Nebkha flow dynamics and shadow dune formation

Hesp, Patrick A.; Smyth, Thomas

doi:10.1016/j.geomorph.2016.12.026

Cited by 81 publications

(82 citation statements)

References 68 publications

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“…It is known that nebkha dunes affect sedimentation by changing the wind flow patterns (Dong et al, 2004;Li et al, 2008). Previous studies have found that with increased dune volume the area where the wind speed is reduced increases, which results in higher sedimentation rates (Hesp, 1981;Hesp and Smyth, 2017). The linear relationship between initial dune volume and dune volume change found for the nebkha dunes in our study indicates that different dune sizes have similar effect on the wind flow pattern per unit of area, which indicates scale invariance (Hallet, 1990).…”

Section: Summer Growthsupporting

confidence: 66%

“…At the lee side of these small clumps of vegetation a shadow dune develops by sand deposition -this shadow dune has a ridge parallel to the wind direction (Clemmensen, 1986;Gunatilaka and Mwango, 1989;Hesp, 1981). Vegetation and shadow dune together are known as nebkha dunes, embryo dunes or incipient foredunes (Hesp, 2002;Hesp and Smyth, 2017). The further development of these nebkha dunes strongly depends on the balance between summer accumulation of sand and vegetation growth and winter erosion of sand and loss of vegetation (Montreuil et al, 2013).…”

mentioning

confidence: 99%

“…However, how coastal nebkha dune growth and erosion rates are influenced by the individual dune characteristics, such as dune size, vegetation and degree of sheltering, are less well studied. Dune size affects the wind flow pattern, thus affecting sand deposition (Walker and Nickling, 2002) -for example increasing height or length of the shadow dune (Hesp, 1981;Hesp and Smyth, 2017). Dune size also influences storm erosion: Claudino-Sales (2008) found that foredunes with a higher volume were less sensitive to erosion.…”

mentioning

confidence: 99%

“…Whether the latter also applies to nebkha dunes, is unknown. Differences in vegetation density between plant species are known to modify sand deposition (Arens, 1996;Hesp, 1983;Keijsers et al, 2014;Zarnetske et al, 2012), storm erosion (Charbonneau et al, 2017;Seabloom et al, 2013), and dune morphology (Du et al, 2010;Hacker et al, 2012;Hesp, 1988). Sheltering by other nebkha dunes can decrease the sand supply but can also reduce erosion by waves (Arens, 1996;Lima et al, 2015;Luo et al, 2014;Montreuil et al, 2013).…”

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confidence: 99%

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Exploring the contributions of vegetation and dune size to early dune development using unmanned aerial vehicle (UAV) imaging

et al. 2017

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Abstract. Dune development along highly dynamic land-sea boundaries is the result of interaction between vegetation and dune size with sedimentation and erosion processes. Disentangling the contribution of vegetation characteristics from that of dune size would improve predictions of nebkha dune development under a changing climate, but has proven difficult due to the scarcity of spatially continuous monitoring data.This study explored the contributions of vegetation and dune size to dune development for locations differing in shelter from the sea. We monitored a natural nebkha dune field of 8 ha, along the coast of the island Texel, the Netherlands, for 1 year using an unmanned aerial vehicle (UAV) with camera. After constructing a digital surface model and orthomosaic we derived for each dune (1) vegetation characteristics (species composition, vegetation density, and maximum vegetation height), (2) dune size (dune volume, area, and maximum height), (3) degree of shelter (proximity to other nebkha dunes and the sheltering by the foredune). Changes in dune volume over summer and winter were related to vegetation, dune size and degree of shelter.We found that a positive change in dune volume (dune growth) was linearly related to initial dune volume over summer but not over winter. Big dunes accumulated more sand than small dunes due to their larger surface area. Exposed dunes increased more in volume (0.81 % per dune per week) than sheltered dunes (0.2 % per dune per week) over summer, while the opposite occurred over winter. Vegetation characteristics did not significantly affect dune growth in summer, but did significantly affect dune growth in winter. Over winter, dunes dominated by Ammophila arenaria, a grass species with high vegetation density throughout the year, increased more in volume than dunes dominated by Elytrigia juncea, a grass species with lower vegetation density (0.43 vs. 0.42 (m 3 m −3 ) week −1 ). The effect of species was irrespective of dune size or distance to the sea.Our results show that dune growth in summer is mainly determined by dune size, whereas in winter dune growth was determined by vegetation type. In our study area the growth of exposed dunes was likely restricted by storm erosion, whereas growth of sheltered dunes was restricted by sand supply. Our results can be used to improve models predicting coastal dune development.

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Section: Summer Growthsupporting

confidence: 66%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Exploring the contributions of vegetation and dune size to early dune development using unmanned aerial vehicle (UAV) imaging

et al. 2017

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“…In addition to storms, other factors such as a high water table, low sediment supply, grain size variability, development of shell lag and climatic conditions may also result in the suppression of the coalescing of coastal foredunes Russell, 2006, 2009;Wolner et al, 2013;Hoonhout and de Vries, 2016;Ruz and Hesp, 2014;Ruz et al, 2017a). Feedbacks between the wind, dune vegetation and sediment transport that are specific to hummocky dunes may also alter the rates of coalescing (Barrineau and Ellis, 2013;Gillies et al, 2014), such as the development of high wind velocity regions located adjacent to hummocky dune forms (Hesp and Smyth, 2017). Work here does not address observations of older foredune ridges that lose their continuous morphology as a result of plant succession, erosion via rain and flow in rivulets, or trampling (Levin et al, 2009(Levin et al, , 2017.…”

Section: Discussionmentioning

confidence: 99%

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

2017

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Abstract. Coastal foredunes form along sandy, low-sloped coastlines and range in shape from continuous dune ridges to hummocky features, which are characterized by alongshore-variable dune crest elevations. Initially scattered dune-building plants and species that grow slowly in the lateral direction have been implicated as a cause of foredune "hummockiness". Our goal in this work is to explore how the initial configuration of vegetation and vegetation growth characteristics control the development of hummocky coastal dunes including the maximum hummockiness of a given dune field. We find that given sufficient time and absent external forcing, hummocky foredunes coalesce to form continuous dune ridges. Model results yield a predictive rule for the timescale of coalescing and the height of the coalesced dune that depends on initial plant dispersal and two parameters that control the lateral and vertical growth of vegetation, respectively. Our findings agree with previous observational and conceptual work -whether or not hummockiness will be maintained depends on the timescale of coalescing relative to the recurrence interval of high-water events that reset dune building in low areas between hummocks. Additionally, our model reproduces the observed tendency for foredunes to be hummocky along the southeast coast of the US where lateral vegetation growth rates are slower and thus coalescing times are likely longer.

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Anchored Dunes

Hesp

Smyth

2019

Aeolian Geomorphology

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Nebkha flow dynamics and shadow dune formation

Cited by 81 publications

References 68 publications

Exploring the contributions of vegetation and dune size to early dune development using unmanned aerial vehicle (UAV) imaging

Exploring the contributions of vegetation and dune size to early dune development using unmanned aerial vehicle (UAV) imaging

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

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