Wind direction and complex sediment transport response across a beach–dune system

Abstract

Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back‐beach and stoss slope of the foredune can be exceedingly complex. The upper‐air flow – measured by a sonic anemometer at the top of a 3·5 m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near‐surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near‐surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37: 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28: 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33: 991–1005, 2008, Geomorphology 105: 139–146, 2010), which shows how near‐surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore‐oblique, attached and deflected; (c) offshore, detached; and (d) offshore‐oblique, attached and deflected. High‐frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near‐surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established. Copyright © 2012 John Wiley & Sons, Ltd.

Vegetation controls on the maximum size of coastal dunes

“…3C). Indeed, Arens et al (17) and Bauer et al (18) reported no transport activity during favorable wind conditions on the windward side of steep dunes (∼27°and ∼22°respectively; 3C). In contrast, flatter foredunes have room to grow, as those in Fig.…”

“…The empirical finding that foredune size correlates with beach type, with foredunes up to 10 times higher on dissipative beaches than on reflective ones (1, 2), combined with conceptual models that predict enhanced aeolian transport on dissipative beaches (1,14,15), has motivated the common assumption that maximum foredune size is primarily controlled by sand supply (1,4,15,16). However, this explanation is problematic based on reports of large foredunes on dissipative beaches with a negligible sand input (17,18) and findings of very small foredunes formed on a reflective beach under high sand supply (19). In addition, there seems to be no general empirical relation between foredune size and beach morphology, i.e., beach width and slope (15,20,21), characteristics traditionally used, along with grain size, as a proxy for potential sand transport (which is notoriously difficult to measure at the beach).…”

Flow deflection over a foredune

“…In a subsequent study at the PEI site, Bauer et al (2012) documented pronounced deflection of oblique incident winds over the foredune, and found that wind directions on the beach during a storm event were far less variable than those on the dune crest for the same incident winds. During alongshore incident flow conditions Lynch et al (2009) found that wind near the dune crest was deflected more offshore compared to the flow along the beach and, for obliquely onshore winds, they found a deflection of ~20° towards shore-normal.…”

“…Davidson-Arnott et al, 2003Walker et al, 2003Walker et al, , 2006Hesp et al, 2005Hesp et al, , 2009Hesp et al, , 2013Bauer et al, 2009Bauer et al, , 2012Delgado-Fernandez and Davidson-Arnott, 2009;Hesp and Walker, 2012;Chapman et al, 2012Chapman et al, , 2013DelgadoFernandez et al, 2013;Ollerhead et al, 2013). The foredune crest is ~10 m above mean water level with a steep stoss slope (20°-25°) and an ENE-WSW crestline orientation.…”

“…Moreover, the topographic profile of the ridge was low. Walker et al (2006; and Bauer et al (2012) report that incident flow is already partly deflected at the base of a foredune. For a separate theoretical case, Bradley (1983) found a deflection of ~4° for an incident oblique wind approaching the ridge at 24°.…”