Conclusions and recommendations

Ruapehu Bulletin, 24 June 1997, Page 6

Conclusions and recommendations

The study has confirmed that significant changes occurred in the Ruapehu crater area following the 1995 and 1996 eruptions. These changes have important implications for

the hazard from future eruptions and lahars when the Crater Lake refills, and the stability and collapse potential of the crater rim. The main conclusions from . the study are as follows: • The inner crater and outlet areas of Ruapehu are covered with tephra from the 1995-96 eruptions, with about 6.4m of permeable bedded ash, blocky scoria, and minor snow and ice now overlying the former outlet channel. A similar situation existed after the 1945 eruption, and as occurred in 1953, the tephra barrier could fail rapidly when the Crater Lake refills in 3-5 years time, assuming there is no further significant eruptive activity. • Collapse of the tephra barrier at the future lake outlet would result in the rapid release of about 1 .46 million m3 of water from Crater Lake, forming a very large lahar in the Whangaehu River. (See above, "possible effects of outlet dam break lahar") • The current situation at the lake outlet poses a greater lahar hazard than in the past. • The 1995-96 eruptions also caused thinning, erosion, and superficial landslides on the southeastern and eastern crater rim from "Stump Saddle" to the notch of Pyramid Peak. Stability analysis indicates that under static conditions, only small superficial failures are likely on the upper, inner, and outer pare of the craterrim. Failures to a level below that to which the lake will eventually rise are unlikely, even (in a major) earthquake. However, the geological history of Mt Ruapehu suggests that the crater rim could collapse during an extreme event, such as dyke or plug intrusion, or a very large volcanic eruption, as has apparently occurred in

recent prehistoric times. • A trench should be excavated in the tephra barrier at the outlet channel to reduce lahar hazards when the lake refills. In order to reach resistant grey lava, the trench should be about 6.5m deep. • Consideration should also be given to continuing a narrow trench about 510m, or even 30m into the lava flows (below the former overflow level) to decrease the future lake volume. This might increase the probability and frequency of small " eruptions depositing ash on the skifields, but would probably reduce the size and effects of the lahar hazard during future volcanic eruptions, possibly with significant benefits to the Whakapapa and other skifields, and rivers draining from Mt Ruapehu. If there is no further significant eruptive activity to change the current crater situation this trench should be excavated in the spring of 1999, or earlier, depending on the rate of - lake level rise. • Crater Lake level, snow buildup below the lake outlet, and crater rim stability should be closely monitored over at least the next 3-5 years while the lake is refilling, and for at least some years afterwards. In particular, the stability of the southeastern crater rim should be geodetically monitored, visually

inspected, and photographed annually during and after lake filling. Both sides of the entire crater rim should be examined during late summer each year for water seepage, or any signs of seepage or erosion-induced instability. However, installation of a real-time monitoring system on the southeastern crater rim to give warning of an impending collapse is not considered justified at present. • Computer modelling and further studies of prehistoric crater rim collapses of Ruapehu should be undertaken to provide a better understanding of the possible causes and effects of future rim collapses, and identification of realistic hazard . mitigation measures.^ • If a trench is not excavated through the 1995-96 tephra, potential flood inundation zones and areas liable to be damaged by a lahar generated by collapse of the tephra barrier need to be defined, and contihgency plans developed. • If further eruptions of Mt Ruapehu occur before the lake refills, any changes to the crater area should be determined and the issue of crater rim stability and future lahar hazards reassessed. Copies of the full report are available, at a cost, from the Department of Conservation.