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Home » Submission to the Department of Conservation in respect of the Draft Assessment of Environment Effects associated with the hazard from Ruapehu Crater Lake

Submission to the Department of Conservation in respect of the Draft Assessment of Environment Effects associated with the hazard from Ruapehu Crater Lake
January 1999


The Department of Conservation is to be congratulated on a very thorough AEE report. It meets the depth of discussion required to enable knowledgeable public debate on the issues.

The Geological Society of New Zealand (GSNZ) feels strongly that, as a special interest group commenting on an issue of wide interest and complex values, its role is to offer opinion within its areas of expertise. It has no mandate to recommend a particular final course of action where this course of action requires evaluation of values in areas where the society has no expertise. The areas where GSNZ can comment are the scientific values of all parts of the Ruapehu volcano and an engineering geology perspective of the proposed courses of action to alleviate a future breakout flood from the growing Crater Lake.


The Geological Society of New Zealand is a member body of the Royal Society of New Zealand that represents the professional interests of New Zealand geologists. It has some 750 members including professional geologists, students and some amateur geologists and branches in 7 major centres. The society has produced a "Geopreservation" inventory of volcanic features which includes listings for all features at Ruapehu considered of local, national or international significance, and this inventory is widely used for purposes of land use planning.

In formulating this submission the President of the Society has formed a committee of three individuals who produced a draft submission which has been then endorsed by the president on behalf of the national committee. The three individuals involved with the draft document are:

Dr Warwick Prebble from University of Auckland, an engineering geologist who was a consultant during the construction of the Tongariro Power Scheme and who performed research into debris avalanches in Tongariro National park;

Professor Vince Neall, Massey University, a world leader on lahars and debris avalanche processes who has focused his research in recent years on the ring plains of Ruapehu and Tongariro; and

Dr Bruce Houghton a physical volcanologist from the Institute of Geological and Nuclear Sciences and first author of the Geopreservation inventory described above.

Professor Neall and Dr Houghton have led the two teams working on scientific studies of the 1995-96 eruption of Ruapehu.

Specific comments on the text of the report:

In the Executive Summary (page 1 - line 4), it is incorrect to say that, after the lake refills, "such changes will produce lahar". They may not and it is misleading to suggest a lahar is certain to happen. The same point applies to page 6 - paragraph 1.

On page 48, there is an assumption that damage at Tangiwai would occur to the western abutments to the road bridge. The topography of the channel at this point forces water and lahars to the east, during the September 1995 eruptions it was the eastern bank upstream of the bridge that was actively eroding, not the western side. This impacts on statements in page 71 - paragraph 1.

There is recognition in the report of the possibility for the overflow into the Tongariro River system (page 51 - Scenarios 2-4). After Chapter 4, scenarios 3 and 4 involving the Whangaehu River waters flowing into the Tongariro River seem to be ignored. For example, in Section 7.5 (page 99) there is mention of 'the blue duck population could be temporarily wiped out from the main stem and the adult trout spawning run or juvenile rearing stage seriously disrupted', and 'Use of water from the Moawhango in particular would be likely to shorten the recovery time'. These ideas of the time scale of recovery ignore the scenario of the acid Crater Lake water continuing to flow in the longer term into the Tongariro River via Waikato Stream. The study by Hancox et al. (1998) bases estimated calculations for spillover on the current topography of the region. Recognition must be made of the likelihood of sudden but highly significant changes, which could occur at the spillover point, leading to permanent capture of a major portion if not all the channel flow. Scenarios 3 and 4 are therefore not as remote as one might read from the report. Hence the Option 2A1 should be recognised as having a 2-fold purpose:

  1. Prevention of lahar overflow into the Tongariro catchment, and also
    2. Prevention of flow of Whangaehu River headwaters into the Tongariro River, with long-term detrimental environment and economic effects.

Crater Lake and the summit area:

Scientific value

This region is of the highest value, on an international scale, to the geological community. The Ruapehu volcano has long served as an international "field laboratory' for the study of growth and evolution of cone volcanoes. The frequency of eruptions through an accessible hot crater lake in an otherwise glaciated summit environment are virtually unique features of the Ruapehu system. There is an increasing frequency of scientific studies at Ruapehu, and an increasing level of involvement of overseas collaborators with this work. Major limitations to understanding the system in the past, like the lack of tools for establishing reliable age determinations, are rapidly being overtaken by evolving new technologies and the future for Ruapehu Crater Lake as an scientific "type volcano", and there are numerous research projects underway or planned for the cone.

Such studies, and the global understanding of volcanic processes, advances most rapidly during and following eruptions such as the 1995-96 episode at Ruapehu. There is some irony that, while such events cause significant hardship in the community, they also help us to better understand and prepare for future events and limit the vulnerability of society to renewed eruptions.

Potential impact of the courses of action contained in the report

There are several areas in which the 1995-96 eruptions have boosted the international value of the Ruapehu system by permitting intense study of aspects of growth of a cone volcano, e.g. initiation and transport mechanisms for lahars, or the processes of ascent of magma and the nature and dimensions of the conduit system beneath the volcano. These studies are based on data collected at the time of the eruption and so are not in any way affected by the proposed courses of action outlined in the draft document. Two other changes, which are consequences of the eruption, are of equal international significance but as on-going studies. The first is a predominantly chemical study of the re-establishment of a crater lake system following its removal during an eruption. The refilling of Ruapehu Crater Lake, is a unique opportunity to trace the development and evolution of the shallow portions of the geothermal system. As the lake refills it will be intensely studied and sampled by a team of geochemists. Physicists and mathematicians will also analyse the physical and especially thermal evolution of Crater Lake.

