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Aeolian Mechanics and Dune Morphodynamics |
Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, UK
Department of Geography, Coventry University, Coventry CV1 5FB, UK
Few empirical measurements have been undertaken of shear velocity, sand transport or dune morphology in the upwind basal regions of dunes. This study compares field measurements in this region from a barchan dune in Oman with calculations from a complex flow model using linearised equations (FLOWSTAR) and measurements in a wind tunnel.
The calculations of shear velocity from wind velocity observations in the field and those predicted by the FLOWSTAR model reflect the observations of previous studies in exhibiting the widely reported upwind reduction in shear velocity. Despite these observations, measurements of sand transport did not reduce at the base of the dune. The implications of these findings for the predictive power of typical saltation flux equations and the dynamics of the dune itself are discussed.
Wind tunnel modelling using a pulse wire probe suggests that the conventional methods of shear velocity derivation are inadequate. The pulse wire measurements exhibited no reduction in shear at the toe. An argument is presented to explain the maintenance of shear velocity in this position. It is suggested that the reduction in shear stress due to the fall in wind velocity is counteracted by an increase in shear stress due to streamline curvature. The adequacy of field methods for determining shear velocity in this respect is considered.
