Flow deflection over a foredune

Hesp, Patrick A.; Smyth, Thomas; Nielsen, Peter; Walker, Ian J.; Bauer, Bernard O.; Davidson‐Arnott, Robin

doi:10.1016/j.geomorph.2014.11.005

Cited by 79 publications

(105 citation statements)

References 46 publications

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“…A more complex model might allow for unequal deposition of sediment across the dune form, and hence varying slope angles, but the general outcome would be similar in terms of overall morphodynamic evolution of the dune form. In this regard, it should be noted that the model is not driven by wind but simply by sediment inputs, and therefore there is no feedback between the evolving form and wind acceleration or steering through time (Hesp et al ., ). The initial foredune height was set at 3 m, which is reasonable for an established foredune and allows for the depiction of the triangular form.…”

Section: Computer Model Of Foredune Evolutionmentioning

confidence: 99%

Sediment budget controls on foredune height: Comparing simulation model results with field data

Davidson‐Arnott

Hesp

Ollerhead

et al. 2018

Earth Surf Processes Landf

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The form, height and volume of coastal foredunes reflects the long‐term interaction of a suite of nearshore and aeolian processes that control the amount of sand delivered to the foredune from the beach versus the amount removed or carried inland. In this paper, the morphological evolution of more than six decades is used to inform the development of a simple computer model that simulates foredune growth. The suggestion by others that increased steepness of the seaward slope will retard sediment supply from the beach to the foredune due to development of a flow stagnation zone in front of the foredune, hence limiting foredune growth, was examined. Our long‐term data demonstrate that sediment can be transferred from the beach to the foredune, even with a steep foredune stoss slope, primarily because much of the sediment transfer takes place under oblique rather than onshore winds. During such conditions, the apparent aspect ratio of the dune to the oncoming flow is less steep and conditions are not as favourable for the formation of a stagnation zone. The model shows that the rate of growth in foredune height varies as a function of sediment input from the beach and erosion due to storm events, as expected, but it also demonstrates that the rate of growth in foredune height per unit volume increase will decrease over time, which gives the perception of an equilibrium height having been reached asymptotically. As the foredune grows in size, an increasing volume of sediment is needed to yield a unit increase in height, therefore the apparent growth rate appears to slow. Copyright © 2018 John Wiley & Sons, Ltd.

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Section: Computer Model Of Foredune Evolutionmentioning

confidence: 99%

Sediment budget controls on foredune height: Comparing simulation model results with field data

Davidson‐Arnott

Hesp

Ollerhead

et al. 2018

Earth Surf Processes Landf

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“…In the past decade, a series of papers describing wind flow over coastal foredunes at Magilligan Strand (Northern Ireland) and the Greenwich Dunes (Prince Edward Island, Canada) have yielded A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 5 significant insight into flow steering and topographic forcing (e.g., Walker et al, 2006Walker et al, , 2009Jackson et al, 2011;Bauer et al, 2012;Delgado-Fernandez et al, 2013;Lynch et al, 2009;Hesp et al, 2009Hesp et al, , 2015. In particular, these field experiments demonstrated that offshore flow can be substantially modified in the lee of a large coastal foredune, to the extent that shore-parallel steering and eddy recirculation occurs on the beach (Lynch et al, 2008;Bauer et al, 2012) as recognized by Hesp (2005).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

“…3) for the crest tower shows that the flow was oriented almost perfectly offshore (southerly) at the start of the measurement interval, veering slightly to the southwest to a maximum of about 35 o by 16:15. At the dune crest surface (St 10), wind direction varied from -15 o (south southeasterly) to +15 o (south southwesterly), essentially crest normal flow for the duration of the period, indicating a localized, topographically-forced steering effect that constrained the near-surface flow to a crest normal orientation (cf.,Hesp et al, 2015). Wind gusts forced small deviations from these mean directions with instantaneous excursions less than about +/-20 o .…”

