Editing Avalanche (section)

== Dynamics ==
When a slab avalanche forms, the slab disintegrates into increasingly smaller fragments as the snow travels downhill. If the fragments become small enough the outer layer of the avalanche, called a saltation layer, takes on the characteristics of a [[fluid]]. When sufficiently fine particles are present they can become airborne and, given a sufficient quantity of airborne snow, this portion of the avalanche can become separated from the bulk of the avalanche and travel a greater distance as a powder snow avalanche.<ref name="final_report">{{cite web |url=http://www.leeds.ac.uk/satsie/docs/final_report.pdf |title=SATSIE Final Report (large PDF file – 33.1 Mb) |page=94 |date=31 May 2006 |url-status=dead |access-date=5 April 2008 |archive-date=12 June 2020 |archive-url=https://web.archive.org/web/20200612122525/http://www.leeds.ac.uk/satsie/docs/final_report.pdf}}</ref> Scientific studies using [[radar]], following the 1999 [[Galtür Avalanche|Galtür avalanche disaster]], confirmed the hypothesis that a [[Saltation (geology)|saltation layer]] forms between the surface and the airborne components of an avalanche, which can also separate from the bulk of the avalanche.<ref>{{cite web |url=http://www.bbc.co.uk/science/horizon/1999/avalanche_script.shtml |title=Horizon: Anatomy of an Avalanche |publisher=[[BBC]] |date=25 November 1999}}</ref>

Driving an avalanche is the component of the avalanche's weight parallel to the slope; as the avalanche progresses any unstable snow in its path will tend to become incorporated, so increasing the overall weight. This force will increase as the steepness of the slope increases, and diminish as the slope flattens. Resisting this are a number of components that are thought to interact with each other: the friction between the avalanche and the surface beneath; friction between the air and snow within the fluid; fluid-dynamic drag at the leading edge of the avalanche; shear resistance between the avalanche and the air through which it is passing, and shear resistance between the fragments within the avalanche itself. An avalanche will continue to accelerate until the resistance exceeds the forward force.<ref>[http://www.avalanche.org/~moonstone/zoning/avalanche%20dynamics.htm Avalanche Dynamics] {{webarchive|url=https://web.archive.org/web/20090224071418/http://www.avalanche.org/~moonstone/zoning/avalanche%20dynamics.htm |date=24 February 2009}}, Art Mears, 11 July 2002.</ref>

=== Modeling ===
Attempts to model avalanche behaviour date from the early 20th century, notably the work of Professor Lagotala in preparation for the [[1924 Winter Olympics]] in [[Chamonix]].<ref name=Ancey>[http://lhe.epfl.ch/pdf/snow-avalanche.pdf Snow Avalanches], Christophe Ancey</ref> His method was developed by A. Voellmy and popularised following the publication in 1955 of his ''Ueber die Zerstörungskraft von Lawinen'' (On the Destructive Force of Avalanches).<ref>Voellmy, A., 1955. ''Ober die Zerstorunskraft von Lawinen''. Schweizerische Bauzetung (English: ''On the Destructive Force of Avalanches''. U.S. Dept. of Agriculture, Forest Service).</ref>

Voellmy used a simple empirical formula, treating an avalanche as a sliding block of snow moving with a drag force that was proportional to the square of the speed of its flow:<ref>[http://www.risknat.org/pages/programme_dep/docs/cemagref_etna/2002_Berthet-Rambaud.pdf Quantification de la sollicitation avalancheuse par analyse en retour du comportement de structures métalliques], page 14, ''Pôle Grenoblois d'études et de recherche pour la Prévention des risques naturels'', October 2003, in French</ref>

::<math> \textrm{Pref} = \frac {1} {2} \, { \rho} \, { v^2} \,\!</math>

He and others subsequently derived other formulae that take other factors into account, with the Voellmy-Salm-Gubler and the Perla-Cheng-McClung models becoming most widely used as simple tools to model flowing (as opposed to powder snow) avalanches.<ref name=Ancey />

Since the 1990s many more sophisticated models have been developed. In Europe much of the recent work was carried out as part of the SATSIE (Avalanche Studies and Model Validation in Europe) research project supported by the [[European Commission]]<ref>{{Cite web |url=http://www.leeds.ac.uk/satsie/ |title=SATSIE – Avalanche Studies and Model Validation in Europe |access-date=5 April 2008 |archive-date=12 June 2020 |archive-url=https://web.archive.org/web/20200612122550/http://www.leeds.ac.uk/satsie/ |url-status=dead}}</ref> which produced the leading-edge MN2L model, now in use with the ''Service Restauration des Terrains en Montagne'' (Mountain Rescue Service) in France, and D2FRAM (Dynamical Two-Flow-Regime Avalanche Model), which was still undergoing validation as of 2007.<ref name="final_report" /> Other known models are the SAMOS-AT avalanche simulation software<ref>{{cite web |url=http://arc.lib.montana.edu/snow-science/objects/issw-2009-0519-0523.pdf |title=Avalanche Simulation with SAMOS-AT |first1=Peter |last1=Sampl |first2=Matthias |last2=Granig |website=Archives and Special Collections – Montana State University Library |url-status=live |archive-url=https://web.archive.org/web/20220824211426/https://arc.lib.montana.edu/snow-science/objects/issw-2009-0519-0523.pdf |archive-date=Aug 24, 2022 }}</ref> and the RAMMS software.<ref>{{Cite web |title=Rapid Mass Movements System RAMMS |url=http://www.naturfare.no/_attachment/534838/binary/859980 |url-status=dead |archive-url=https://web.archive.org/web/20160304060348/http://www.naturfare.no/_attachment/534838/binary/859980 |archive-date=4 March 2016 |access-date=19 May 2015}}</ref>