INITIATION OF MOTION FOR WIND BLOWN SAND: A NEW LOOK AT AN OLD MODEL

Predicting aeolian sand transport rates has proven problematic for aeolian geomorphologists studying terrestrial or extraterrestrial surfaces. In a recent paper, Sherman et al. (2013) recalibrated the empirical constants for the most frequently used mass transport equations. They concluded the Lettau and Lettau (1978) model with an apparent von Kármán parameter (κ_a: Li et al., 2010) to estimate shear velocity, is the most robust. An integral component of the Lettau and Lettau model is threshold shear velocity (u_*t). It is commonplace to use Bagnold’s (1936) equation to calculate u_*t, which includes A or the square root of Shield’s parameter. Typical values for A range between 0.08 or 0.1 (depending on active or fluid threshold). Sherman et al. (2013) also found the corrected models still substantially over-predict mass transport rates. They suggest the error could be reduced with better quantification of u_*t.

Here, we present data from a pilot study designed to assess the variability of A to improve estimations of u_*t, which in turn, will improve transport rate predictions. We measured or calculated all parameters within the Lettau and Lettau model and u_*t equation. In Jericoacoara, Brazil (BRA) and Esposende, Portugal (POR) wind velocities were measured with cup anemometer towers and transport rates measured using traps for 27 data runs lasting 120 to 900 seconds each. Mean grain size was 0.30 mm (BRA) and 0.31 mm (POR). Mean shear velocities were calculated as 0.59 m/s (BRA) and 0.42 m/s (POR) from the measured wind speeds using the Law of the Wall equation and κ_a . Adjusted A values ranged from 0.07 to 0.23 with a mean and standard deviation of 0.15 and 0.04. There is no statistical difference between A values derived at the different field sites. No relationship exists between measured A and mass transport rates or grain size. The mean value of 0.18 is approximately twice that usually assumed for the dynamic threshold.