However, it remains unclear how the flux of particles in aeolian saltation-the wind-driven transport of sand in hopping trajectories-scales with wind speed, largely because models do not agree on how particle speeds and trajectories change with wind shear velocity. Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. Though both fluid and impact thresholds are likely important for high-frequency saltation prediction, we find that impact threshold primarily governs the long-term occurrence of wind-driven sand and dust transport. As saltation activity increases, potential threshold crossings are increasingly governed by impact threshold, whose value is only 80% of fluid threshold shear velocity. We show that statistically-determined "effective" threshold stress decreases linearly with the fraction of time that saltation is active. Based on comprehensive high-frequency field saltation measurements, we provide the first field-based demonstration of distinct fluid and impact thresholds, and we determine the respective importance of these thresholds for modeling wind-blown sediment flux. However, the coexistence of distinct "fluid" and "impact" thresholds for the respective initiation and cessation of aeolian saltation, which is suggested by laboratory and numerical experiments, produces ambiguity in wind-driven transport predictions. Wind-blown sand and dust transport models depend sensitively on selection of the threshold wind stress. Limits imposed by noise levels in the signal and interferences from extraneous light sources were also identified.ĭespite the results presented being specific to the OGD model tested, much of the approach outlined is applicable to any OGD-type device (including Wenglor®) if the signal of the photo detector can be accessed directly. Practical implications of this are that there is potential for extracting size distribution information. Subsequent tests revealed that the response of the phototransistor (light sensor) can be linear when operated within certain workable limits. Inter-comparison among eight identical units of the OGD showed excellent repeatability (R² > 0.98 for 7 of 8 units). The OGD device provided particle counts and total signal response that were well correlated with sand trap data (R² between 0.66 and 0.88). The approach uses the highly time resolved signal from these sensors, which consist of a light emitter and a photosensitive sensor.Ī specific OGD that is manufactured by Optek (Carrollton, Texas, USA) was tested in a sediment transport wind tunnel alongside trap-style devices.
Despite continuing improvements, currently available devices for field measurement of sand movement have limitations.Īn optical gate device (OGD) for detecting the movement, size, and possibly speed of individual sand grains during aeolian sediment transport was analyzed. Through sand blasting during saltation, large amounts of dust are ejected into the atmosphere and transported long distances, impacting climate and human health. Movement of sand in response to wind is the most important feature of aeolian sediment transport on Earth and other planets.
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