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| Titel |
A Real Two-Phase Mechanical Model for Rock-Ice Avalanches |
| VerfasserIn |
S. P. Pudasaini, M. Krautblatter |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060657
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| Zusammenfassung |
| Rock-ice avalanches in high mountain permafrost environments are a hazardous and poorly
understood process. Their hazard potential derives from the large volume, high velocities, the
potential entrainment of large amounts of rock-debris, ice, snow and water during the flow,
high impact pressures, and unpredictable flow paths and deposition patterns. In contrast to the
usual single-phase model of rock avalanches, the solid phase (ice) in rock-ice avalanches can
transform to fluid (water or slurry) during the course of the debris-avalanche and
fundamentally alter the multiple mechanical processes. We postulate that a real two-phase
debris flow model could much better address the dynamic interaction of solid (rock and ice)
and fluid (water, snow, slurry and fine particles) rather than a simple single-phase
Voellmy- or Coulomb-type model. For this, we enhance the general two-phase debris
flow model proposed by Pudasaini (2011) by additionally introducing two new
mechanical aspects typical for the rock-ice avalanches: (a) the dynamic strength
weakening including the internal fluidization and basal lubrication, as well as (b) the
internal mass and momentum exchanges between the phases. In these models, the
effective basal and internal friction angles are variable and are described in terms of
evolving effective solid volume fraction (rock and ice), friction factors, volume
fraction of the ice, true friction coefficients and the lubrication and fluidization
factors. These factors are functions of several physical parameters and mechanical
and dynamical variables, including the volume fractions of the solid, shear-rate
and the normal stresses. Rock-ice avalanches are a unique scenario in geophysical
mass flows, where phase exchange and material strength weakening occurs and can
dominate the flow dynamics. Here, we present an innovative approach to model and
simulate these two special aspects. Additionally, in the model, the inertial terms
include the hydraulic pressure gradients and the virtual mass. The source in the solid
momentum includes gravity, the Coulomb friction, slope gradient, buoyancy, and the
generalized drag. The source term for the fluid momentum includes gravity, fluid
pressure and topographic gradients, enhanced non-Newtonian viscous stresses,
and the drag. There are strong couplings between the solid and fluid momentum
transfer.
The new enhanced two-phase model can better explain dynamically changing frictional
properties of rock-ice avalanches that occur internally and along the flow path. Both mass and
momentum exchanges allow for a much more realistic simulation, especially during the
critical initial and final stages of avalanche propagation. Benchmark numerical simulations
demonstrate that the dynamics of permafrost rock-ice avalanche is fundamentally different
form that of pure rock avalanches. The model simulations reveal special features of
rock-ice avalanche propagation form and dynamics, similar to those observed, e.g., in
the 2007 Bliggspitze rock-ice avalanche event. Numerical results also reveal that
mass and momentum exchange between the phases and the associated internal
and basal strength weakening offer a new explanation for the exceptionally long
run-out distances leading to higher flow mobility typical for high-mountain rock-ice
avalanches. These new results substantially improve modelling run-out distances
and inundation areas, and could significantly contribute to hazard prediction and
mitigation in high-mountain permafrost environments. Here we show that the new
two-phase rock-ice avalanche model can yield a novel and enhanced representation of
multiple processes that lead to the high and changing mobility of rock-ice-avalanches. |
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