Saltation (geology)

:"For definition, and other use disambiguation, see Saltation"

In geology, saltation (from Latin, "saltus", "leap") is a specific type of particle transport by fluids such as wind, or the denser fluid water. 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.

A recent study finds that saltating sand particles induce a static electric field by friction. Saltating sand acquires a negative charge relative to the ground which in turn loosens more sand particles which then begin saltating. This process has been found to double the number of particles predicted by previous theory. [ [http://www.eurekalert.org/pub_releases/2008-01/uom-esf010708.php Electric Sand Findings, University of Michigan Jan. 6, 2008] ] This is significant in meteorology because it is primarily the saltation of sand particles which dislodges smaller dust particles into the atmosphere. Dust particles and other aerosols such as soot affect the amount of sunlight received by the atmosphere and earth, and are nuclei for condensation of water vapor.

ee also

*Saltation (biology)
*Saltatory conduction
*Aeolian processes
*The Physics of Blown Sand and Desert Dunes
*Bagnold formula
*Aeolian landform

External links

* [http://www.weru.ksu.edu/new_weru/multimedia/movies/dust001.mpg Dune sand saltation video, Kansas State University]
* [http://www.weru.ksu.edu/new_weru/multimedia/movies/dust003.mpg Close up of dune sand saltation, Kansas State University]
* [http://www.lbk.ars.usda.gov/wewc/biblio/bar.htm The Bibliography of Aeolian Research]

References