The second is the evaluation of the evolution of the geomorphology and architecture of the summit region after a moderate-sized eruption- i.e. studies of the processes and rates of erosion and removal of the 1995-96 products, the influence of the eruption on erosion of older portions of the rim of the Crater Lake basin. It has often been remarked that the previous eruption of approximately this size, in 1945, has left few permanent recognisable products in the summit region, but no-one has detailed information about how such products were destroyed and at what rate this transformation occurred. The 1995-96 eruption will probably supply the only opportunity of this kind in New Zealand for the next 50-100 years. Any collapse or more gradual erosion of the 1995-96 tephra in the area of the former outlet forms part of this process of re-adaptation of the volcano.

The evolution of the growing Crater Lake is not affected in any serious way by any of the proposed options. Option 1 in fact will enhance the value of the study by constraining the time scale over which processes are happening. Options 3,4 and 5, by limiting the final volume of the lake, would have a minor impact on the duration of the geochemical study.

In a similar way the observations needed for monitoring the development of Crater Lake under Option 1 will be invaluable to the scientific study of the processes of modification and evolution of the vent area after the eruption. Option 2 in no way impacts on this study but Options 3 to 5 would have a major impact by reducing the number of potential courses that erosion could affect the outlet area. However, these interventions should not be weighed against the scientific values alone as the scientific consequences alone are not sufficient to either rule-out or promote these courses of action.

Whangaehu fan:

Scientific value

The Whangaehu fan is nationally a unique landscape and ecosystem being the only active lahar aggradation fan in New Zealand. It is also of international significance principally because it is the active portion of an exceptionally well studied and understood andesitic ring plain. Numerous splays of laharic debris have constructed the fan as a complex array of varying sedimentary products, soils and vegetation associations. Processes in 1995 and 1996 were the focus of intense scientific study and these investigations are now largely complete. A lower level of focus has been the longer-term re-adjustment of the fan to the changes during 1995-96. Once Crater Lake is re-filled to the point of discharge, or should further explosive eruptions commence, there will be intense scientific interest in any sediment transport on to the fan- either by lahars or by stream flow.

Potential impact of the courses of action contained in the report

Continuing lahar flows are part of the genesis of the landscape and any damage or burial to the vegetation is, in this context, a natural process. Option 1 will not impact in any way on this natural evolution of the Whangaehu fan. Option 2 has a major impact on the otherwise natural growth of the lower ring plain and may limit the scope of future research. However, some of the potential long term consequences of not carrying out some combination of options 2 through 4, in particular, the potential for long term contamination of the Tongariro River, should the Whangaehu River avulse into this catchment, may far outweigh any value attached to the scientific merit of this research. Options 3,4 and 5 have no direct physical impacts on the fan but do limit the range of potential options for natural fan growth.

Engineering geology perspective of the proposed courses of action:

The new deposit of tephra, which blocks the former outlet of the Crater Lake, is soil in the geotechnical sense and is described as "weak, poorly consolidated and likely to be prone to internal erosion" (p35). Assertion that there is a "high probability of a major type 3a lahar from the outlet area of the crater" (p36 and referred to again on p38) would seem to be well founded and is compatible with the engineering geological nature of the tephra barrier as judged from the description in the report. The immediate concern is the inherent instability of the tephra barrier. It is also evident from the report that the underlying lava sill is apparently "structurally sound" and has evidently not been weakened (p36).

A range of engineering options to mitigate the lahar breakout hazard posed by the new tephra barrier have been presented and considered in the report. This submission is not intended to be, and cannot be, a geotechnical review of the engineering works proposed under the various options listed and summarised on p15 of the AEE report as Table 1, Risk Mitigation Options, 1 to 5. Critical assessment of these is a matter for geotechnical engineering review and is not possible in this brief submission. The options outlined in the report have precedents in recent engineering work at overseas volcanoes. Similar containment dams and bunds to those being proposed have been built in Java and Japan. Crater Lake rim modifications have been implemented in Indonesia.

Option 1 is entirely consistent with geotechnical perspectives of hazard monitoring and risk reduction. Option 2A does not involve engineering works at the crater rim and so avoids the engineering problems and uncertainties associated with many of the options proposed in that area.

It is essential for the public to understand that the proposed works only minimise the effects of a lahar caused by collapse of the unconsolidated 1995-96 materials overlying the former overflow point of the Crater Lake. The proposals are not designed to mitigate any larger type collapse, which could occur, involving failure of the crater rim below the former overflow point, or of any rain - triggered lahars on the upper flanks of the volcano.

A final point from an engineering geological perspective is the most unusual opportunity which now exists for observing the geomorphic development of the new crater rim in the outlet area, and especially the natural performance of the tephra barrier under load from the filling and rising lake. Intervention at the crater rim outlet area would affect this situation.

Concluding statement:

Future hazards associated with the refilling of Ruapehu Crater Lake can be forecast in a manner that is almost unique among geological hazards. We commend the action taken by the Department of Conservation with respect to risk mitigation. Establishing acceptable levels of risk and vulnerability is a complex process and decisions in this regard can only follow extended consultation. The Geological Society of New Zealand remains available for further input to the process, if requested.

For comment on this submission, contact the President, Dr Jarg Pettinga