12:15 PM - 12:30 PM
★ [HCG35-12] Modeling runout and deposition of dense granular flows and its application to pyroclastic density currents in nature
Keywords:pyroclastic density currents, granular flow, modeling, shallow water theory
Recent dome collapse events at Soufriere Hills volcano provide good examples to study the dynamics of pyroclastic density currents and to examine granular flow models, because of abundant geological and geophysical data. Here the 20 May 2006 lava dome collapse and resultant deposit are focused. The total collapse volume of the 2006 event was 97 Mm3, of which about half was dense granular components (Trovimovs et al., 2012). The seismograms showed a prolonged buildup of increasingly large block and ash flows lasting ~90 min. The most intense phase that produced the main deposit occurred in 35 min with two marked peaks. Based on the collapsed volume and the duration of seismicity, an average collapse rate can be estimated to be 4.7*104 m3/s. On the other hand, the submarine deposit is characterized by an elongated shape in flow direction and by levee-like facies. The deposit shape was very different from that produced in the 2003 event where collapsed materials 170 M m3 was released in 2.6 hours with an average collapse rate 1.8*104 m3/s, and the shape of proximal submarine deposit was characterized by semicylindrical, steep-sided lobes.
To investigate the factors controlling the shape of the deposit in 2006, the 2D shallow water model of granular flows with different types of Coulomb-type friction models is applied to the terrain of the Soufriere Hills. One had a constant friction coefficient, and another had a friction coefficient that depends upon the dimensionless inertial number (I) of the motion. For source condition, a constant mass or a line source with a flux function were used. Parameter studies were carried out within possible ranges of parameters such as volume, grain-size, and friction angles. When the I-dependent friction model was applied with a flux function and specific values of the parameters were used, the characteristics of deposit shape can be reproduced. The I-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. Our results suggest that coupling effects of discharge rates, slope and granular friction properties may explain the different shapes of the deposits produced by dome collapse events at Soufriere Hills volcano.
References: Maeno, F. et al. (2013) Physics of Fluids, 25, 023302, doi:10.1063/1.4792707, Trofimovs, J. et al. (2012) Bull. Volcanol., 74, 391-405.