Aeolian sand transport: Length and height distributions of saltation trajectories

We report wind-tunnel measurements on aeolian sand transport aiming at characterizing the distribution of the length and height of trajectories of the saltating particles. We employ a simple horizontal sand trap device to assess the distribution of saltation length while the distribution of saltation height is inferred from the measurements of the particle lift-off velocity by means of particle velocimetry tracking techniques. Our measurements reveal that the saltation length and height present a continuum distribution which decreases monotonously and exhibits a long tail that can be well described by a lognormal law. Interestingly, these distributions are found almost invariant with the flow strength. As a consequence, the mean saltation length ((l) over bar) and height ((h) over bar) are independent of the flow strength confirming previous indirect measurements. The influence of the flow strength is only seen through the tail of the saltation length distribution: the higher the Shields number, the flatter the distribution tail. Finally, experiments carried out with sand of different sizes show that the mean saltation length and height are not related to the sand grain size through a simple manner but depend instead linearly with the height z(f) of the Bagnold focus point: (l) over bar approximate to 6z(f) and (h) over bar approximate to 0.6z(f). This last result emphasizes that the focus height is an important characteristic length scale of the saltation transport. (C) 2014 Elsevier B.V. All rights reserved.