Please use this identifier to cite or link to this item: http://hdl.handle.net/2122/5526
AuthorsCagnoli, B.* 
Quareni, F.* 
TitleOscillation-induced mobility of flows of rock fragments with quasi-rigid plugs in rectangular channels with frictional walls: A hypothesis
Issue Date2009
Series/Report no./103 (2009)
DOI10.1016/j.enggeo.2008.07.009
URIhttp://hdl.handle.net/2122/5526
Keywordspyroclastic flows
rock avalanches
Subject Classification04. Solid Earth::04.08. Volcanology::04.08.08. Volcanic risk 
AbstractWe present here a new hypothesis to explain the high mobility of same rapid mass movements of rock fragments. We suggest that oscillations of flows with a quasi-rigid plug can result in reduction of their apparent coefficient of friction. This coefficient is computed as the ratio between drop in elevation and horizontal distance of travel. In our model, the effective friction during a downhill journay is a combination of the friction forces acting on the plug during the ascending and descending parts of its slope-normal oscillations. As a consequence of oscillations, the decreased contact with ground surface reduces the apparent coefficent of friction. Channel lateral surfaces can also support a portion of plug weight giving another contribution in the reduction of this coefficient. The support of lateral surfaces requires a relatively narrow channel such as a gully or the presence of levees whereas the reduced basal contact can be important also in larger channels that do not provide lateral support. We suggest that slope-normal oscillations are generated by ground asperities. The true coefficent of friction are larger than the apparent one because they account energetically for the oscillations that reduce basal contact. Thus we can say that our model is able to explain long runout distances as long as the energy dissipated by oscillations is accounted for by the true coefficents of friction that enter the calculations. Field and experimental investigation of several ideas discussed in this paper constitutes important aspects of future research that will improve the understanding of granular flows mobility.
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