On the aeolian saltation bed shear stress and saltation roughness length

Abstract

The aerodynamic surface roughness length, z 0 , is an input parameter in models to calculate the bed shear stress from the geostrophic wind. The bed shear stress is a fundamental variable to link flow conditions to sediment transport and dust emission. This study examines by numerical integration of the equations of motion for grains how the cloud of grains in transport modifies the wind profile. Transport parameters as the ejection angle, the lift-off velocity of the grains and the transport rate are systematically varied and the resulting air borne bed shear stresses profile τ a (z) and effective roughness length z 0s during saltation are evaluated. The results for z 0s and τ a (z) are compared with the analytical model of Raupach. Our grain-scale analysis also provides valuable information about the splash function which is an important input for numerical models. Using a constant sand transport rate estimated from wind tunnel data yields different τ a (z) and z 0s dependent on the mean saltation height and mean ejection angle. The higher the mean saltation height of the grains, the higher is To and the lower is z 0s . Assuming proportionality of the vertical grain velocity to the shear stress above the saltation layer, as often done in numerical modelling, is shown to be inconsistent.