Eruption safety upgrade looms for $25m gondola

Ruapehu Alpine Lifts’ new $25 million gondola may have to strengthen any gondola towers found to be in a lahar path.

RAL chief executive Jono Deans said four of the 14 towers were potentially in the path of a lahar (volcanic mudslide) generated by an eruption of Mt Ruapehu.

The gondola, which opened in July and received a $10m loan from the Government’s Provincial Growth Fund, has so far carried 247,000 passengers, about half its expected annual capacity.

Massey University is doing modelling of the lahar path and advice from a structural engineer would help determine whether any strengthening work was needed, Deans said.

It could involve physically strengthening towers or earthworks to create a bund that would deflect the mud flow of a lahar.

Asked why the lahar work was not done before building began, Deans said resource consents were issued based on previous modelling done after the 1995 Ruapehu eruption and consultation with volcanic experts.

The number of towers in the lahar path was reduced during the planning stages of the lift, which aims to greatly increase summer sightseeing on the mountain.

Deans said more advanced modelling technology had become available after construction commenced, but the lahar protection work was factored into the requirements set out in approvals from the Department of Conservation.

The work has taken longer than expected due to its complexity, the need for collaboration across multiple disciplines and to consult DOC and local iwi.

DOC’s Tongariro operations manager, Connie Norgate, confirmed RAL had to get the assessment done as part of risk mitigation work, and the outcome would determine next steps in terms of lahar protection.

‘‘The likelihood that it will get knocked out of action is low, however there is a risk.’’

Former DOC vulcanologist Dr Harry Keys said that two years ago he had given RAL the ‘‘worst-case scenario’’ that the towers would need to be able to withstand.

‘‘It’s a fast moving dense lahar, a volcanic flood coming down mountain at, say, 100kmh with boulders and stuff in it.’’

He believed four towers were in the lahar path, and two of those might need strengthening.

Deans said any work required would be completed before the coming winter.

TOM LEE/STUFF Ruapehu Alpine Lifts should know soon whether it will need to strengthen any gondola towers to withstand the force of a lahar produced in a volcanic eruption.