Wetting in Granular Flows: Debris Flows & Ice Avalanches

Barbara Turnbull (The University of Nottingham)

Frank Adams 1, Alan Turing Building,

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 ex-
plore 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 coat-
ing 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 mod-
elling 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.

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