University of Cambridge > Talks.cam > Fluid Mechanics (CUED) > Wetting in Granular Flows: Debris Flows & Ice Avalanches

Wetting in Granular Flows: Debris Flows & Ice Avalanches

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If you have a question about this talk, please contact Dr Henry Burridge.

There are many cases of geophysical granular flows where fluid mediates the grain contacts: here we focus on two of them ranging from fluid volume fractions of hundredths of a percent to 60%.

Firstly we focus upon the hazard posed by avalanches of rock & ice. These can arise from collapsing glacier séracs, rock-faces previously stabilised by permafrost or the activity of an ice-capped volcano. The extraordinary mobility of the extreme event at Karmadon, Russian Caucasus, in September 2002 brought such ice-bearing flows into sharp focus. Here, we explore the hypothesis that localised melting within such ice-bearing flows may significantly alter the dynamic characteristics compared to a classical dry granular shear flow. Building sandcastles on the beach as children, we relied on the mechanical strength of moisture coating the sand grains to hold our structures together. Furthermore, when we ice skate it is in fact a microscopic ‘pre-melted’ water film between blade and ice that allows us to glide over the surface. So, when considering the granular mechanics of ice, can we expect these two phe- nomena to interact? The dynamical effects of melting processes that lead to wetted particle surfaces are here investigated in a laboratory experiment.

In contrast, when we move to higher fluid volume fractions, the flow separates into dry and wet regimes. This can be seen in debris flows of water, sediments and rocks and volcanic lahars. In these flows dry rocks collect the front of the flow forming a dry snout that is pushed along by the viscoplastic water/fines mixture in the flow’s body. Despite the largest, most energetic rocks and boulders typically forming part of the granular snout, debris flow modelling has been largely focussed on the viscoplastic behaviour in the body, with relatively little consideration given to the sometimes extreme deviations in predicted impact pressures. Here we discuss a method for characterising these different flow regimes using data from laboratory-scale chute flows of glass bead, water and glycerol mixtures.

This talk is part of the Fluid Mechanics (CUED) series.

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