Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold

Abstract

Reliably predicting the geomorphology and climate of planetary bodies requires knowledge of the dynamic threshold wind shear velocity below which saltation transport ceases. Here we measure this threshold in a wind tunnel for four well‐sorted and two poorly sorted sand beds by visual means and by a method that exploits a regime shift in the behavior of the surface roughness caused by momentum transfer from the wind to the saltating particles. For our poorly sorted sands, we find that these measurement methods yield different threshold values because, at the smaller visual threshold, relatively coarse particles do not participate in saltation. We further find that both methods yield threshold values that are much larger (60–250%) for our poorly sorted sands than for our well‐sorted sands with similar median particle diameter. In particular, even a rescaling of the dynamic saltation threshold based on the 90th percentile particle diameter rather than the median diameter cannot fully capture this difference, suggesting that relatively very coarse particles have a considerable control on the dynamic threshold. Similar findings were previously reported for water‐driven sediment transport. Our findings have important implications for quantitative predictions of saltation transport‐related geophysical processes, such as dust aerosol emission.

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

“…All data shown in the figures of this review can be found in the following references: Peyneau and Roux (2008a Bagnold (1937), Chepil (1945), Ho (2012), Martin and Kok (2018), and Zhu et al (2019). We thank Michael Church and three anonymous reviewers for their critical reviews and numerous insightful comments and suggestions.…”

“…However, it is unclear whether this is also true for sediment transport cessation. While the continuous rebound mechanism (see section 4.2) is clearly a size-selective process (coarser particles are less accelerated during their trajectories), impact entrainment may not be Zhu et al, 2019). Furthermore, in heterogeneous sediment beds, relatively…”

“…Predictions of the rebound threshold, (dashed lines in (a)) and (solid lines in (a) and (b) and dashed lines in (b)), from (a) the IPTM and (b) the correlation‐based model by Pähtz and Durán () and its IPTM analog as a function of the Stokes‐like number for five different density ratios (2.65, 40, 190, 2200, and 250000) corresponding to five different fluvial or aeolian conditions (Water, Venus, Titan, Earth, and Mars). Symbols correspond to threshold measurements (or measurement compilations) from various studies (Bagnold, ; Buffington and Montgomery, ; Chepil, ; Ho, ; Loiseleux et al, ; Martin and Kok, ; Ouriemi et al, ; Zhu et al, ) and methods (see text). Ouriemi et al () did not report single measurement values but a constant threshold for a large range of viscous conditions, indicated by the dotted square.…”

“…For viscous bedload transport driven by water‐oil mixtures, the visual incipient motion measurements by Yalin and Karahan () and Loiseleux et al () and cessation threshold measurements by Ouriemi et al () are shown (for viscous bedload transport, the differences between transport initiation, rebound, and impact entrainment threshold are very small (Pähtz and Durán, )). For aeolian saltation transport, a few studies (e.g., Ho, ; Zhu et al, ) carried out an indirect extrapolation to vanishing transport to obtain using a proxy of : the surface roughness (see Appendix for its definition in the absence of transport), which undergoes a regime shift when saltation transport ceases. Furthermore, visual measurements of by Bagnold () and Chepil () are shown, obtained from successively decrementing until intermittent saltation transport stops.…”

“…All data shown in the figures of this review can be found in the following references: Peyneau and Roux (), da Cruz et al (), Kamrin and Koval (), Houssais et al (), Allen and Kudrolli (),Allen and Kudrolli (), Diplas et al (), Valyrakis (), Williams et al (), Iversen and White (), Burr et al (), Swann et al (), Beladjine et al (), Vowinckel et al (), Salevan et al (), Buffington and Montgomery (), Yalin and Karahan (), Loiseleux et al (), Ouriemi et al (), Bagnold (), Chepil (), Ho (), Martin and Kok (), and Zhu et al (). We thank Michael Church and three anonymous reviewers for their critical reviews and numerous insightful comments and suggestions.…”