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Managing hazards from future Ruapehu lahars

Hon Sandra Lee
Minister of Conservation


Conservation Minister releases report on managing hazards from future Ruapehu Crater Lake lahars

The Minister of Conservation and Associate Maori Affairs Minister Hon Sandra Lee today announced a new early warning system is to be set up to reduce the risk to lives and property of future floods of water and volcanic debris from Ruapehu's Crater Lake.

Releasing the final Environmental and risk assessment for mitigation of the hazard from Ruapehu Crater Lake report today, Ms Lee said she had agreed that a $100,000 proposed alarm system should be put in place on the eastern side of Mt Ruapehu "as soon as possible".

The Department of Conservation has warned the sudden failure of an ash dam blocking the lake's natural outlet, formed during eruptions in 1995-96, might trigger a flow of this debris mix, or lahar, sometime after 5-years from now given the rate at which the lake is re-filling.

"A lahar is a natural event, but we need to take every reasonable precaution now to prevent any repeat of the 1953 Tangiwai rail disaster which was the tragic consequence of a similar event," said Ms Lee.

The final assessment report is the outcome of four years of intensive expert study and consultations, including a draft report for comment which drew submissions from key stakeholders such as the Electricity Corporation, the Ngati Rangi Trust, Transit New Zealand, Tranz Rail, Trans Power, Tuwharetoa Trust Board, and several local authorities and environmental organisations.

The new alarm system would warn of any lahar flow eastwards off the mountain down into the Whangaehu Valley. There is currently no system giving general warning to the public of lahars down this river, although Tranz Rail and Genesis Power Ltd operate small alarm systems on it for their own purposes.

"The early warning alarm will be designed to enable the Police and other agencies to alert people along the Desert Road (State Highway One) and at Tangiwai, in order to provide the authorities with time to secure bridges, roads and power supplies that might be threatened," said Ms Lee.

"The report states the proposed system would give at least 45 minutes warning of an impending lahar reaching the Desert Road and almost two hours warning for Tangiwai."

She said the proposed alarm system was in addition to an Eruption Detection System installed last year to provide advance warning of eruption lahars down the northern side of the mountain.

Ms Lee says the Department of Conservation had assured her that it would be at least five years before risk of a lahar hazard from collapse of the ash dam emerged.

Ms Lee said the final assessment examined a range of options for action including
 the excavation of a trench in the ash dam using bulldozers and
 construction of a $5M earth dam.

"As an interim step, I have agreed to the installation of a warning/response system and revised hazard planning in lahar run-out zones," the Minister said.. "I have decided to seek an independent expert review of the supplementary measures proposed by the final assessment report for additional containment dam construction in the lahar run-out zones, and in the meantime I am keeping an open mind on options involving engineering intervention," she said."

(The Hon Sandra Lee has approved all the recommendations in the Executive Summary attached to this News Release. A copy of the final Environmental and risk assessment for mitigation of the hazard from Ruapehu Crater Lake report can be obtained from DOC.)


Dr Harry Keys (the author of the final report, for technical support information) Office 07 386 9250

Executive Summary

Eruptions of Mt. Ruapehu in 1995 and 1996 resulted in several changes to its crater. The most significant change is a deposit of tephra (ash, scoria and rocks) which is blocking the former outlet of Crater Lake (cover photo and frontispiece). After the lake refills sometime between late 2000 and 2006 such changes will probably produce lahar (“mudflow”) and possibly other hazards on the eastern side of the mountain and in run-out paths downstream. These hazards would be minor or negligible if the tephra dam is eroded before the lake rises up to it or if the dam is overtopped and down-cutting slowly re-establishes the former outlet channel. However sudden collapse of the tephra dam when the lake has refilled to some height above the normal overflow level as in the 1953 precedent is seen as more likely. Collapse when the lake had filled close to the top of the tephra dam would produce a dangerous lahar down the Whangaehu River with peak discharge at Tangiwai about 54% larger than the 1953 event.

The main effects of such a lahar could be on infrastructure outside Tongariro National Park. Scientists predict that the lahar resulting from a tephra dam collapse could be large enough to damage and possibly destroy the road and rail bridges over the Whangaehu River at Tangiwai. The public toilets and Tangiwai memorial there plus the Strachan’s Road bridge downstream would be destroyed and there would also be scouring at the approaches to the road bridge. Power pylons between the Wahianoa aqueduct and State highway 1 would probably be severely damaged. On State highway 1 a culvert could be washed out and the bridge over Waikato Stream might be damaged or destroyed due to a small overflow from the main lahar. Such lahar effects would disrupt road and rail traffic on State highways 1 and 49 and the main trunk railway, and possibly power, some communication services, people, industries and services that depend on them. Likely loss of the walking bridge over the Whangaehu in Tongariro National Park would cause a relatively minor inconvenience to trampers there. The towns of Turangi and Waiouru are not threatened and the substation and pulp mill at Tangiwai do not appear to be threatened.

