Analysis of velocity profile measurements from wind-tunnel experiments with saltation

Investigations of wind-field modification due to the presence of saltating sediments have relied heavily on wind tunnels, which are known to impose geometric constraints on full boundary layer development. There remains great uncertainty as to which portion of the vertical wind-speed profile to analyze when deriving estimates of shear velocity or surface roughness length because the lower sections are modified to varying degree by saltation, whereas the upper segments may be altered by artificially induced wake-like effects. Thus, it is not obvious which of several alternative velocity-profile parameterizations (e.g., Law of the Wall, Velocity Defect Law, Wake Law) should be employed under such circumstances. A series of experimental wind-tunnel runs was conducted across a range of wind speed using fine- and coarse-grained sand to collect high-quality, fine-resolution data within and above the saltation layer using thermal anemometry and ruggedized probes. After each run, the rippled bottom was fixed with fine mist, and the experiment repeated without saltation. The measured wind-speed profiles were analyzed using six different approaches to derive estimates of shear velocity and roughness length. The results were compared to parameter estimates derived directly from sediment transport rate measurements, and on this basis, it is suggested that one of the six approaches is more robust than the others. Specifically, the best estimate of shear velocity during saltation is provided by the logarithmic law applied to the profile data within about 0.05 m of the bottom, despite the fact that this near-surface region is where profile modification by saltating sediments is most pronounced. Uncertainty remains as to whether this conclusion can be generalized to field situations because progressive downwind adjustments in the interrelationship between the saltation layer and the wind field are anticipated in wind tunnels, thereby confounding most analyses based on equilibrium assumptions. (C) 2003 Elsevier B.V. All rights reserved.