Velocity profiles measured on the windward slope of a transverse dune

Abstract

AbstractVelocity gradients measured on the windward slope of a transverse dune show that flow acceleration strongly affects the turbulence structure of wind. Wind profiles measured below a height of 1.6m are nonlogarithmic and the altered nature of velocity profiles varies over the windward slope of a dune. These findings suggest that calculated values of shear velocity are largely dependent upon the height and location of wind speed measurements, as well as the strength of the prevailing wind. Moreover, if Bagnold‐type transport equations are used to calculate rates of sand transport on a dune, wind speeds must be measured within 0.2 m of the dune surface. If wind speeds are measured above this height, the resulting errors in calculated shear velocity will manifest themselves as errors in calculated rates of sand transport. If rates of sand transport are derived from wind speeds measured above 0.2 m the calculated rates may underestimate sand transport on the windward face of a dune as a result of flow acceleration. In contrast, if a dune has a separate crest upwind of the brink, the expansion and subsequent deceleration of flow downwind from the crest may result in an overestimation of sand transport.