Dynamics of skimming flow in the wake of a vegetation patch

Mayaud, Jerome R.; Wiggs, Giles F.S.; Bailey, Richard M.

doi:10.1016/j.aeolia.2016.08.001

Cited by 40 publications

(37 citation statements)

References 72 publications

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“…Grasses and shrubs are collectively defined as elements with growing points that are mainly located at ground level, while trees are distinguished as having a distinctive trunk below a canopy [ Leenders et al ., ]. In the cases of grasses and shrubs, it has been shown in field and wind tunnel experiments [e.g., Bradley and Mulhearn , ; Vigiak et al ., ; Leenders et al ., ; Gillies et al ., ; Wu et al ., , Mayaud et al ., , ] that the surface wind velocity in the wake ( u surf ) is lower than in the absence of plants ( u ref ) and exponentially recovers with increasing downwind distance. Therefore, in the ViSTA model a zone of reduced wind velocity in the wake of grasses and shrubs is parameterized using an exponential curve:

u_{surf} = ((),, u_{ref} - u_{0}) . ((),, 1 - e^{- b \frac{x}{h}}) + u_{0}

where u 0 is the minimum wind velocity in the direct lee of the nearest upwind element,

\frac{x}{h}

is the downwind distance from the nearest element in terms of element height, and b is a fitted coefficient.…”

Section: Model Descriptionmentioning

confidence: 99%

A coupled vegetation/sediment transport model for dryland environments

2017

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Dryland regions are characterized by patchy vegetation, erodible surfaces, and erosive aeolian processes. Understanding how these constituent factors interact and shape landscape evolution is critical for managing potential environmental and anthropogenic impacts in drylands. However, modeling wind erosion on partially vegetated surfaces is a complex problem that has remained challenging for researchers. We present the new, coupled cellular automaton Vegetation and Sediment TrAnsport (ViSTA) model, which is designed to address fundamental questions about the development of arid and semiarid landscapes in a spatially explicit way. The technical aspects of the ViSTA model are described, including a new method for directly imposing oblique wind and transport directions onto a cell‐based domain. Verification tests for the model are reported, including stable state solutions, the impact of drought and fire stress, wake flow dynamics, temporal scaling issues, and the impact of feedbacks between sediment movement and vegetation growth on landscape morphology. The model is then used to simulate an equilibrium nebkha dune field, and the resultant bed forms are shown to have very similar size and spacing characteristics to nebkhas observed in the Skeleton Coast, Namibia. The ViSTA model is a versatile geomorphological tool that could be used to predict threshold‐related transitions in a range of dryland ecogeomorphic systems.

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u_{surf} = ((),, u_{ref} - u_{0}) . ((),, 1 - e^{- b \frac{x}{h}}) + u_{0}

where u 0 is the minimum wind velocity in the direct lee of the nearest upwind element,

\frac{x}{h}

is the downwind distance from the nearest element in terms of element height, and b is a fitted coefficient.…”

Section: Model Descriptionmentioning

confidence: 99%

A coupled vegetation/sediment transport model for dryland environments

2017

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Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…However, whilst windbreak studies can be useful proxies for vegetation elements, the three-dimensional nature of live plants (as opposed to the two-dimensional problem represented by a fence) means that findings may not be completely transferable [46,54,64,77]. The recovery length downwind of a vegetation patch or fence has been shown to be significantly longer than in the case of single vegetation elements [46,54,62,64,77,81,83,91,98,102]. The difference in velocity recovery length has been attributed to a lack of flow moving laterally around a patch [77] or a fence where the obstacle width is assumed to be infinite [63,86,99,103], in contrast to isolated vegetation elements where faster-moving airflow mixes with slower-moving airflow in the wake via counter-rotating vortices [59].…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…The recovery length downwind of a vegetation patch or fence has been shown to be significantly longer than in the case of single vegetation elements [46,54,62,64,77,81,83,91,98,102]. The difference in velocity recovery length has been attributed to a lack of flow moving laterally around a patch [77] or a fence where the obstacle width is assumed to be infinite [63,86,99,103], in contrast to isolated vegetation elements where faster-moving airflow mixes with slower-moving airflow in the wake via counter-rotating vortices [59]. shown to be significantly longer than in the case of single vegetation elements [46,54,62,64,77,81,83,91,98,102].…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…This results in stress being transferred to the inter-canopy surface, and potentially greater sediment transport [52,75]. In the case of skimming flow (>~40% cover [43]), the increased drag from the vegetation acts to displace z 0 upwards (establishing a zero-plane displacement height, d), which simultaneously extracts momentum from the surface wind and increases wind shear stress above the canopy (e.g., [47][48][49][50][51][52]76,77]). The absorption of additional stresses by the plants therefore decreases the erosion potential at the surface, despite the additional above-canopy shear stress induced by the increased surface roughness.…”

Section: Introduction: the Drylands Contextmentioning

confidence: 99%

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Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

Mayaud

Webb

2017

Land

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Drylands are characterised by patchy vegetation, erodible surfaces and erosive aeolian processes. Empirical and modelling studies have shown that vegetation elements provide drag on the overlying airflow, thus affecting wind velocity profiles and altering erosive dynamics on desert surfaces. However, these dynamics are significantly complicated by a variety of factors, including turbulence, and vegetation porosity and pliability effects. This has resulted in some uncertainty about the effect of vegetation on sediment transport in drylands. Here, we review recent progress in our understanding of the effects of dryland vegetation on wind flow and aeolian sediment transport processes. In particular, wind transport models have played a key role in simplifying aeolian processes in partly vegetated landscapes, but a number of key uncertainties and challenges remain. We identify potential future avenues for research that would help to elucidate the roles of vegetation distribution, geometry and scale in shaping the entrainment, transport and redistribution of wind-blown material at multiple scales. Gaps in our collective knowledge must be addressed through a combination of rigorous field, wind tunnel and modelling experiments.

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A field‐based parameterization of wind flow recovery in the lee of dryland plants

Mayaud

Wiggs

Bailey

2016

Earth Surf Processes Landf

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Wind erosion is a key component of land degradation in vulnerable dryland regions. Despite a wealth of studies investigating the impact of vegetation and windbreaks on windflow in controlled wind-tunnel and modelling environments, there is still a paucity of empirical field data for accurately parameterizing the effect of vegetation in wind and sediment transport models. The aim of this study is to present a general parameterization of wind flow recovery in the lee of typical dryland vegetation elements (grass clumps and shrubs), based on their height (h) and optical porosity (θ). Spatial variations in mean wind velocity around eight isolated vegetation elements in Namibia (three grass clumps and five shrubs) were recorded at 0.30 m height, using a combination of sonic and cup anemometry sampled at a temporal frequency of 10 seconds. Wind flow recovery in the lee of the elements was parameterized in an exponential form,The best-fit parameters derived from the field data were u 0 = u ref (0.0146θ À 0.4076) and b = 0.0105θ + 0.1627. By comparing this parameterization to existing models, it is shown that wind recovery curves derived from two-dimensional wind fence experiments may not be suitable analogues for describing airflow around more complex, three-dimensional forms. Field-derived parameterizations such as the one presented here are a crucial step for connecting plant-scale windflow behaviour to dryland bedform development at landscape scales.

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Dynamics of skimming flow in the wake of a vegetation patch

Cited by 40 publications

References 72 publications

A coupled vegetation/sediment transport model for dryland environments

A coupled vegetation/sediment transport model for dryland environments

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

A field‐based parameterization of wind flow recovery in the lee of dryland plants

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