Risks to human life are assessed as low to high. The risk is considered tolerably low if specific warning systems are developed and if people heed warnings. Anyone walking or working beside or near the Whangaehu River or rafting or kayaking it, at the time of the lahar would be at great danger unless they could get out of the way fast enough. Similar risks in the Tongariro would not be as large but anglers in the river could be at risk. Road and rail users of the threatened bridges over the Whangaehu River and Waikato Stream could be at considerable risk unless they are pre-warned. Therefore installation of specific automatic lahar warning and response systems is very important and does not seem particularly expensive. Such installations would also have long-term safeguards against far larger lahars caused by possible collapse of the original crater rim or explosion lahars larger than those occurring this century. Such events are of low probability in the next 10 years or more but are of greater probability on a time scale of decades or more.

There would also be significant ecological damage if as predicted the lahar spilled over into the catchment of the Tongariro River. Blue duck and other aquatic life in the Tongariro River, including trout, would be displaced or destroyed by such an overflow lahar. Water quality in Lake Taupo might also be impaired temporarily. These effects would disrupt angling and local economies until after the fishery and water quality recovered. This would be in the order of probably days for Taupo water quality to weeks or months for the adult trout to return. Blue duck could take more than a decade to recover and recovery might not be complete by the time the next lahar occurs. Because of continued evolution of the Whangaehu outwash fan further spill-over events may be more likely to occur in future and in addition the possibility of longer term contamination and hence environmental and social impacts in the Tongariro-Taupo catchment can not be ruled out.

These risks and options for mitigating them are assessed in this Assessment of Environmental Effects (AEE). A draft report was prepared at the request of the Minister of Conservation to assist decision-making. He has noted that a comprehensive assessment is required before we can be confident any possible mitigation work is necessary. The Minister has a responsibility to carefully and properly consider whether or not to intervene with management action including possible engineering works. There is no particular course of action in relation to a natural hazard which is not open to him. All possible options have to be considered in the contexts of management of both Tongariro National Park and natural hazards.

Engineering works, especially any at the crater of Ruapehu, would have major implications for national park, cultural and other important conservation values. Resource consents would need to be obtained if engineering work was planned (unless an emergency situation suddenly arose).

The issue is of major public interest and the park is a World Heritage Area under both natural and cultural criteria. The Crater Lake is very important to those criteria as well as to tangata whenua Ngati Rangi and Ngati Tuwharetoa and many Pakeha.

The draft AEE was released publicly in October 1998 and submissions were called for. The draft was the basis for consultation by providing information, seeking feedback, and highlighting the best forms of mitigation of the hazards. Consultation has been crucial for this issue. This includes discussion on the critical aspect of management intervention in natural processes in a national park. There is sufficient time for proper consultation but not an unlimited amount of time before options will start becoming precluded. This final AEE has taken account of this consultation and should become a statutory basis for decision-making.

As a minimum this report concludes that option 1 is required because it has significant short and long term benefits, and the least effects, impacts and risks. This involves no work at the crater, but development of a warning/response system and revised hazard planning in lahar run-out zones. A consensus has already emerged that option 1 is a necessity. The major advantages of this option are:
• vulnerable national park, world heritage and cultural values are not directly threatened;
• risks to human life from the expected lahar from the collapse of the tephra dam can be minimised to a very large extent and appear tolerable;
• the risks to infrastructure are regarded as tolerable by most asset owners and the public;
• much larger risks from future larger lahar events can also be minimised;
• natural processes and conservation values in TNP would not be interfered with;
• no change will be required to the TNP Management Plan.

Its major disadvantages are:
• safety of individual members of the public could not be guaranteed because of potential failure of some component of the warning system or failure to heed warnings (as in the 1995 eruptions) despite warnings that would be given to ensure that this risk was very low;
• in the worst case risks to nationally important infrastructure and conservation values outside the national park may not be avoided and damaged infrastructure could cost millions of dollars to repair or replace. However most asset owners except Transit New Zealand prefer a reactive approach to these risks;
• in the worst case there could be significant local to regional economic effects and impacts for a period of several days to weeks;
• there could be significant damage to blue duck on the Tongariro River which would take several years to recover. The damage would aggravate previous impacts from hydroelectricity generation, lahars and climatic events and there is some risk that recovery would be incomplete before the next eruptions.

