyed by vegetation. © 2018 John Wiley & Sons, Ltd.

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Estimation of the bed shear stress in vegetated and bare channels with smooth beds

Cited by 77 publications

References 40 publications

The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

Predicting Bed Shear Stresses in Vegetated Channels

Delta size and plant patchiness as controls on channel network organization in experimental deltas

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