New Zealand Journal of Geology and Geophysics abstracts
Rheological properties of a remobilised-tephra lahar associated with the 1995
eruptions of Ruapehu volcano, New Zealand
V. MANVILLE1,2
K. A. HODGSON2,3
J. D. L. WHITE1
1Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
2Wairakei Research Centre
Private Bag 2000
Taupo, New Zealand
3Present address: 2 Pinfold Rd, RD1, Palmerston North, New
Zealand.
Abstract Volcanic activity at Ruapehu during 1995/96
represents the most voluminous volcanic activity in New Zealand this century.
Early in the eruption sequence, lahars were generated by the ejection of water,
lake sediments, and pyroclastic material from the summit Crater Lake. Lahars
(volcanic debris flows) generated by the remobilisation of pyroclastic fall
deposits on the upper slopes of the volcano followed the two largest
tephra-producing eruptions in October 1995, representing a previously
unrecognised hazard. Prediction and mitigation of such hazards requires
quantitative knowledge of initiation processes, lahar mechanics (i.e.,
rheology), and appropriate hydrodynamic routing for channelised flows. The
rheological properties of a remobilised tephra lahar in the Mangatoetoenui
valley were estimated using techniques developed for determining the properties
of debris flows based on the geometry of their deposits. Calculated maximum
flow velocities of 12-27 m/s, volumetric flow rates of
1300-2000 m3/s, shear strengths of 1.9-3.4 x
103 Pa, and plastic viscosities of 29-93 Pa.s compare well
with measured and estimated examples cited in the literature, and may aid in
understanding and constraining this new class of hazard at Ruapehu volcano.
Keywords lahars; debris flows; hazards; velocity; rheology;
shear strength; viscosity; Froude number; modified Reynolds number
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 157-164
0028-8306/98/4102-0157 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (1381K); (scanned from paper original: notes about this process)
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