Flow Deflection and Deceleration Across a Simple Foredune

There are few field observations of flow deflection across foredunes in a range of incident wind conditions. This study reports observations of (1) flow deflection across the seaward slope of a foredune during alongshore and oblique onshore incident winds; and (2) associated sand transport. We compare these with modelled results from Hesp et al. (2015). Sonic anemometers and sand traps were deployed along, a transect across a simple foredune at St. Kilda Beach, Dunedin, New Zealand. Representative sections of each record were isolated for a range of incident wind directions, and for each section we determined flow deflection and relative change in wind velocity. The incident winds, observed on a 3m mast on the beach, ranged from 228 degrees (35 degrees oblique onshore) to 268 degrees (5 degrees oblique offshore) and 6 - 14 ms(-1) mean wind speed (over 5-15 minute intervals). Gusts reached 28 ms(-1). Onshore deflection varied between 19 degrees and 23 degrees for incident winds between 228 degrees and 249 degrees and 9-14 ms(-1). Our results are only partly consistent with those of Hesp et al. (2015) for a similar foredune and similar strength winds. We also found deflection increases with increasing obliquity, the degree of deflection increases towards the crest, and that topographically-forced flow acceleration is strongly influenced by the angle of approach of the incident winds. In general, however, we measured more flow deflection than was modelled, including significant deflection for highly oblique winds. Relative wind velocity up the profile showed deceleration up-slope for all sections because the anemometers were placed within the vegetation.