Grain shape effects on aeolian sediment transport
Particle shape is a parameter which has been largely neglected in the study of sediment transport by wind. Many methods of measuring shape have been published. Those which characterize shape in pebble and sand sized sediments are reviewed here. In order to test the influence of shape on aspects of particle movement and on sediment transport rate, two very differently shaped populations were used, (a) a reasonably equant quartz dune sand and (b) a platy dune sand composed largely of shell fragments. Recommendations are made for reasonably fast and accurate methods of shape measurement, including Sneed and Folk's Maximum Projection Sphericity and Winklemolen's Rollability. The possible effects of shape on saltation were examined in terms of grain trajectories and the grain's interaction with the bed at collision using video and high speed photography. The latter enabled individual particles (from a coarse, medium or fine size fraction) to be followed as they impacted the bed, through to ricochet and the possible ejection of previously stationary bed grains. Experiments were conducted over horizontal and sloping beds, the latter representing different parts of the stoss face of a ripple. It was found that shape, in terms of the sphericity of the particles has a marked effect on collision. The near spherical quartz sand causes more dislodgements and more ejecta per collision than the much less spherical platy shell sand. The quartz sand is also more likely to approach and ricochet from the bed at higher angles to the horizontal than the shelly sand, and to rebound more vigorously. Thus, the bed activity generated by collision increases with an increase in particle sphericity. However, high speed photography of grain dislodgement by wind action alone indicates that, as sphericity decreases, there is a greater probability that a grain will be entrained aerodynamically. Shape also influences trajectories. Video films show that grain paths become longer and flatter as sphericity decreases. These observations indicate that the transport rate for grains with a low sphericity will be greater than those with a high sphericity, both in terms of aerodynamic entrainment and of longer trajectories. However, once collision becomes important in the dislodgement of surface grains, the greater bed activity seen with more spherical particles means that their transport rate will increase. These findings are supported by the observations of Williams (1964) and of Willetts, Rice and Swaine (1982), that sediment transport is promoted at low windspeeds by less spherical grains, while the opposite occurs at high windspeeds.