Crater hazard can be avoided by digging lake outlet trench

Ruapehu Bulletin, Volume 15, Issue 691, 17 June 1997, Page 1

Crater hazard can be avoided by digging lake outlet trench

Scientists and engineers have recommended that before the Ruapehu Crater Lake refills, a trench be dug to avoid volcanic material from the recent eruptions forming a dam. The rim of the Mt Ruapehu crater has been found to be stable according to a scientific report released Sunday by the Minister of Conservation, Nick Smith. However, ash and scoria from the 1995 and 1996 eruptions of Ruapehu are blocking the former outlet of the Crater Lake. Unless this barrier is removed first, scientists believe it could lead to a flood, twice as large as that which caused the Tangiwai Disaster of 1 953, when the lake refills in 3-5 years time. No immediate threat "There is no immediate threat from Ruapehu," stressed Dr Smith when visiting the site on Sunday, explaining that the lake won't refill for 3-5 years. The report on the stability of the crater rim was commissioned following the 1995 eruptions. "We have time to look at all the options of dealing with the problem. The Department of Conservation will be working closely with the local communities, tangata whenua and other downstream stakeholders on how best to eliminate risk to people and property from this potential hazard." Dr Smith said supportive agencies in the region had jointly funded an investigation by the Institute of Geological and Nuclear Sciences (IGNS) to examine the crater

rim. The Department had recognised the eruptions had changed the area but needed scientific advice on the scale and nature of the problem and options for managing it. The Electricity Corporation, Manawatu Wanganui Regional Copncil, the Army, Winstone Pulp International, Trans Power, Tranz Rail, IGNS and commercial helicopters joined with the Department to fund the study. The study concludes that the outlet barrier is the main concern following the eruptions. The former outlet channel of Crater Lake is covered by 6-7 metres of loose ash, scoria and rocks from the 1995 and 1996 eruptions. Elsewhere the crater rim is sufficiently stable to be able to contain the lake when it refills, even in a large earthquake.

This present situation poses a greater lahar (volcano flood) hazard than in the past. The worst-case scenario would be, once the lake had refilled, for a sudden failure of the outlet barrier, releasing about 1 .5 million cubic metres of water from the lake. The resulting flood would threaten bridges, roads, the main trunk railway, the national grid power lines and facilities of the Tongariro Power Scheme, as well as causing environmental and property damage in the Whangaehu and Tongariro rivers. Trench recommended "The report recommends that a trench is excavated through the 1995-96 material at the outlet channel area," said Dr Smith "and suggests that this might best be done by a bulldozer during spring when snow cover would prevent tracking damage on route up to the crater. I recognise that the summit area of Ruapehu is a sacred area and very important to many New Zealanders. Therefore solutions to the problem need to be worked through by the regional community. In the meantime the rising lake level and the rim situation will continue to be closely monitored". An alternative could be to fly a small bulldozer up to the crater by helicopter. This has been done during ski lift construction at Turoa. At present the lake is still very low at 100m below eventual overflow level. The outlet situation poses no threat at all to the skifields and while the lake is so low there is negligible risk of lahars to them if volcanic activity restarts.