Testing the suitability of frictional behaviour for pyroclastic flow simulation by comparison with a well-constrained eruption at Tungurahua volcano (Ecuador)
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We use a well-monitored eruption of Tungurahua volcano to test the validity of the frictional behaviour, also called Mohr–Coulomb, which is generally used in geophysical flow modelling. We show that the frictional law is not appropriate for the simulation of pyroclastic flows at Tungurahua. With this law, the longitudinal shape of the simulated flows is a thin wedge of material progressively passing, over several hundreds of metres, from an unrealistic thickness at the front (<<1 mm) to some tens of centimetres. Simulated deposits form piles which accumulate at the foot of the volcano and are more similar to sand piles than natural pyroclastic deposits. Finally, flows simulated with a frictional rheology are not channelised by the drainage system, but affect all the flanks of the volcano. In addition, their velocity can exceed 150 m s−1, allowing pyroclastic flows to cross interfluves at bends in the valley, affecting areas that would not have been affected in reality and leaving clear downstream areas that would be covered in reality. Instead, a simple empirical law, a constant retarding stress (i.e. a yield strength), involving only one free parameter, appears to be much better adapted for modelling pyroclastic flows. A similar conclusion was drawn for the Socompa debris avalanche simulation (Kelfoun and Druitt, J Geophys Res 110:B12202, 2005).
KeywordsPyroclastic flows Numerical simulation Rheology Tungurahua
Those studies have been funded by the French Institut de Recherche pour le Développement (IRD). We thanks the Japan International Cooperation Agency (JICA) and Dr Hiroyuki Kumagai for the use of seismic data. The paper was improved by Fran van Wyk de Vries and by the useful comments of A. Neri, M. Bursik, an anonymous reviewer and the editor. The authors deeply thank the staff of the Tungurahua Volcano Observatory (IG-EPN), especially those in charge during the July 14th and August 16th eruptions.
Thickness of pyroclastic flows at Tungurahua simulated using the frictional model. Same parameters and colour scale as for Fig. 6. The red points above the crater indicate pyroclastic flow genesis (duration, 40 min). (AVI 6018 kb)
Thickness of pyroclastic flows at Tungurahua simulated using the constant retarding stress model. Same parameters and colour scale as for Fig. 9. The red points above the crater indicate pyroclastic flow genesis (duration, 40 min). (AVI 8375 kb)
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