Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

Abstract

Wind‐blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind‐driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field‐based estimates of distinct fluid and impact thresholds from high‐frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near‐continuous transport conditions. Although both thresholds are thus important for high‐frequency saltation prediction, our results suggest that the time‐averaged saltation flux is primarily governed by the impact threshold.

Large Effects of Particle Size Heterogeneity on Dynamic Saltation Threshold

“…Based on their experiments and a physical parametrization of near‐surface particle dynamics, Creyssels et al () proposed a linear relationship between the nondimensionalized transport rate and the Shields number (the reason for parametrizing aeolian transport by Θ becomes apparent shortly), where ρ p is the particle density, g the gravitational constant, and d 50 the median particle diameter: where is the dynamic threshold Shields number and C Q a proportionality factor. Such a linear transport law is currently favored among most aeolian transport physicists (Creyssels et al, ; Durán et al, ; Ho et al, ; Kok et al, ; Martin & Kok, , ). However, discrete element‐based simulations of saltation transport suggest a nonlinear transport law because of midair collisions (Carneiro et al, ), which can be parametrized via (see Figure S1 in the supporting information, which shows data from numerical discrete element method‐based simulations of sediment transport that have been experimentally validated in a number of recent studies; Durán et al, , , ; Pähtz & Durán, , , ) …”

“…Saltation transport laws, such as equations and , assume that saltation transport is saturated (or continuous; Pähtz & Durán, ). That is, if we defined u t indirectly through a saltation transport law (which is the assumed definition whenever one uses a saltation transport law to predict Q ), u t should convey information about the saturated state even though transport near u t is intermittent and thus undersaturated (Martin & Kok, ). According to recent studies (Pähtz & Durán, , ), transport saturates because splash entrainment of bed sediment supplies the transport layer nearly continuously with bed sediment until the flow becomes so strongly suppressed by the negative feedback of the particle motion that it can no longer compensate energy losses of rebounding particles, resulting in a sudden strong increase of deposition that compensates splash entrainment.…”

“…Note that Martin et al () identified u t in an equivalent manner from measuring the relationship between the average wind velocity at a constant large elevation and u * . Further, note that this method exploiting properties of saturated transport does not imply that transport at the associated dynamic threshold is saturated because equation is effectively extrapolated to vanishing transport described by equation , which neglects the transitional region that occurs near the dynamic threshold because transport is intermittent (Martin & Kok, ).…”

“…In contrast to the static threshold above which saltation can be initiated (e.g., Bagnold, ; Burr et al, ; Chepil, ; de Vet et al, ; Gillette et al, ; Iversen & Rasmussen, ; Iversen et al, ; Merrison et al, ; Nickling, ; Raffaele et al, , and references therein), u t has only rarely been systematically studied in controlled laboratory settings, especially in recent history (there are numerous poorly controlled field studies though; Barchyn & Hugenholtz, ; Li et al, ; Martin & Kok, , ; Martin et al, , and references therein). Even today, we largely rely on the old data sets by Bagnold () and Chepil (), who measured u t by visual means.…”

Scales of collective entrainment and intermittent transport in collision-driven bed load

“…One possible mechanism for collective motion is collisional impulses. Collisions are widely recognized as drivers of bed load transport in aeolian systems where separate thresholds for entrainment without collision, the 15 fluid threshold, and with collisions, the impact threshold, have been defined (Bagnold (1941); Martin and Kok (2016)). In aeolian systems these collisions are accompanied by dramatic 'splash' events where numerous particles are ejected at once (i.e.…”