HR: 0800h
AN: V31E-0704    [Abstracts]
TI: Experimental and Numerical Investigation of the Boundary Conditions of Over-Water Pyroclastic Flows
AU: * Wexler, J S
EM: jswexler@berkeley.edu
AF: Department of Earth and Planetary Science, UC Berkeley, McCone Hall, Berkeley, CA 94720-4767, United States
AU: Dufek, J
EM: dufek@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, UC Berkeley, McCone Hall, Berkeley, CA 94720-4767, United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, UC Berkeley, McCone Hall, Berkeley, CA 94720-4767, United States
AB: In over-water pyroclastic flows the ability of particles to sink through the water interface influences the speed of the flow and the distance it travels. In order to understand how the water interface affects flow mobility, we performed lab measurements to characterize the dynamics of particles at the water interface and then used these experimental results in numerical simulations. To quantify the dynamics of particles that impact the water surface we perform a series of 150 experiments of pumice-water collisions. We measure the fraction of particles that bounce and the energy lost following the collision using high-speed photography. We vary the collision angle, impact speed, particle mass and shape. We find that the restitution coefficient (the fractional decrease of speed) is insensitive to the initial speed and particle mass, but does depend on the collision angle. The amount of energy lost during the collision is proportional to the amount of time that the particle spends in contact with the water during the collision. We fit empirical models for the fraction of particles that sink and the fraction of energy lost during the collision to the experimental data. Similar experiments are currently being conducted for collisions with a bed of pumice particles. In order to determine how far and how fast flows will travel we use the results of continuum numerical simulations, similar to those in Dufek and Bergantz, 2007 (Journal of theoretical and computational fluid dynamics). We integrate probability distributions for sinking and energy lost over the velocity distribution at the water interface to determine the sink terms in the mass, momentum and energy equations. Flows that have what we think are realistic boundary conditions travel less than ~ 10 % farther and faster than a completely leaky end-member due to the high rate of energy dissipation; they have however ~ 50-60 % less runout distance than equivalent overland flows.
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting