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

1A1_3G-O2Room A4

Date/Time: July 21 9:15-9:30

Modeling dynamics and sedimentation of dilute pyroclastic density currents
Amanda B Clarke1 , Brittany D Brand2 , Barry Voight3
1Arizona State University, School of Earth and Space Exploration, USA, 2 Boise State University,
Department of Geosciences, USA, 3 Penn State University, College of Earth and Mineral Sciences, USA
E-mail: amanda.clarke@asu.edu

Dilute pyroclastic density currents (dilute PDCs, or surges) are lethal and destructive types of flows generated
by volcanic eruptions. Such flows are typically unsteady, density-stratified and turbulent, producing a range ofdeposit characteristics. Here we discuss models aimed at improving our understanding of the first order dynamics
that influence dilute PDC runout and damage potential. We also explore the mechanisms of sediment transportand deposition in order to link depositional characteristics with current dynamics. Model (1) comprises simulationof axi-symmetric flow and sedimentation from a steady-state, vertically uniform density current. Equations forconservation of mass, momentum, and energy are solved simultaneously, and effects of atmospheric entrainment,
particle sedimentation, basal friction, temperature changes, and variations in current thickness and density areexplored. The Rouse number and Brunt-Väisäla frequency are used to estimate the wavelength of internal gravity
waves in a density-stratified current, in order to predict deposit bedform characteristics. The model predicts realistic
runout distances and bedform wavelengths for several well-documented field cases, although results are heavily
dependent on source conditions, grain-size characteristics, and entrainment and friction parameters. For instance,increasing particle settling velocity, by increasing particle size and/or decreasing total particle concentration,
decreases both runout distance and bedform wavelength. Model (2) uses a one-dimensional hydraulic balanceof sedimentation of clasts and entrainment of air for an idealized ash-cloud surge current moving away from thechanneled PDC "avalanche" source. The idealized ash-cloud surge is assumed to flow normal or obliquely to anavalanche track along a series of 1D flow paths defined by digital topography and broken into segments of constantslope and arbitrary length. In its existing form a series of starting points for 1D surge calculations are selectedalong the PDC channel, or arbitrarily specified within the modeled pyroclastic flow inundation limits, while mass flux
is provided as an input parameter.
In both models, the surges move laterally while entraining air and sedimenting particles until their bulk densities
fall below that of ambient air, signifying lift-off and defining the surge deposit limit. Deposit limits are used todefine the possible area that may be inundated by surges. For both models, preliminary runs for test cases
are consistent with observations, but because our existing formulations are based on depth-averaged equations,assumptions about density stratification and corresponding Brunt-Väisäla frequencies are oversimplified. Future
model development seeks to quantify controls on density profile, and make use of experimental observations of
sedimentation mechanics in controlled laboratory experiments.

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