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| Titel |
A numerical study of pyroclastic flow dynamics: A shallow-water model for gravity currents with wide ranges of density differences |
| VerfasserIn |
Hiroyuki Shimizu, Takehiro Koyaguchi, Yujiro J. Suzuki |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130712
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| Publikation (Nr.) |
 EGU/EGU2016-11008.pdf |
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| Zusammenfassung |
| During explosive volcanic eruptions, gravity currents of mixtures of volcanic particles and
gas flowing on the ground surface (pyroclastic flows) are commonly generated. The
pyroclastic flows are characterized by strong density stratification with wide ranges of density
ratio ρc∕ρa = 100−3, where ρc and ρa are the densities of the currents and ambient,
respectively. We aim to understand the dynamics of pyroclastic flows, such as flow
velocity and run-out distance. For this purpose, we have developed a new numerical
model based on shallow-water equations for gravity currents with a wide range of
ρc∕ρa.
In order to calculate gravity currents with a wide range of ρc∕ρa, the balance between the
driving force and the resistance of ambient at the flow front (i.e., front condition) needs to be
correctly taken into account. In previous works, two types of numerical models have been
proposed to solve the front condition: Boundary-Condition (BC) model and Artificial-Bed
(AB) model. In BC model, the front condition is calculated directly as a boundary condition
at each time step. In AB model, on the other hand, the front condition is calculated by setting
a thin artificial bed ahead of the front. We have verified these numerical models by comparing
their results with exact analytical solutions which are available for a simple case of
homogeneous currents. The results show that AB model provides good approximations
of the exact solutions for ρc∕ρa ≳ 102, given a sufficiently small artificial bed
thickness, whereas it fails to reproduce the exact solutions when ρc∕ρa ≲ 102.
On the other hand, the results of BC model agree well with the exact solutions
when ρc∕ρa ≲ 102, whereas it tends to overestimate the speed of the front position
when ρc∕ρa ≳ 102. It is, therefore, suggested that AB model is applicable to the
currents of ρc∕ρa ≳ 102, whereas BC model should be used for the currents of
ρc∕ρa ≲ 102.
On the basis of the present results, we have developed a two-layer model for a
pyroclastic flow with a strong density stratification, where a dilute overriding part with
ρc∕ρa = 100−1 is calculated by BC model, and a dense basal part with ρc∕ρa = 102−3 is
calculated by AB model. Our preliminary results show that the choice of appropriate
models for the calculation of the front condition is essential in order to reproduce the
dynamics of pyroclastic flows and the resultant sediments from the pyroclastic flows. |
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