Coordinated management of Ruapehu lahar issue
1 November 2001 Media Statement
Coordinated management of Ruapehu lahar issue
Cabinet has authorised the setting up of a ministerial committee to work with the Minister of Conservation on coordinating government responses on the potential Ruapehu lahar issue.
The Committee comprises Conservation Minister Sandra Lee, Civil Defence and Police Minister George Hawkins, Maori Affairs Minister Parekura Horomia and Defence Minister Mark Burton.
"Public safety has always been a prime consideration in decision-making on this issue," said Ms Lee. "If the Ruapehu Crater Lake continues filling at the current rate, a lahar is expected to occur sometime between late 2002 and the end of 2005."
"While the volcanic nature of New Zealand is part of what makes this country special, the downside is that lahars and eruptions are natural occurrences that can be managed but not controlled, just as we can not control nature. However we do have an enormous range of technology and information to help us predict, prepare and minimise risk to public safety. Lahars do occur from time to time on Ruapehu, and what we are putting in place will help address all potential lahar incidents in the future."
Ms Lee said the Committee would not only help coordinate planned responses to a lahar but also provide advice on the need, if any, for actions such as intervention at the crater rim which has been sought by some interested parties but opposed by others, and it will continue to monitor work on new warning systems as their installation proceeds. The Minister of Conservation is expected to decide by the end of November whether there should be crater rim intervention.
"The advice from other Ministers, along with the considerable other information I have already received, will help ensure that future decisions I am required to make under the National Parks Act on mitigating a potential lahar are as well informed as is possible," Ms Lee said.
Last year, the Conservation Minister approved the installation of an alarm system to provide advance warning of a lahar. The Minister also decided DOC should proceed with the construction of a bund (stop bank) on the lower slopes of the mountain to reduce any likelihood of the lahar leaving the Whangaehu River and flowing into the Tongariro River.
Ms Lee said emergency response planning is "well advanced", involving a number of agencies including the Police, local authorities and DOC.
A major exercise of the alarm system and the response is scheduled for April 2002.
Background information on the Ruapehu Crater Lake is at http://www.doc.govt.nz
A media backgrounder is attached.
Mt. Ruapehu (2797m) is one of several volcanoes that make up Tongariro National Park. They have shaped the central North Island through successive eruptions over thousands of years. These volcanoes include Ruapehu, Tongariro, Ngauruhoe and Pihanga, which are revered by Ngati Tuwharetoa and Ngati Rangi for their spiritual and cultural links with ancestors and gods.
In 1990, Tongariro National Park gained World Heritage status in recognition of its outstanding natural values. In 1993, its associated cultural values were also accorded the same status.
It is one of only 23 sites in the world with this dual World Heritage classification.
Ruapehu is the southernmost volcano of the Taupo Volcanic Zone, which stretches northeastward through Lake Taupo, which, with the Rotorua Lakes are also volcanic craters, to White Island and out into the South Pacific. The first volcanic activity that took place where Ruapehu stands today began 250,000 years ago. The 1995-1996 eruptions are the latest activity from Ruapehu but they won't be the last.
In 1945, a series of eruptions emptied Mount Ruapehu's Crater Lake and dammed its outlet with tephra (material of varying size ejected from the volcano) and glacial ice, allowing it to refill to a much greater depth than before. In 1953, part of the crater rim's ice wall collapsed and water poured from the lake over a two-hour period. It flowed down the mountain as a mudflow (lahar) and swept away the Tangiwai Rail Bridge. Tragically, the Auckland Express reached the demolished bridge site minutes later and 151 lives were lost. The 1953 lahar from Crater Lake had not been predicted, nor was there any alarm system in place.
The 1995-1996 Eruptions
The 1995-1996 eruptions of Mt.Ruapehu again emptied the Crater Lake and deposited about seven metres of unconsolidated tephra over its former outlet, creating a weak dam. Unlike the tephra dam of 1945, the current one has no ice component, as the glaciers of Ruapehu are less prominent today.
