Mount Ruapehu Crater Lake Lahar
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 Executive Summary is provided below.
The full report is available in two sections:
The Main Body of the report (pdf 725 KB)
The Appendices (pdf 600 KB)
The joint Minister's media statement (doc) issued on 25 November 2002.
Overview Comment
by John Norton
Director of Civil Defence & Emergency Management
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.
Key Issues
- 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.
25 November 2002
Executive Summary
This is the report of an assessment of the residual risks associated with the anticipated lahar which will occur when the tephra barrier deposited on the rim of the crater of Mt Ruapehu in the 1995/96 eruptions collapses under the weight of the water in the crater lake. It has been carried out by TTAC Limited in cooperation with numerous other parties for the Ministry of Civil Defence and Emergency Management, to assist in development of appropriate response arrangements for this event.
The assessment considers:
- the nature and scale of the anticipated lahar
- how the lahar flow will develop as it travels down the mountain
- the risk to key assets and infrastructure in the path of the lahar
- the risk to people if those assets are damaged
- the effectiveness and reliability of warning and response systems.
Conclusions are drawn as to the likely residual risk, a) with arrangements as they currently stand, and b) with what should realistically be achievable, given all the planned and proposed improvements to asset protection, readiness, warning and response arrangements discussed in the report. The residual risk is estimated in terms of the likelihood that the lahar will lead to one or more fatalities (ie directly answering the key question “How likely is it that someone will die as a result of this lahar?”).
An important issue throughout the assessment has been the high levels of uncertainty surrounding this lahar, and the potential for different interpretations and perceptions to develop in the face of that uncertainty. Care has been taken to expose uncertainty, to understand what matters most in the face of uncertainty wherever possible, and to draw conclusions which are robust, notwithstanding the uncertainties. Risk estimates presented in this Executive Summary are upper values, given all the associated uncertainties. That is, when we say a risk would be “No more than 10%”, we mean “There can be a high degree of confidence this risk would be no more than 10%”. The risk may actually be a lot lower, but we cannot definitively say so with confidence in the face of all the uncertainties.
The principal conclusions are:
1. There is a wide range of possible sizes of the anticipated lahar, but experts believe that the range of possibilities can be bracketed by considering a range of 2x2x2x3 (= 24) scenarios corresponding to different:
- levels of the crater rim below the tephra dam Note – the term tephra dam is used loosely throughout this report to describe the tephra barrier, although the barrier will not technically become a “dam” until the crater lake rises part way up it, which is many months away at present.
- heights of the crater lake up the dam at the time of failure
- rates of failure of the dam once it does fail, and
- bulking factor as the lahar flows down the upper mountain.
2. There is high uncertainty as to when the tephra dam will fail and the lahar will happen, but general agreement among experts that:
(a) it will fail quickly when it does fail, and
(b) the lake is likely to reach a level high up the dam before failure.
3. The anticipated flows down the mountain span a wide range of possible sizes of lahar. Most scenarios involve flows above, and travel times below, those experienced in the 1953 lahar which led to the Tangiwai rail disaster. Some scenarios involve lower, less damaging flows.
4. The bund constructed at the bottom of the Whangaehu Gorge to prevent flow crossing over to the Waikato Stream is considered highly effective in preventing such crossover.
5. The Tangiwai Memorial area and Strachan’s Bridge are virtually certain to be severely damaged by the lahar; the rail and road bridges at Tangiwai, and the Tirorangi Marae Bridge, are at high risk (30 to 80% chance of severe damage). There is a high level of uncertainty as to the risk to SH1 in the vicinity of the Wahianoa Aqueduct, while the level of risk to SH1 at the Waikato Stream crossings is significantly lower because of the effectiveness of the bund.
6. With arrangements in their present state (with ERLAWS part commissioned, Transit automated barriers on SH49 not yet in place, and rudimentary local response plans) the residual risk of somebody being killed as a result of the lahar could be as high as 30 or 40% or as low as a few per cent. This uncertainty is unlikely to be reduced much by further analysis.
7. With measures already in progress or planned, and other measures identified in this assessment put in place, we can be confident of reducing this residual risk to a less than 10% chance of somebody being killed as a result of the lahar. The risk is uncertain and may be much lower, but we cannot say with confidence that it would be much less than 10%.
8. The main contributors to the risk of there being a fatal accident are road accidents associated with the lahar, and events at the Tangiwai Memorial. The likelihood of a passenger train accident is significantly lower (less than 0.1% with current arrangements, realistically reducible to less than 0.01%).
9. The measures required to progress from the “could be 40%” to the “less than 10%” state are described in detail in Section 10.2 of the report. The associated challenges and costs should not be underestimated. In summary, the measures involve:
(a) removing risk on SH1 by further measures to protect the road from lahars
(b) closing the high risk Tangiwai Memorial site, car park and toilets
(c) enhancing warning and response arrangements at Tangiwai itself
(d) enhancing readiness, warning and response arrangements downstream of Tangiwai, and
(e) reducing possible risk to people in the Tongariro River associated with Genesis Power’s response actions to lahars.
10. Priorities for emergency response after the lahar event should focus on 9 (c) and (d), and not on attempting to send personnel high up on Mt Ruapehu to warn and evacuate people. Risk management on the mountain should focus on managing access to at-risk areas.
11. Risk reduction to provide high assurance of residual risk levels down to about 1% chance of a fatal accident could be achieved, but at a much higher price. Further reduction would require additional warning systems over and above ERLAWS, and sophisticated response arrangements and systems well beyond any currently envisaged.
12. If risk reduction to a less than 1% chance of a fatal accident is required, or the costs of achieving the “less than 10%” or “around 1%” levels are considered excessive, then the decision to allow the lahar to happen when nature dictates it would have to be revisited.
13. Some significant factors relevant to the decision to allow the lahar to happen have been revealed in the course of this assessment. In particular:
(a) different people had interpreted previous qualitative risk assessments very differently
(b) there does not appear to have been a shared appreciation of what is involved, and what it costs, to provide high reliability warning and response arrangements to protect life, and
(c) false perceptions that “we will know then this lahar is going to happen” may have led some people to overestimate the likely effectiveness and reliability of response arrangements.
