Vertical mixing in coastal canopies

Abdolahpour, Maryam; Ghisalberti, Marco; Lavery, Paul S.; McMahon, Kathryn

doi:10.1002/lno.10368

Cited by 22 publications

(42 citation statements)

References 63 publications

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“…Vertical mixing in rigid coastal canopies is characterized by a coupled contribution of both shear‐ and wake‐driven mixing (Abdolahpour et al ) and can be quantified by

D_{t, z} = 0.043 normalΔ {UL}_{D} + 0.58 \sqrt[3]{\frac{w}{L_{D}}} U_{c}^{rms} w

…”

Section: Resultsmentioning

confidence: 99%

“…The first ratio ( B ) is that of the buoyancy force to rigidity, expressed for rectangular blades as:

B = \frac{F_{normalB}}{F_{normalR}} = \frac{12 ((), ρ_{w} - ρ_{s}) {gl}^{3}}{{italicEt}^{2}}

where ρ w , ρ s , l , t , and E are the density of water, the density of seagrass, the blade length, the blade thickness and the modulus of elasticity of the blade, respectively (Luhar and Nepf ). The second dimensionless ratio, the Cauchy number ( Ca ), is defined as the ratio of the drag to rigidity, and is expressed for rectangular blades as:

italicCa = \frac{F_{normalD}}{F_{normalR}} = \frac{6 ρ_{w} C_{D} l^{3} {(U_{c})}^{2}}{{italicEt}^{3}}

where U c is the amplitude of the in‐canopy orbital velocity (Ghisalberti and Nepf ; Luhar and Nepf ) and C D ≈ 1 (after Abdolahpour et al ). The Cauchy number sets the plant posture; at high Ca , the hydrodynamic forcing is large enough to overcome blade stiffness, and the blade more readily adjusts its posture in response to the flow (Luhar and Nepf ).…”

Section: Methodsmentioning

confidence: 99%

“…In the flexible canopies, vertical measurements were taken every 1 cm (starting at 2 cm from the bed) for z ⩽ 16 cm, and continued every 2 cm for z > 16 cm. This allowed focus on the near‐bed region, where the drag, turbulence, and shear were expected to be greatest (Lowe et al ; Abdolahpour et al , b ).…”

Section: Methodsmentioning

confidence: 99%

“…KH-vortices are only generated at the top of the canopy when KC is sufficiently large, i.e., KC > 5 (and Re > 1000) (Ghisalberti and Schlosser 2013). Although shear-driven vortices dominate vertical exchange in steady flows, mixing in coastal canopies is dictated by a coupled contribution of both shear-and wake-driven mixing (Abdolahpour et al 2017a). Wake turbulence generated behind each canopy element increases in strength with the in-canopy velocity (Nepf 1999;Abdolahpour et al 2017a).…”

Section: Hydrodynamics Of Oscillatory Canopy Flowsmentioning

confidence: 99%

“…There has been growing interest in the structure of oscillatory (Pujol et al 2013a) and turbulent flows (Reidenbach et al 2007;Pujol et al 2013b) in submerged canopies, as well as in understanding vertical mass (Nishihara et al 2011;Abdolahpour et al 2017a) and momentum transport (Ghisalberti and Schlosser 2013). The majority of physical and numerical modeling efforts have used rigid cylinders to simulate canopy elements, as this allows the canopy geometry to be invariant over time and therefore easily quantified.…”

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confidence: 99%

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The impact of flexibility on flow, turbulence, and vertical mixing in coastal canopies

