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Published: 2001-05-25 00:00:00

Collapse of the dam holding back the Ruapehu crater lake could generate a lahar half as big again as the one which caused the Tangiwai Disaster – and it’s likely to happen in the next few years.

This month’s Environment Waikato Environment Committee meeting heard the eruptions of 1995 and 1996 created a tephra dam which is now blocking the former lake outlet. The dam is likely to collapse between 2003 and 2008, spilling a lahar - a mudflow of volcanic ash and water – to flood the Whangaehu River flood plain and possibly spill into the Waikato Stream.

The Department of Conservation has recommended a warning system be installed by autumn next year, with half the costs coming from local government. The system will include three sets of acoustic sensors, radio telemetry links to the Genesis Power communication system and automated warning messages for key emergency personnel. A stopbank is also being considered across the catchment divide.

The risks for the Waikato Region include a major lahar entering the Tongariro River, State Highway One’s road bridge and culverts at risk from scouring damage, and risks for people using the river for recreation or crossing State Highway One. The Tangiwai road and rail bridges could also be damaged.

The cost of the warning system is about $370,000 over four years, with the Crown paying half. DoC is seeking half the costs from major stakeholders, including Environment Waikato and Taupo District Council. Transrail says their existing system is sufficient and the risk is not significant enough, while Transit New Zealand is still considering a funding contribution.

The Committee recommended that Environment Waikato support the project, providing management support to enable all stakeholders to look at project cost distribution.

It also recommended apportioning the remaining 50 percent of the costs among itself, Horizons mw, Transrail, Transpower, Transit New Zealand, Taupo District Council and Genesis Power. It also recommended making an initial payment of $7000 to enable DoC to second a project manager.