Aeolian sand transport over gobi with different gravel coverages under limited sand supply: A mobile wind tunnel investigation

- The blown sand flux profile over gravel beds is non-monotonic.
- Sand transport over gravel beds can be expressed by an Owen-type saltation equation.
- Gravel beds can evidently reduce sand transport.
- Gravel size contributes to the reduction in sand transport.

Transport is one of the important aeolian processes on earth. Here we report results of systematic tests using a mobile wind tunnel to examine aeolian sand transport over different gravel beds and at different wind speeds. The gravel beds differ in terms of gravel size and spacing. The results reveal that the blown sand flux profile over gravel beds is non-monotonic such that sand transport increases with height above the surface for the first 5-8 cm before exponentially decreasing. The height at which the maximum sand transport rate occurs tends to increase with increasing both the experimental wind velocity and gravel coverage. Furthermore, the total sand transport rate in the upper exponentially decreasing zone of the sand flux profile scales as u∗2-u∗t2. However, sand transport over gravel beds with different coverages within the 0-20 cm layer can be well expressed by an Owen-type saltation equation: q=g(C)ρgu∗(u∗2-u∗t2) where q is the total sand transport rate, u∗ is the friction velocity, u∗t is the threshold friction velocity, g is the gravitational acceleration, ρ is the air density, g(C) is a cubic polynomial equation of gravel coverage C. In addition, gravel beds can obviously reduce sand transport compared with the same surface without the tested gravels, and the increase in gravel size benefits the reduction in sand transport.