The influence of bed conditions on sand transport behaviours in aeolian desert environments.

Abstract:

In aeolian environments, the principal mode of sand transport is saltation. Typically, grains travel with ballistic trajectories and when they impact onto the sand surface they produce a splash of new ejected grains. The transport flux is therefore controlled by three principal variables; the wind strength (velocity or shear velocity), the sand characteristics (grain size and density) and the boundary conditions at the surface (e.g. sediment availability). The scaling of sand flux with above threshold wind strength has been long established with the majority of transport laws invoking a cubic relationship between shear velocity and sand flux accounting for major variations in sand transport. However, recent wind tunnel and field studies have suggested that the characteristics of the surface boundary might provide a first-order control on the exponent of the transport law.

Here we present the first field measurements of sand flux at differing wind velocities over distinct erodible and non-erodible surfaces to determine the influence of the bed boundary conditions on sand transport. We quantify saltation height using terrestrial laser scanning and wind velocity using both hotwire and 3D sonic anemometers. In addition, Wenglor optical gate sensors coupled with Bagnold-type sand traps provide saltation flux and height.

Our analysis focuses on two key features of the saltation system, saltation layer height and sand flux. We find that wind speed has a stronger influence on the saltation layer height when grains interact with non-erodible surfaces. This strong coupling provides a cubic dependence of the sand flux with shear velocity. On erodible surfaces our field data show that this coupling is weaker and so the dependence of the sand flux with shear velocity becomes quadratic. This variability in coupling of sand transport characteristics with surface erodibility is discussed as a potential mechanism for the origin and development of proto-dunes.