Alongshore Variations in Aeolian Sediment Transport: Carrick Finn Strand, Ireland

Abstract

Past studies of coastal aeolian sediment transport have adopted a two-dimensional perspective relying on data collected from single survey transects normal to the shoreline to represent wind speed and sediment flux. Although this approach provides a useful approximation to the aeolian system, it presumes that air flow is steady and uniform and that single-point data or transect samples represent broader beach conditions. These conditions are frequently not satisfied because beaches are three-dimensional systems with substantial alongshore variation.

This study focuses on spatial variations in sediment transport and in surface sediment characteristics that affect transport during offshore winds. The sediment flux was sampled over 15 minute intervals with vertical traps positioned to monitor small-scale (0-5 m), and medium-scale (0-50 m) alongshore variations. Variability in trapped sediment flux is of the order of + /- 30% of the mean flux at both spatial scales. Alongshore variations for sediment size (+/- 10% of the mean) and carbonate content (+/- 7% of the mean) are too small to explain entirely the variability in sediment flux. Variations in moisture content (+/- 35% of the mean) are large enough to account for flux variability, especially when coupled with the potentially important rule of wind gustiness. The combined effect of wind gustiness and moisture variation produces non-uniform sediment transport, manifested as streamers of sand that move across the beach. The irregular streamer movement results in selective interception by traps producing variable sediment trapping rates.