Surface wind flow characteristics over foredune and beach topography: 3-D model and high resolution field investigations

Understanding airflow behaviour over three-dimensional foredune terrain allows detailed examination of the aeolian transport environment between beaches and dunes. A spatial array of twenty four ultrasonic anemometers was deployed in 2010 over a beach and foredune system at Magilligan, Northern Ireland to capture airflow data under a range of incident offshore directions. Computational Fluid Dynamics (CFD) modelling was also conducted using input data from anemometer field measurements, running over a surface mesh generated from LiDAR and DGPS surveys. Aeolian sediment transport measurements were also undertaken using high resolution (20Hz) load cell traps. Results demonstrate the heterogeneity of surface (sub-metre) airflow speed and direction when emerging from the foredune crest position. Distinctive airflow steering and reverse flow directions were noted as being dependent on incident wind direction at the crest with maximum reversal under near perpendicular offshore winds. As the incident winds veer around to a more oblique offshore direction they adapt a steered rather than reversed flow behaviour. A zonation pattern of surface shear stress was apparent across the foredune toe and back beach area and contained flow that was above threshold for aeolian sediment entrainment. Measurement of sediment flux also showed transport events under these steered and reversed flow scenarios. Localised undulations in the foredune crest topography are likely to have played a significant role in dictating the heterogeneous patterns of surface flow and shear stress. These findings show that reversed flow behaviour can induce significant aeolian flux and therefore demonstrates its importance in aeolian sediment budgets for leeside coasts (offshore winds dominant) where there are particular foredune configurations present.