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

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 3·5m 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 20cm 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.