Aeolian fetch distance and secondary airflow effects: the influence of micro‐scale variables on meso‐scale foredune development

Lynch, Kevin; Jackson, Derek W.T.; Cooper, J. Andrew G.

doi:10.1002/esp.1582

Cited by 56 publications

(48 citation statements)

References 47 publications

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“…The net movement, however, was seen to be onshore with ripple crest orientation and dune foot trap data presented as evidence. Further to this work, Lynch et al (2008) have reported on post-storm recovery at this site over a 2-year period -under characteristically dominant offshore winds.…”

Section: Wind Approach Angle Thresholds For Airfl Ow Separationmentioning

confidence: 91%

Coastal foredune topography as a control on secondary airflow regimes under offshore winds

Lynch

Jackson

Cooper

2010

Earth Surf Processes Landf

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Aeolian sediment transport under offshore winds is usually discounted from sediment budget considerations of coastal dunes. Results presented here indicate however, that depending on wind orientation and foredune morphology, positive contributions to the sediment budget can arise from offshore winds. Lee-side airfl ow patterns, seaward of the foredune crest, under offshore winds were studied at three sites of varying foredune geometry along Magilligan Strand, Northern Ireland. Ultrasonic anemometers recorded 1 Hz wind velocity data at 1° intervals. Descriptive statistics and wind rose diagrams were used in the analysis of these data. Under perpendicularly offshore winds each site exhibited a different type of lee-side airfl ow pattern. A tall (11·4 m), sharp-crested foredune produced fl ow separation with a recirculation cell. A lower (6·6 m), more rounded, foredune crest exhibited lee-side fl ow that remained attached but was defl ected parallel to the crest, while fl ow over a low incipient foredune crest resulted in attached fl ow with no defl ection in the lee. The morphology of the lee slopes best explained these airfl ow adjustments. More obliquely offshore winds were also found to affect the secondary airfl ow patterns, with a smooth transition from fl ow separation to fl ow attachment as the incident angle decreased. At higher wind velocities this smooth transition was replaced with a more abrupt switch between the secondary airfl ow types. This threshold was located at similar approach angles to those recorded for desert dunes (90° +/− 20°). These fi ndings have signifi cant implications for the post-storm recovery and long-term evolution of beach-dune systems where the predominant winds are offshore, and offer an explanation for the presence of many beach-dune systems in lee-coast locations.

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Section: Wind Approach Angle Thresholds For Airfl Ow Separationmentioning

confidence: 91%

Coastal foredune topography as a control on secondary airflow regimes under offshore winds

Lynch

Jackson

Cooper

2010

Earth Surf Processes Landf

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“…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: 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 approach is still being developed in the literature and remains the only way to improve knowledge of the coastal processes and interactions (Stive et al, 2002). Even if these theoretical links are difficult to substantiate (Sherman, 1995;Lynch et al, 2008), they should be an important focus of future research by coastal geomorphologists.…”

Section: Discussionmentioning

confidence: 99%

“…K.F. Nordstrom et al, (1996) indicate an abscence of sand transport around the dune toe while K. Lynch et al (2008) highlight a change in flow direction and a possible reverseal. Nevertheless, both cases investigated involved high dunes (>10 m) which are not directly comparable with the low dune systems of the erosional shores of Camargue.…”

Section: The Southern North Sea Coast: Beach/dune/shoreface Articulatmentioning

confidence: 99%

Morphodynamics of beach/dune systems: examples from the coast of France

Sabatier¹,

Anthony²,

Héquette³

et al. 2009

geomorphologie

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Aeolian fetch distance and secondary airflow effects: the influence of micro‐scale variables on meso‐scale foredune development

Cited by 56 publications

References 47 publications

Coastal foredune topography as a control on secondary airflow regimes under offshore winds

Coastal foredune topography as a control on secondary airflow regimes under offshore winds

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

Morphodynamics of beach/dune systems: examples from the coast of France

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