1995 Ruapehu lahars in relation to the late Holocene lahars of Whangaehu River, New Zealand

Abstract

Observations of active flows, their deposits, and the effects of lahars generated during the 1995 eruptions of Ruapehu are compared to those of the last 2000 years. The 1995 lahars were generated by similar mechanisms, and had similar volumes and flow rheologies, to those of the last 135 years. However, the large number of lahars in the 1995 sequence and the eventual emptying of the entire Crater Lake on Mt Ruapehu by lahars is distinctive in the context of the historic record. The 1953 and 1975 lahars, although of smaller total volume than the largest 1995 lahar, had higher peak discharges and consequently greater effects on life and property within 57 km from the source. This implies that failure of a lake dam is the most efficient mechanism for generating a fast lahar with high peak discharge, and that eruptions through Crater Lake are more efficient at generating a lahar with high peak discharge when the lake is full. The scant 1995 lahar deposits preserved indicate that the geological record of lahars is likely to be incomplete and biased to events of greatest magnitude. Given this limitation, lahars which formed deposits of the prehistoric (>135 years old) Onetapu Formation were of larger magnitude in all respects, and often more concentrated with sediment, than those of the last 140 years (including 1995). In order to generate such events, catastrophic total emptying of Crater Lake is required. Alternatively, some of the events may have been the result of small flank collapses on the eastern slopes of Ruapehu. These larger events are less frequent than small eruption‐triggered lahars on Ruapehu, but they may be preceded by little or no warning. Future lahar hazard mitigation strategies in the Whangaehu River should begin with improving the detection and early warning of such flows.