Ruapehu Lahar New Zealand 18 March 2007: Lessons for Hazard Assessment and Risk Mitigation 1995-2007
Harry J. R. Keys and Paul M. Green
Department of Conservation, Private Bag, Turangi, NZ
A moderately large lahar caused by a collapse of a tephra dam over the outlet of Crater Lake occurred 11 years and 4 months after the situation that led to it was first recognized. This allowed sufficient time for the considerable period needed by emergency managers and the Government to learn from scientists about the hazard, assess risks in pragmatic and robust ways, debate and progressively mitigate them. Early involvement of a wide range of agencies and the media was encouraged, enabling transfer of knowledge, development of interagency relationships and provided fora for a mitigation strategy to evolve in the face of sometimes controversial mitigation options. Decisions about options were based primarily on maximizing public safety and long-term utility in a valley subject to frequent, sometimes large, past and future lahars. These decisions, which had a significant political component also considered option suitability in a national park setting. A best-practice alarm system was installed based on acoustic flow monitors. A training levee structure was built to protect a highway and other vulnerable infrastructure and a river system near-by, and a key highway bridge raised and strengthened. Response agencies each prepared their own emergency plans which were then integrated and practiced over 3-4 years, incorporating an increasing readiness triggered by a Warning Level system linking to rising lake levels. Execution of the plans progressed smoothly on 18 March. The outcome showed that community resilience to lahars was achieved by adopting long-term solutions, rather than short-term ones, and carefully considering cultural, social, and technological perspectives, especially in a national park setting. Decision-making procedures that give weight to impartial scientific information, as well as values and legislation, are required for effective disaster mitigation, including an acceptance of an appropriate degree of residual risk.
-  W. R. Hackett and B. F. Houghton, “A facies model for a Quaternary andesitic composite volcano: Ruapehu, New Zealand,” Bull. Volcanology, 51, pp. 51-68, 1989.
-  H. J. R. Keys, “Lahars of Ruapehu Volcano, New Zealand: risk mitigation,” Annals Glaciology, 45, pp. 155-162, 2007.
-  B. F. Houghton, J. H. Latter, and W. R. Hackett, “Volcanic hazard assessment for Ruapehu composite volcano, Taupo Volcanic Zone, New Zealand,” Bull. Volcanology, 49, pp. 737-751, 1987.
-  M. Gillon, R. Fell, H. J. R. Keys, and M. Foster, “A contribution to public safety studies —failure modes and likelihood analysis for a tephra dam on an active volcano,” Bulletin of ANCOLD (Australian National Committee on Large Dams), 134, pp. 11-22, 2006.
-  G. T. Hancox, H. J. R. Keys, and M. G. Webby, “Assessment and mitigation of dam-break lahar hazards from Mt Ruapehu Crater Lake following the 1995-96 eruptions,” Procs of the NZ Geotechnical Society Symposium — Engineering in Hazardous Terrain, Christchurch, August 2001, NZ Geotechnical Society, Institution of Professional Engineers New Zealand, Wellington, pp. 385-409, 2001.
-  J. Lecointre, K. Hodgson, V. Neall, and S. Cronin, “Lahartriggering mechanisms and hazard at Ruapehu Volcano, New Zealand,” Natural hazards, 31-1, pp. 85-109, 2004.
-  I. A. Nairn, C. P.Wood, and C. A. Y. Hewson, “Phreatic eruptions of Ruapehu, April 1975,” New Zealand J. of Geology and Geophysics, 22, pp. 155-173, 1979.
-  A. C. Beck, “Volcanic activity at Mt Ruapehu from August to December 1945,” New Zealand J. of Science and Technology, B31, pp. 1-13, 1950.
-  B. E. O’Shea, “Ruapehu and the Tangiwai disaster,” New Zealand J. of Science and Technology, B36, pp. 174-189, 1954.
-  Department of Conservation, “Environmental and risk assessment for the mitigation of the hazard from Ruapehu Crater Lake — Assessment of Environmental Effects,” Department of Conservation, Tongariro Taupo Conservancy, Turangi, 142 pp., 1999.
-  V. Manville and S. J. Cronin, “Breakout lahar from New Zealand’s Crater Lake,” EOS, 98-43, pp. 441-442, 2007.
-  T. Taig, “Ruapehu lahar residual risk assessment,” Ministry of Civil Defence and Emergency Management, Wellington, New Zealand, 80 pp., 2002.
-  S. J. Cronin, K. A. Hodgson, V. E. Neall, A.S. Palmer, and J. A. Lecointre, “1995 Ruapehu lahars in relation to the late Holocene lahars of Whangaehu River, New Zealand,” New Zealand J. of Geological and Geophysics, 40, pp. 507-520, 1997.
-  K. Hodgson, J. LeCointre, and V. Neall, “Onetapu Formation: the last 2000 yr of laharic activity at Ruapehu volcano, New Zealand,” New Zealand J. of Geology & Geophysics, 50, pp. 81-99, 2007.
-  T. C. Hale, “The geology of the summit area, Mt Ruapehu,” Unpublished BSc dissertation, Canterbury University, New Zealand, 123 pp., 2000.
-  R. I. Tilling, “Volcanic hazards and their mitigation: progress and problems,” Reviews of Geophysics, 27, pp. 237-269, 1989.
-  T. C. Pierson and K. M. Scott, “Downstream dilution of a lahar: transition from debris to hyperconcentrated streamflow,” Water Resources Res., 21, pp. 1511-1524, 1985.
-  S. A. Fagents and S. M. Baloga, “Toward a model for the bulking and debulking of lahars” J. Geophys Res, 111, B10201, 2006.
-  J. L. Carrivick, V. Manville, and S. J. Cronin, “A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu, New Zealand,” Bull. Volcanology, DOI 10.1007/s00445-008-0213-2, 2008.
-  V. Manville, “Palaeohydraulic analysis of the 1953 Tangiwai lahar, New Zealand’s worst volcanic disaster,” Acta Vulcanologica. 16/1-2, pp. 137-151, 2004.
-  V. Manville, “Modelling the predicted break-out lahar from the Crater Lake of Mt Ruapehu, North Island, New Zealand,” GNS Science Report 2007/03, 29pp., 2007.
-  S. J. Cronin, J. Procter, and I. Fuller, “An assessment of the changes to channel capacity at the Whangaehu bund site, Mt Ruapehu,” Massey University report, Palmerston North prepared for the Department of Conservation, 30pp., 2005.
-  G. K. C. Clarke, “Hydraulics of subglacial outburst floods: new insights from the Spring-Hutter formulation,” J. Glaciology, 49, pp. 299-313, 2003.
-  R. G. LaHusen, “Detecting debris flows using ground vibrations,” US Geological Survey Fact Sheet 236-96, 1996.
-  G. S. Leonard, D.M. Johnston, D. Paton, A. Christianson, J. Becker, and H. Keys, “Developing effective warning systems: ongoing research at Ruapehu volcano, New Zealand,” J. of Volcanology and Geothermal Research, 172, pp. 199-215, 2008.
-  I. Galley, G. S. Leonard, D. M. Johnston, R. Balm, and D. Paton, “The Ruapehu lahar emergency response plan development process: an analysis,” Australasian J. of Disaster and Trauma Studies, 2004-I, pp. 1-33, 2004.
-  M. Dittmer, “The clockwork lahar,” unpublished Masters thesis in Communication Management, Massey University, Palmerston North, New Zealand, 235 pp., 2008.