Ruapehu Lahar Residual Risk Assessment
The report assesses residual risks associated with a lahar (fluid and debris flow event) generated on Ruapehu, a volcanic mountain in the centre of the North Island of New Zealand. It was undertaken for the Ministry of Civil Defence and Emergency Management in 2002 and addresses response and management issues arising from the range of possible lahar flow scenarios.
The full report is available in two sections:
by John Norton, Director of Civil Defence & Emergency Management
25 November 2002
The Ruapehu Lahar Residual Risk Assessment report was commissioned by the Ministry in May 2002. It is part of an ongoing role to monitor the lahar risk and coordinate response planning on the part of Civil Defence and Emergency Management agencies and utilities with assets at risk from the lahar.
The report was prepared by Dr Tony Taig, a United Kingdom risk management expert. It addresses response and management issues arising from the range of possible Lahar flow scenarios and the intended mechanisms to manage the consequent risk.
The objectives of the report were to provide a quantitative basis for response planning and to provide a basis for ongoing decision-making in relation to managing the overall lahar risk.
The focus has been on risk to life and measures necessary to protect that and on coordinating with utilities for their management of risk to their infrastructure.
For the first time the risk to life has been quantified. This has been necessary to provide a sound basis for response planning particularly given the short timeframe (around 90 minutes) from the Crater Lake dam break to the lahar wave reaching road and rail bridges at Tangiwai, 40 km away. It also provides a basis for considering whether further measures should be implemented to manage the lahar risk.
There is a wide range of possible sizes of the anticipated lahar. These have been modelled by a scientific panel with a range of 24 scenarios addressing variations in:
- Levels of the crater rim outlet below the tephra dam
- Heights of crater lake up the dam at time of failure
- Rates for failure of the dam once it does fail
- Bulking factor as the lahar flows down the upper mountain
The report addresses levels of risk to life given the intended mechanisms for dealing with the lahar. This is compared in the report with the risk assessed at the time of the report’s writing in June 2002 (warning systems and response planning incomplete).
The lahar is not expected to occur before the summer of 2003/04 and could be as late as 2005/06. It will occur as the Ruapehu crater lake rises against a 7m high tephra dam formed over the crater rim outlet during the 1995 eruptions. Currently the crater lake is still 14m below the 1995 outlet.
- There is a high degree of uncertainty as to the potential size of the lahar and its timing.
- For the upper bound level event the probability of loss of life is less than 10%.
- For the expected level event (or lower), the probability of loss of life is less than 1%.
- There is uncertainty as to the likelihood of the upper bound level event versus the expected level event (or lower).
- The primary risk to life occurs lower down the mountain at the Tangiwai road and rail crossings and below.
- These levels of probability assume that as far as practicable, a substantial and robust response plan would be in place to prevent people being in the path of the lahar at the time it passes through.
- Risks associated with the upper bound level event cannot be discounted at this point.
- Risks to life could be substantially reduced if the timing of the lahar could be managed or identified.
Where to from here:
Work with the Civil Defence and Emergency Management agencies continues with refining the Ruapehu Lahar Response Plan.
Work with the Department of Conservation continues to:
- Increase understanding of the dam failure and Lahar flow mechanisms
- Investigate whether monitoring might allow timing of the Lahar flow to be pinpointed
- Investigate low impact options for initiating the Lahar flow at a time of our choosing
- Continue monitoring the overall risk.
A peer review of the risk assessment methodology is being initiated. Any further decisions will be taken before May 2003.