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.
ENDS
Background information on the Ruapehu Crater Lake is at http://www.doc.govt.nz
A media backgrounder is attached.
Eruption History
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.
Risk Assessment
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.
Decisions
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 future
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.
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