Sediment entrainment to the lee of roughness elements: Effects of vortical structures

This paper investigates the role of vortical structures shed by a roughness element, particularly the paired counterrotating vortices found in the element's wake, in the initiation of sand transport under conditions of both fluid and impact entrainment. Patterns of particle entrainment by fluid stress only were observed in wind tunnel studies employing narrow spanwise strips of sediment ( 277 mm mean diameter), which minimized dislodgement by particles transported from upwind. Composite images of the erosional patterns clearly illustrate regions of the bed where shear stress is above, equal to, and below fluid threshold in the vicinity of the roughness elements. The results clearly show that elevated shear stress allows entrainment to occur in the vicinity of the horseshoe and paired counterrotating vortices at wind speeds otherwise insufficient for entrainment on a smooth bed. Impact entrainment was observed using the same methodology, with a supply of saltators introduced to the airstream through an upwind particle feed. The grain-borne momentum of the saltation cloud greatly reduced the influence of the vortices on the rate of entrainment. However, the vortices were seen to influence the proportion of grains entering the different modes of transport ( saltation, reptation, and traction). The obstruction of the saltator's trajectory by the roughness elements allowed sand tails to form to their lee under saltation, as compared to the clean air experiments in which the sand tails did not form.