Changes in grain size of sand in transport over a foredune

Abstract

Suspended sand is sampled at several heights and positions on a beach and foredune, providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport. Grain-size analysis is used to study the changes in grain-size distribution during landward transport. Mean grain size and sorting decrease during transport. Changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune. Sediment trapped on the seaward slope at a height of 30-50 cm above the surface closely resembles the sediment trapped landward, which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles. The movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune, leading to a change from saltation on the beach to modified saltation and suspension on the foredune. Small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward, resulting in a gradual decrease in grain size of the landward-deposited sediment.

Constructing notches in foredunes: Effect on sediment dynamics in the dune hinterland

“…Under such conditions many particles are no longer transported in saltation but in modified saltation or even short-term suspension. Arens et al (2002) confirms that turbulent flow over vegetated foredunes may cause a change from saltation on the beach to modified saltation and suspension on the foredunes. Hesp (2002), Christiansen and Davidson-Arnott Hesp et al (2005), and Petersen et al (2011) also describe the occurrence of modified saltation and suspension above foredunes during heavy wind storms.…”

Aeolian sediment transport over gobi: Field studies atop the Mogao Grottoes, China

“…This result was completely different from that the mean grain size of sediment flux over sand surfaces decreased monotonously (Arens et al, 2002), increased (Van der Wal, 2000) or first decreased then increased with height (Farrell et al, 2012;Tan et al, 2014;Williams, 1964). Theoretically, it is generally believed that grain size decreases with height above the bed surface, while the coarsening trend of mean grain-size with height requires that the coarser grains have higher launch velocities than finer grains.…”

The harmonious character in equilibrium aeolian transport on mixed sand bed

“…4, the mean grain size increases with height at lower wind velocity. In some field experiments, the mean grain size was also found to increase with height (Arens et al, 2002). Some researchers explained the increase in mean grain size with height by the observation that smaller grains are influenced more strongly by air drag than larger grains and larger grains saltate higher than smaller grains in the saltation transport (Bagnold, 1941;Arens et al, 2002).…”