Explosives release water from dam at Mangapoike River landslide

Explosives were detonated yesterday to cut a two-metre deep trench to release water from a dam formed by a Mangapoike River landslide.

The landslide was likely triggered by a small earthquake in the area earlier this month, resulting in the formation of a dam which was more than 40 metres deep and rising 60cm a day.

With rainfall predicted over Easter, there was a high risk water levels would rise and potentially damage a bridge at Paparatu and block access to Mangapoike Station.

Council’s principal scientist Dr Murry Cave says if that were to happen over the long weekend, there was less ability to respond rapidly.

“Council identified a window to give nature a hand before Easter and undertook the blasting operation to allow for a controlled release of water from the dam into the basin behind it,” he says.

Council staff have been working with Hawke’s Bay Regional Council and scientists from Auckland University to monitor the landslide.

Their assessments led to the conclusion that if a channel was cut between the lake and a low point in the dam, there was a good chance the water levels could be safely reduced.

“This operation was completed successfully with overall water levels dropping giving Gisborne, Wairoa and Hawkes Bay councils time to plan for a spillway on the downstream side of the spillway if this should be necessary,” Dr Cave says.

A video of the explosion can be found on the GDC Facebook page.

Council will continue to monitor the situation over the next few weeks until the long-term stability of the dam is established and lake water level stabilizes to a point that eliminates the risk to Mangapoike Station.

The area affected by the landslide is now approximately 33 hectares.

Paparatu Road was closed to the public last week.

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