Construction of a bund at the spill-over point between the Whangaehu and Tongariro rivers appears to be the only practical or desirable part of option 2 (engineering works in lahar run-out zones). The major advantages of this sub option (2A1) are:
• effects, impacts and risks in the Tongariro River and Lake Taupo would probably be avoided relatively cheaply;
• the most important and vulnerable national park and cultural values are not threatened or compromised;
• risks to human life from the expected lahar from the collapse of the tephra dam can be further minimised;
• risks from some future lahar events of similar or larger magnitude may also be minimised in the Tongariro catchment;
• risks from longer term capture of the Whangaehu headwaters by the Tongariro would also be minimised;
• effects on other ecosystems and biodiversity would be small.

Its major disadvantages are that:
• by interfering with natural processes it would threaten national park, scientific and ecological values;
• safety of individual members of the public could still not be guaranteed although risks would be less than for option 1 alone and they would be confined mainly to a relatively lightly used footbridge and track in Tongariro National Park;
• some of the work would impair army training opportunities;
• there would be an additional financial cost.

There is scientific justification for option 2A1 because without it environmental effects and social impacts in the Tongariro and Taupo could be substantial and in the event of longer term contamination may far outweigh scientific values of the outwash fan. Determining whether the other impacts of sub-option 2A1 would be greater or less than the potential consequences of not implementing it involve value judgements which are difficult to make.

This report concludes that option 4A is not required. Of eight methods examined for achieving option 4 (engineering works at crater rim) flying in light bulldozers would appear to be the best overall method of excavating a trench through or partly through the 1995-96 tephra deposits at the former outlet of Crater Lake. This option (option 4A) is not required because:
• it would involve work at the crater, the most special part of Tongariro National Park, with consequential major impact on cultural, philosophical and some other values;
• it would set a precedent for such works at the crater in future and would raise other concerns about precedence;
• risks to human life can be managed tolerably without it;
• risks to infrastructure are generally tolerable without it and any damage that does occur can be repaired;
• it would not provide any further long-term protection for future large lahars and may actually increase the risk in the longer term;
• there is little public support for this option.

These conclusions draw on the risk assessments carried out by asset owners as well as the priority which they, local government and the wider community assign to long-term protection against inevitable future lahars. Most asset owners do not consider their major assets are at such significant risk that pro-active management is required. Risks to other national assets (e.g. highway bridge approaches) are regarded as only moderate in financial terms. Engineering work, especially at the crater, might give a false sense of security from future large lahar events and might create even larger risks in the future. The majority of submissions indicate that residual risks to people would be tolerably low if a suitable warning system is in place. Minor assets such as a tramping bridge in Tongariro National Park and local use bridges are likely to be destroyed. Like blue ducks, the Tongariro fishery and other potentially effected values in the Tongariro-Taupo area, such minor assets are not seen as important as World Heritage national park and cultural values by the majority of people who have been involved with this issue.

The minority view point shared by district councils and Transit New Zealand is that risks not avoided by option 1 can and should be avoided. A significant proportion of submissions, though still a minority, considered that sub-option 2A1 in particular is required because blue duck, trout and Lake Taupo are important.

The level of the lake must continue to be monitored to determine more closely when the lahar hazard develops. While the current best estimate is that this will not be before mid 2002 - August 2003, continued monitoring will allow accurate warnings of immediate risks to be issued. It will also determine the state of the ash dam, including its rate of erosion, and deposition or erosive effects of any further volcanic activity. There is a clear advantage in not implementing any potential engineering work too soon. On the other hand, the level of the lake will also guide the timetable for decision-making as options will be foreclosed progressively once the level rises above 2520 m and particularly above 2530 m. Monitoring of the crater rim is also required to determine how it changes with time, in particular how it responds to the filling lake and whether it poses additional significant threats. It will also allow the probability of larger lahars to be better assessed.

The report recommends the following:

1. A reliable real-time lahar warning system needs to be installed high on the eastern side of Mt. Ruapehu
2. An acoustic-based system and practical alternatives to it should be further investigated
3. Funding needs to be provided for ongoing monitoring of the rising lake level and the condition of the crater rim
4. Regional and asset hazard management needs to take into account very severe hazards in the medium to longer terms
5. Where necessary local government needs to begin a re-zoning process to take account of these newly understood hazards through district and regional plans and reviews.
6. Contingency plans for warning and response actions need to be prepared by responsible agencies, including councils, asset managers and DOC.
7. Parties at risk, especially those owning or managing public assets should form an Emergency Management Group. Discussions on whether to pursue the option of preventing spill-over into the Tongariro catchment should be a priority for this group.
8. Transit and other agencies involved in planning a new bridge to replace the present one over Waikato Stream on State Highway 1 should advance the consent and funding processes so that it can be built before the lahar would be expected. The culvert to the south should also be replaced by a structure better able to pass large lahar flows.

The Minister of Conservation will be making the decision on which is the preferred option for mitigation of the lahar hazard.


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