Initiation of motion of quartz sand grains

Summary

Direct observation at a limited range of wind speeds of grain behaviour near the upwind edge of a sand deposit (by means of high-speed film) shows that disturbed grains usually, but not always, roll before taking off into reptation or saltation. Time-averaged description of the population of initial motions is deficient because of the importance of pronounced flurries of grain activity which occur at intervals. The flurries are clearly associated with flow features in the clean air wind because they occur too close to the leading edge to be impact generated.

An adapted saltation model is used to explore the development of activity downwind which results from a sequence of first dislodgements (into rolling or take-off) recorded on film. The portrayal of the development of a saltation layer is plausible and can be checked by means of flux profile measurements. However, it emphasises the need for more information about turbulence in the grain laden layer.