Crater rim strength to be assessed before lake refills

Ruapehu Bulletin, Volume 13, Issue 635, 7 May 1996, Page 1

Crater rim strength to be assessed before lake refills

An investigation of the strength and safety of the rim of Mt Ruapehu's crater is needed, the Department of Conservation told a workshop it convened last week to discuss the management of the 1995 eruption of Mt Ruapehu. A study is needed because the eruption modified the rim which may have implications for the drainage and containment of the new Crater Lake, said Dr Harry Keys, advisory scientist for the Department of Conservation. "At present the lake level is still 80-90 metres below the former overflow point and it is expected to take three to four years to refill back up to that point. In the meantime the strength of the rim needs to be assessed to ensure that it will still be able to contain the refilled lake". Last year' s eruption caused significant erosion

in the crater area, lowering and thinning the crater rim quite noticeably in places. Twenty to 30 metres has been eroded off the south eastern part of the rim. Although this means this part of the rim will still be about 40 metres higher than the lake surface when it refills and 100 metres thick, it has re-emphasised observations made at least 50 years ago and well recognised by scientists since. Professor Charles Cotton, a famous New Zealand geographer, noted in 1946 that this part of the rim was potentially unstable. However, a subsequent engineering study did not identify any immediate threat. Scientists have noted that in the very worst case scenario a collapse of the rim could cause a large mudflow (lahar) to flood down the Whangaehu and Tongariro rivers threatening bridges, roads, electrical generation and

transmission facilities, and possibly even parts of Waiouru and Turangi. "This scenario is not likely" said Paul Green, regional conservator for the Department of Conservation "but we need to know if there could be a problem and if there is, what could be done about it". Elsewhere ashfall has built up parts of the crater area, including at the outlet of the former Crater Lake where about five metres of new material has been deposited. "In this former outlet area we need to know if the new material will gradually wash away or whether it will form a temporary blockage," said DrKeys. Because the area is in a national park and the mountain is such a special place, DoC has Turn to Page 2

Crater rim strength to be assessed before lake refills

FROMP1 developed a strategy for addressing potential concerns about the crater rim. Local iwi have been consulted about the strategy, which they endorsed, says DoC. The workshop outlined the strategy to representatives from regional and local councils and other stakeholders including the Electricity Corporation, T ransit New Zealand Transpower and Tranz Rail. The Conservation Department will be approaching them for contributions to help fund the required study. Once the results are available a decision will be made to decide what, if any, further action may be needed.