Abdolahpour

Ghisalberti

McMahon

et al. 2018

Limnology & Oceanography

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Physical modeling of canopy‐flow interactions has mostly employed rigid model vegetation, whereby the canopy geometry (i.e., its height and frontal area) is invariant and easily quantified. Here, we demonstrate that embedding realism in model vegetation, in the form of buoyancy and flexibility, can profoundly impact the structure of the flow and rates of vertical mixing in wave‐dominated conditions. A laboratory investigation was undertaken with two types of model canopy: (1) rigid canopies consisting of wooden dowels, and (2) flexible, buoyant model plants designed to mimic meadows of the seagrass Posidonia australis. To isolate the impact of flexibility, the maximum heights and frontal areas of the two types of canopy were matched. These canopies were subjected to oscillatory flows with a realistic range of wave heights and periods. Drag reduction caused by the reconfiguration of flexible canopies leads to a greatly diminished velocity attenuation in the canopy (by, on average, 65%). The reduced average height of flexible canopies shifts the canopy shear layer toward the bed, resulting in significantly enhanced levels of near‐bed turbulence. Finally, a decreased vertical diffusivity (by approximately 35%) was observed in the flexible model canopies, relative to the rigid analogues. Thus, while the use of dynamically scaled vegetation adds complexity to modeling efforts, it represents a step toward a more accurate quantitative understanding of flow and mixing in these environments.

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“…Vertical mixing in rigid coastal canopies is characterized by a coupled contribution of both shear‐ and wake‐driven mixing (Abdolahpour et al ) and can be quantified by

D_{t, z} = 0.043 normalΔ {UL}_{D} + 0.58 \sqrt[3]{\frac{w}{L_{D}}} U_{c}^{rms} w

…”

Section: Resultsmentioning

confidence: 99%

“…The first ratio ( B ) is that of the buoyancy force to rigidity, expressed for rectangular blades as:

B = \frac{F_{normalB}}{F_{normalR}} = \frac{12 ((), ρ_{w} - ρ_{s}) {gl}^{3}}{{italicEt}^{2}}

italicCa = \frac{F_{normalD}}{F_{normalR}} = \frac{6 ρ_{w} C_{D} l^{3} {(U_{c})}^{2}}{{italicEt}^{3}}

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Hydrodynamics Of Oscillatory Canopy Flowsmentioning

confidence: 99%

mentioning

confidence: 99%

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The impact of flexibility on flow, turbulence, and vertical mixing in coastal canopies

Abdolahpour

Ghisalberti

McMahon

et al. 2018

Limnology & Oceanography

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The wave‐driven current in coastal canopies

2017

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Wave‐driven flows over canopies of aquatic vegetation (such as seagrass) are characterized by the generation of a strong, shoreward mean current near the top of the canopy. This shoreward drift, which is observed to be up to 75% of the RMS above‐canopy orbital velocity, can have a significant impact on residence times within coastal canopies. There have been limited observations of this current and an accurate formulation of its magnitude is still lacking. Accordingly, this study aims to develop a practical relationship to describe the strength of this current as a function of both wave and canopy characteristics. A simple model for the Lagrangian drift velocity indicates that the magnitude of the wave‐driven current increases with the above‐canopy oscillatory velocity, the vertical orbital excursion at the top of the canopy, and the canopy density. An extensive laboratory study, using both rigid and (dynamically scaled) flexible model vegetation, was carried out to evaluate the proposed model. Experimental results reveal a strong agreement between predicted and measured current velocities over a wide and realistic range of canopy and wave conditions. The validity of this model is also confirmed through available field measurements. Characterization of this wave‐induced mean current will allow an enhanced capacity for predicting residence time, and thus key ecological processes, in coastal canopies.

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Wave-induced steady currents by submerged canopy-oscillatory flow interaction

Caceres¹,

Orfila²,

Maza³

et al. 2021

Preprint

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A new analytical model is presented in order to better understand the depth-dependent wave-induced steady current caused by submerged aquatic canopy-oscillatory flow interaction. The analytical model takes into account the wave and canopy properties. The model is developed by determining the dominant terms in the momentum equation by means of dimensional analysis and satisfying the mass conservation. The dimensional analysis reveals that the pressure gradient (due to wave decay) is of the same order of magnitude as the drag force, wave stress and Reynolds stress terms. In addition, the balance between the pressure gradient and mass conservation induces a seaward current above the canopy, and the presence of the pressure gradient in the momentum equation contributes to intensify the skimming flow at the top of the canopy. Finally, given that the model follows a polynomial function it can be easily implemented in large scale models such as phase average models.

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Vertical mixing in coastal canopies

Cited by 22 publications

References 63 publications

The impact of flexibility on flow, turbulence, and vertical mixing in coastal canopies

The impact of flexibility on flow, turbulence, and vertical mixing in coastal canopies

The wave‐driven current in coastal canopies

Wave-induced steady currents by submerged canopy-oscillatory flow interaction

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