The Crater Lake is refilling and is currently about 56% full.
Barring another eruption, it is expected to reach the top of the tephra dam between late 2002 and the end of 2005.
Assessment of Environmental Effects
In 1997, the Department of Conservation was asked to prepare an Assessment of Environmental Effects (AEE) in response, for the then-Minister of Conservation Dr Nick Smith. DOC staff, engineering and scientific consultants thoroughly researched 25 options for mitigating the effects of a lahar. They included a range of engineering options at the crater rim, alarm systems, dams and diversion walls further downstream. The draft AEE was independently reviewed and released for public consultation. 45 submissions were received before the final document was submitted to the Minister. Despite having nearly six months in which to do so, Dr Smith did not make any decisions on the AEE in his remaining time as Conservation Minister before the 1999 general election.
Organisations with infrastructures likely to be affected by a lahar flowing down the Whangaehu River were asked to carry out their own risk assessments. The NZ Army, TransPower, TranzRail and ECNZ/Genesis concluded that the risks to their infrastructure did not require engineering work at the crater.
They favoured the installation of an alarm system. Transit NZ favoured both options.
The Minister of Conservation Sandra Lee accepted the AEE's recommendations and in May 2000 approved the installation of an alarm system. In December 2000, she also recommended the construction of a bund (stop bank) on the lower slopes of the mountain to reduce any likelihood of the lahar leaving the Whangaehu River and flowing into the Tongariro River. In making her recommendations, the Minister has stressed the importance of public safety as a paramount consideration.
She has also kept the possibility of further mitigation options open.
What could happen at the
Crater Lake outlet?
* A lahar (volcanic mudflow) from the Crater Lake down the Whangaehu River will occur when lake waters either percolate through the tephra or if the dam should fail suddenly.
* The size of the lahar will depend on the water level of the lake and whether the water seeps through the dam or causes a major collapse.
* A major lahar has the potential to damage the road and rail bridges at Tangiwai, parts of State Highway 1 and possibly spill over into the headwaters of the Tongariro River. It will not threaten houses on the Whangaehu or Tongariro Rivers.
It is difficult to predict the size of the
lahar but there are several scenarios:
* The lake water could percolate through the loosely compacted tephra,
creating a tunnelling effect that empties the overfill as a dribbling lahar, of no threat to anything and anyone.
* The lake could erode through when the levels have reached only half the maximum height (7m) and create a moderate lahar.
* The worst case would be for the tephra dam to collapse when the water level reaches its top, releasing the full volume of overfill into the Whangaehu. Planning for the consequences of a lahar has been based on this eventuality.
What happens now?
* The alarm system (Three sets of alarms placed at different locations, from the Lake outlet down the Whangaehu River) is a long-term solution that will provide warning of this and other future lahar events. The alarms will give an hour's warning at State Highway 1 and up to two hours at Tangiwai Rail Bridge.
* The district councils and the Police are preparing an Emergency Response Plan that will ensure that all roads are closed once the lahar begins. Areas where people might be at risk will be checked and cleared to minimise the risk to human life.
* A stop bank will be constructed at a point in the Whangaehu River where a major lahar could overtop the bank and flow into the Tongariro River or its headwaters, eventually reaching Lake Taupo. The 1995 lahars raised the riverbed of the Whangaehu by several metres in some places.
* DOC and other agencies are
monitoring the Crater Lake levels and will
erect signposts in the area, warning of the lahar danger as the lake reaches the old outlet level and beyond.
* The alarm system and embankment will be in place by April 2002.
The massive forces of nature are still shaping New Zealand's landscape, manifesting themselves as earthquakes and volcanic eruptions. Eruptions at Mount Ruapehu are occurring at about 50 year intervals but they could happen at any time. The risks to people and property have long been recognised and discussed extensively with community representatives in the area. Their majority view as expressed in submissions is that the risk of property damage is outweighed by the costs of engineering at the crater rim and by the environmental and cultural disturbance that such work would create.