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

Abstract

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 airflow 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 airflow pattern. A tall (11·4 m), sharp-crested foredune produced flow separation with a recirculation cell. A lower (6·6 m), more rounded, foredune crest exhibited lee-side flow that remained attached but was deflected parallel to the crest, while flow over a low incipient fore- dune crest resulted in attached flow with no deflection in the lee. The morphology of the lee slopes best explained these airflow adjustments. More obliquely offshore winds were also found to affect the secondary airflow patterns, with a smooth transition from flow separation to flow attachment as the incident angle decreased. At higher wind velocities this smooth transition was replaced with a more abrupt switch between the secondary airflow types. This threshold was located at similar approach angles to those recorded for desert dunes (90° +/- 20°). These findings have significant 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. © 2010 John Wiley & Sons, Ltd.