Field characterization of three-dimensional lee-side airflow patterns under offshore winds at a beach-dune system

Full characterization of three-dimensional airflow remains elusive within a variety of environments, and is particularly challenging over complex dune topography. Most work describes lee-side flow dynamics in two dimensions over dunes that present angle of repose lee sides. However, the presence of vegetation in coastal dunes creates topographic differences and irregular shapes that interfere with flow separation at the crest and significantly modify lee-side airflow patterns and potential transport. This paper presents the first three-dimensional field characterisation of airflow patterns at the lee-side of a subaerial dune. Flow information was obtained using an array of 3D ultrasonic anemometers deployed over a beach surface during 7 offshore wind events. Data were used to measure cross-shore and alongshore lee-side airflow patterns using the three dimensions of the wind vector. Distances to re-attachment were similar to previous studies but the range of transverse incident wind directions resulting in flow separation (0+/-35°) was almost double than those previously reported (0+/-20°). Airflow reversal took place with winds as low as 1 m s-1. Transverse offshore winds generated areas of opposing wind directions both within the reversed zone and beyond re-attachment, contrary to consistent deflection in only one direction found in transverse desert dunes. Patterns of flow convergence-divergence have been reported in fluvial studies. However, while convergence was associated with weak reversal in fluvial settings it appeared to be related to strong flow reversal here and could be produced by pressure differentials at the dune crest.