Fire in the hole!

Explosives release water from dam. Picture by Murry Cave

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

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

Gisborne District Council principal scientist Dr Murry Cave said if that were to happen over the long weekend, there was less ability to respond rapidly.

“The 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 said.

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 Hawke’s Bay councils time to plan for a spillway on the downstream side of the spillway if this should be necessary,” Dr Cave says.

The 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 stabilises 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.