IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A2_3G-O11

Room A4

Date/Time: July 21 12:00-12:15

Friction properties controlling deposit shape of dense pyroclastic flows: insights fromrecent dome collapse events in Soufriere Hills Volcano, Montserrat
Fukashi Maeno1 , R Stephen J Sparks2 , Andrew J Hogg3 , Peter J Talling4
1Earthquake Research Institute, University of Tokyo, Japan, 2 School of Earth Sciences, University ofBristol, UK, 3 School of Mathematics, University of Bristol, UK, 4 National Oceanography Centre,Southampton, UK
E-mail: fmaeno@eri.u-tokyo.ac.jp

Capturing the major characteristics of dense pyroclastic flows generated by the collapsing of lava domes isimportant to assess volcanic activities and hazards, but is a significant challenge because the mechanics of thegrains and their interactions are incompletely understood. One approach has been to exploit the thinness of theflows relative to their length by employing a depth-averaged description. A key issue is the granular friction law
that is introduced into the models. Recent laboratory studies on dense granular flows suggest that rheology canbe described by a friction coefficient. Variation of this coefficient with shear rate and pressure is captured througha dimensionless inertial number. Under the shallow water assumption how well this friction model works remains
unclear when applied to pyroclastic flows.
Recent dome collapse events in Soufriere Hills volcano, Montserrat, provide good examples to study the dynamics
of dense pyroclastic flows and to examine granular flow models, because of abundant geological and geophysicaldata. In this study, the July 2003 and May 2006 dome collapse events and resultant pyroclastic flow deposits areinvestigated. The most intense phase of the 2003 event produced the deposit 170 M m3 in 2.6 hours, and the shapeof proximal submarine deposit offshore Montserrat is characterized by semicylindrical, steep-sided lobes. The 2006
event produced 97.8 M m3 in 35 min and the deposit is characterized by a more elongated shape in flow direction
than the 2003 deposit and by channel and levee-like facies (Trofimovs et al., 2012, BV). Geophysical observationsuch as seismic and strain records also constrain the variation of discharge rates of pyroclastic flows during theevents.
To investigate the factors controlling the shape of pyroclastic flow deposit, we used a 2D shallow water modelwith two types of Coulomb-type friction models. One had a constant friction coefficient, and another had a friction
coefficient that depends upon the dimensionless inertial number of the motion. The models are applied to a simple
system or the terrain of Soufriere Hills volcano. When the latter friction model was examined, the variation ofdeposit shape such as channel and levee-like facies was reproduced, depending on initial mass, discharge rate orslope angle. Also our numerical results suggest that the inertial number dependent friction model works better after
the flow passing a slope break point where slope angle is equal to the friction angle at zero shear rate. Coupling
effects of discharge rates, slope and granular friction properties may explain the different shapes of the pyroclatic
flow deposits produced by dome collapse events in Soufriere Hills volcano.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

948