Rheological properties of a remobilised‐tephra lahar associated with the 1995 eruptions of Ruapehu volcano, 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 × 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.