Changes in the saltation flux following a step‐change in macro‐roughness

Abstract

3The effect of a step change in macro-roughness on the saltation process under sediment supply 4 limited conditions was examined in the atmospheric boundary layer. For an array of roughness 5 elements of roughness density λ=0.045 (λ=total element frontal area/total surface area of the 6 array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness 7 element heights into the array. This matches the value predicted using an empirical design 8 model and provides confidence that it can be effectively used to engineer roughness arrays to 9 meet sand flux reduction targets. Measurements of the saltation flux characteristics in the 10 vertical dimension, including: saltation layer decay (e-folding) height and particle size, revealed 11 that with increasing distance into the array, the rate of mass flux change with increasing height 12 decreased notably, and (geometric) mean particle diameter decreased. The distribution of the 13 saltation mass flux in the vertical remains exponential in form with increasing distance into the 14 roughness array, and the e-folding height increases as well increasing at a greater rate as 15 particle diameter diminishes. The increase in e-folding height suggests the height of saltating 16 particles is increasing along with their mean speed. This apparent increase in mean speed is 17 likely due to the preferential removal, or sequestration, of the slower moving particles, across 18 the size spectrum, as they travel through the roughness array. 19Keywords: saltation flux, macro-roughness, e-folding height, mean particle size changes 20 Introduction 21Wind blowing over a surface with loose sand that exceeds a critical shear stress on the surface 22 will entrain these particles and move them downwind as bed-load in three recognized modes of 23 transport: creep, reptation, and saltation. The characterization of the saltation flux, in which 24 particles move in a series of repeated ejections and travel in ballistic trajectories followed by 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w 2 impact on the surface has garnered considerable research effort in the earth sciences. The 26 saltation process plays critical roles in aeolian bedform development across multiple scales 27 (e.g., Anderson, 1987; Claudin and Andreotti, 2006; Dúran et al., 2011; Gillies et al., 2012; 28 Parteli et al., 2014; Schmerler et al., 2016) and is a dominant mechanism driving the emission of 29 dust-sized particles, which represent the suspended load in aeolian transport (e.g., Shao et al., 30 1993; Shao, 2001; Kok et al., 2012). 31It is now generally acknowledged that the horizontal mass flux of wind-driven sand decreases 32 exponentially with elevation above the surface (e.g., Bagnold, 1941; Chepil, 1945; Williams, 33 1964; Sørensen, 1985; Namikas, 2003; Farrel...

Laboratory Experiments of Volcanic Ash Resuspension by Wind

“…Aerodynamic effects arise from the flow properties that are influenced by the degree or scale of the roughness (Raupach et al, ; Shao & Yang, ). Physical effects are related to the interaction of the moving sediment with the roughness (Gillies et al, ; Gillies & Lancaster, ; Wolfe & Nickling, ). As this laboratory‐based study only tested surfaces in the absence of large roughness elements, we will not address the effects of superposed roughness on ash resuspension and emission rates but recognize that this may be an important aspect to consider for real‐world conditions.…”

Observations and modeling of shear stress reduction and sediment flux within sparse dune grass canopies on managed coastal dunes

“…In these sparsely vegetated scenarios, limitations of the existing Raupach et al (1993) approach become more apparent for both laboratory and field cases. Of course, utilizing either shear coupler approach ignores physical interaction, turbulent kinetic energy reduction, and other mechanisms that contribute to spatial changes in the sediment transport rate (Gillies et al2018).…”

“…The wind tunnel study by Buckley (1987) measured sediment fluxes within canopies of real vegetation with cover ranging from 0 to 17% and found the sediment flux could be well predicted by a Bagnold-type transport model modified to include the cover. The field studies conducted by Gillies et al (2006) found that the sediment flux decreased exponentially with distance into a canopy of 5-gallon buckets in an open sandy bed before reaching a limit related to the roughness density (the frontal area perpendicular to flow divided by the ground area) (Gillies & Lancaster, 2013;Gillies et al, 2006Gillies et al, , 2015Gillies et al, , 2018.…”

Wind Tunnel Test of Sand Particle Size Distribution along Height in Blown Sand

“…At present, the focus of most domestic studies on wind-blown sand has been on the saltation of sand [3][4][5], lift-off speed and lift-off angular velocity [6][7][8][9], movement trajectory [10] and sand flux [11,12]. Additionally, the sand transport rate is the most direct representation of the strength of regional aeolian sand movement and a critical physical quantity in the research on wind-blown sand two-phase flow [13].…”