Profiles Analysis of Sand Dunes and their Steady State Signification

Sand dunes that are subjected to different wind directions attain different steady state profiles. Two fundamental types of profile are known: the convex asymmetric one, characterizing barchans and transverse dunes, and the symmetric triangular one of seif dune. The distribution of wind velocity along these two fundamental profiles indicates that the convex profile of barchan and transverse dunes are in a steady state to uni-directional winds. A steady state profile having a separate crest and brink ensues a higher advance of slip-face relative to the windward slope.A triangular sharp-edged profile of seif dunes is not in a steady state under long-term perpendicular uni-directional wind. Steady state can be attained, however, by bi-directional winds of oblique approach to the dune. In this case seif dunes are in a steady state with respect to profile oblique to dune axis and parallel to wind direction. A sharp-edged profile causes a substantial separation of perpendicular flow over the lee slope. The separation vortex reduces the wind-velocity's rate of increase near the crest line which keep its sharp profile during a short period of perpendicular wind. In the case of the wind shifting to a perpendicular direction for a longer period, the triangular shape will turn into a convex asymmetric one.