Paper No. 2
Presentation Time: 8:20 AM


ELLIS, Jean, Marine Science Program and Department of Geography, University of South Carolina, Columbia, SC 29208, LI, Bailiang, Environmental Sciences, Xi'an Jiaotong-Liverpool University, Suzhou, 215123, China and SHERMAN, Douglas J., Geography, University of Alabama, 202 Farrah Hall, University of Alabama, Tuscaloosa, AL 35487,

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 (κa: 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 κa . 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.