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confidence: 95%

Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

et al. 2015

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Flow dynamics and sediment transport responses over a large, vegetated foredune at Prince Edward Island, Canada, during an offshore wind event are examined. Data were collected along an instrumented transect that extended from the dune crest, down the lee-side (seaward) slope of the dune, across a wave-cut scarp, and on to the back-beach. When the wind direction at the dune crest was approximately crest-normal (less than about 15 o deviation), mean near-surface flow directions along the dune slope and on the back beach were generally onshore, indicating reversed (onshore) flow relative to the regional (offshore) wind direction. Although flow patterns were consistent with a lee-side recirculation eddy, large excursions in flow direction were also prevalent, suggesting that the eddy was unstable and alternated with highly turbulent wake flow. As wind direction at the crest veered to greater than 20 o from crest-normal, lee-side winds shifted toward strongly alongshore flow with minimal directionally variability. On the dune slope, the wind vectors were slightly offshore whereas on the back-beach they were slightly onshore.Wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer. Mean particle counts (averaged over a 15-minute interval) derived from laser sensors positioned at the crest were large (7.76 per second) in comparison to those measured in the immediate lee of the crest (0.52 per second) and farther down the dune slope (< 0.13 per second).In contrast, the values were as large as 25.62 per second on the middle of the back-beach, declining rapidly to a value of only 0.24 per second at the dune toe. Transport intensity was highly variable with the largest Activity Parameter (AP = 0.5) values at the dune crest and on the back-beach, and with the smallest values (AP < 0.1) on the lee-side dune slope down to the top of the scarp. Calculations of sediment (particle) flux divergence between instrument stations show that deposition was significant A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 immediately downwind of the dune crest but negligible across most of the lower dune slope.Deposition was also prevalent on the dune ramp below the scarp.These results demonstrate that sediment transport across the beach-dune system was spatially discontinuous during this offshore wind event. Rebuilding of the dune ramp at the toe of the scarp occurred quite independently of, and with a different sediment source than, the broadening of the dune crest, which was fed with sediment from the landward side of the foredune. Such process 'decoupling'is an example of the complexity by which foredunes evolve or are maintained, and as with previous research reinforces the importance of offshore and alongshore wind events to beach-dune morphodynamics.

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“…This is particularly the case within trough blowouts, which have narrow and topographically constrained 'Vand U-shaped' morphologies. McKenna Neuman et al, 1997;Walker, 1999;Hesp et al, 2015;Smyth and Hesp, 2015) and turbulent kinetic energy (Smyth et al, 2014), these measurements alone are not direct indicators or predictors of whether sediment is being eroded, transported or deposited, which, in turn, limits their value for understanding the evolutionary dynamics of the system. However, the precise measurement of flow dynamics and resulting patterns of topographic change have not yet been performed in detail.…”

Section: Introductionmentioning

confidence: 99%

Topographic change and numerically modelled near surface wind flow in a bowl blowout

Smyth

Hesp

Walker

et al. 2019

Earth Surf Processes Landf

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A number of studies have measured and numerically modelled near surface wind velocity over a range of aeolian landforms and made suppositions about topographic change and landform evolution. However, the precise measurement and correlation of flow dynamics and resulting topographic change have not yet been fully realized. Here, using repeated high‐resolution terrestrial laser scanning and numerical flow modelling within a bowl blowout, we statistically analyse the relationship between wind speed, vertical wind velocity, turbulent kinetic energy and topographic change over a 33‐day period. Topographic results showed that erosion and deposition occurred in distinct regions within the blowout. Deposition occurred in the upwind third of the deflation basin, where wind flow became separated and velocity and turbulent kinetic energy decreased, and erosion occurred in the downwind third of the deflation basin, where wind flow reattached and aligned with incident wind direction. Statistical analysis of wind flow and topographic change indicated that wind speed had a strong correlation with overall topographic change and that vertical wind velocity (including both positive and negative) displayed a strong correlation with negative topographic change (erosion). Only weak or very weak correlations exist for wind flow parameters and positive topographic change (accretion). This study demonstrates that wind flow modelling using average incident wind conditions can be utilized successfully to identify regions of overall change and erosion for a complex aeolian landform, but not to identify and predict regions where solely accretion will occur. © 2019 John Wiley & Sons, Ltd.

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Flow deflection over a foredune

Cited by 79 publications

References 46 publications

Sediment budget controls on foredune height: Comparing simulation model results with field data

Sediment budget controls on foredune height: Comparing simulation model results with field data

Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

Topographic change and numerically modelled near surface wind flow in a bowl blowout

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