Dynamics of particles in aeolian saltation

In aeolian transport saltation is an important link by which momentum is transmitted from the air to the bed through grain impact, but momentum transfer, impact and subsequent entrainment take place in a very shallow layer at the air-bed interface with large velocity gradients. Consequently, experimental evidence on the impact-entrainment processes has been obtained from few and simplified studies of the splash (e.g. Willetts & Rice 1986, 1989, Mitha et al. 1986), from investigating the vertical intensity and distribution of the saltation cloud including the wind profile within the saltation layer (e.g. Williams 1964, Iversen & Rasmussen 1999, Liu & Dong 2004), and finally from theoretical reasoning and numerical modelling (Owen 1964, Sørensen 1985, Anderson & Haff 1988, 1991, McEwan & Willetts 1991, 1993, Shao & Li 1999, Spies & McEwan 2000). Direct measurement of particle trajectory (e.g. White and Schultz 1977), Mitha et al. 1986) or particle speed (Rasmussen 2002, Dong et al. 2002) can be used to verify the (plausible) assumptions contained in such models, but data are sparse. The present paper reports from a series of wind tunnel measurements where the vertical variation of grain speed has been recorded using a Doppler method above beds of quarts grains of different diameter. It focuses on results of the horizontal component of grain velocity for one size class and for a range of friction speeds and also compares the experimental results to values predicted using a analytical model of saltation.