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Saltation of sand

In geology, saltation (from Latin, saltus, "leap") is a specific type of particle transport by fluids. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface. Examples include pebble transport by rivers, sand drift over desert surfaces, soil blowing over fields, or even snow drift over smooth surfaces such as those in the Arctic or Canadian Prairies.

At low fluid velocities, loose material rolls downstream, staying in contact with the surface. This is called creep or reptation. Here the forces exerted by the fluid on the particle are only enough to roll the particle around the point of contact with the surface.

At higher speeds, the lift and moment exerted by the fluid on the particle is enough to pull it away from the surface and into the flow. Initially the particle moves quite rapidly compared to the flow and so has high lift, moving it away from the surface. As the particle moves into the faster flow away from the bed, the velocity difference between particle and flow decreases, and so lift decreases. When the particle weight is greater than the lift force, the particle sinks back towards the surface. During its descent, the particle keeps some of the speed it picked up in the faster moving flow, and so returns to the surface at higher speed than the fluid near the surface. This gives the particle a parabolic trajectory through the fluid, which is the defining characteristic of saltation.

Depending on the surface, more loose material could be dislodged by the impacting particle, the particle might disintegrate on impact, or the particle could continue bouncing downstream. In rivers, this process repeats continually, gradually eroding away the river bed, but also transporting in fresh material from upstream.

Suspension generally affects small particles ('small' means 100 micrometres or less for particles in air). These particles experience lift forces which are similar in magnitude to the weight of the particle. These smaller particles are carried by the fluid in suspension, and convected down stream. Particles are carried until the fluid decelerates, when the weight is greater than the lift and so particles settle out.

• Dune sand saltation video, Kansas State University
• Close up of dune sand saltation, Kansas State University
• The Bibliography of Aeolian Research