Field Estimates of Thresholds for Aeolian Sand Transport

The transport of sand by wind has been the subject of extensive research. Despite numerous efforts, however, substantial discrepancies persist among theoretical models, wind tunnel experiments, and field observations concerning the magnitude of the critical threshold necessary to induce sand movement over heterogeneous surfaces.

In this study, we conducted field experiments to quantify surface shear velocities and wind velocities across seven heterogeneous surfaces characterized by a broad range of grain sizes. Surface shear velocities were measured using flush-mounted Irwin sensors (IS), while wind velocities were measured using a thermal anemometer (TA) positioned 30 mm above the surface and a 3-D ultrasonic anemometer (UA) 0.6 m above the surface. A high-resolution video camera monitored the surface to document sand motion. IS and TA sensors were emplaced within the view field of the camera. Reptating sand grains were captured using adhesive-coated plastic sticks. A total of 46, quality-controlled, threshold event data sets were acquired from the IS and TA measurements and 39 from the UA measurements.

For each event, surface shear velocities and wind velocities were block-averaged at 1-second intervals. Maximum and minimum values of shear (IS) and wind velocities (TA) were determined for each threshold event. Maxima are defined as the fastest shear and wind velocities without sand movement in the 5 seconds before an event, and minima are defined as the slowest velocities with movement in the 10 seconds before an event. Shear velocities in the constant stress layer (UA) were estimated with 2 minute-averaged Reynolds stress derivations. Median grain sizes for each event were estimated via photosieving. This approach enabled the estimation of threshold conditions corresponding to specific grain sizes, bracketed by their respective maximum and minimum values.

Our results from surface and near-surface data indicate no statistically-significant relationship between threshold shear or wind velocities and median grain sizes ranging from 131 to 475 µm. The mean of the maximum and minimum surface shear velocities for the 1-second block averages is 0.17 m/s (standard deviation, 0.02 m/s). The respective wind velocity threshold is 5.28 m/s (standard deviation, 1.64 m/s). Testing longer averaging intervals (1.5, 2, and 3 seconds) yielded consistent results, with a reduction in the differences between maximum and minimum values as the interval increased, but yielding no significant relationships. The shear velocity estimates from the ultrasonic anemometer data indicate a statistically significant, Bagnold-type relationship for the threshold, but with an “A” coefficient of 0.14 (standard deviation, 0.05) for the fluid threshold.

These findings support the concept of equal mobility, whereby a single threshold shear or wind velocity can initiate sand motion for a range of grain sizes on heterogeneous surfaces. The findings also indicate a decoupling of threshold conditions as measured at or near the surface and those as measured typically in the constant stress layer.