Two-Dimensional Analysis of Dune Profile and the Effect of Grain Size on Sand Dune Morphology

Theoretical consideration shows that the steady-state profile of a dune subjected to an unidirectional wind is flat, low, and convex with the ratio of the dune height (h) to its upwind length (L) h/L = 0.05. This profile can be found in many barchans and transverse dunes. A steeper profile or a triangular-shaped profile cannot remain in a steady state under such conditions, as such a profile is only assured by bidirectional or multidirectional winds.Grain size is another factor governing dune profile. A coarse-grained sand dune has a greater difference in threshold velocity between the crest area and its lower levels than a finer-grained sand dune of a similar profile. A coarse-grained sand dune will have much longer periods of erosion taking place only at or near the crest, thus lowering the dune profile. This explains why all dunes composed of biomodal coarse-mode grains such as whaleback and “zibar” are flat and low.