A large landslide on the Mangapoike River remains a significant hazard and people are warned to stay away from it. Photo/Supplied.
A large landslide on the Mangapoike River remains a significant hazard and people are warned to stay away from it. Photo/Supplied.
Analysis of a landslide that created a lake between Wairoa and Gisborne has found it is unlikely there will be a sudden release of the six million cubic metres of water.
Last Thursday, Hawke's Bay Civil Defence Emergency Group announced it was working with local authorities to monitor the rising body of water. It is the result of a landslide likely triggered by a small earthquake earlier this month.
Hawke's Bay Civil Defence Emergency Management Group Manager Ian Macdonald said specialised engineers had been at the Mangapoike River, a small tributary of the Wairoa River, for the past few days to establish potential risks.
"We know that there will be some impact for the first 15km downstream of the small river but there are no properties in the way and it's all on private land.
"Once this water reaches the Wairoa River there's no impact on the level of the river, or it's a very minor impact. If it does go there will obviously be some sedimentation and debris so that could possibly have impacts further downstream."
Macdonald said it was certain that the water would continue to rise until it overtopped the lake, an event expected to take place around the Easter period, and local authorities were working to establish controlled management plans.
"The key thing was the landslide itself is relatively stable, it's a lot of material, but there's always the possibility that it might fail.
"The water could, once it starts to move through, liquefy the material and could have a failure. However it happens over a period of hours, it won't just fail. It will happen over a number of hours, if at all."
A landslide likely triggered by a small earthquake earlier this month resulted in about 80 million tonnes of material forming a large dam on the Mangapoike River. Photo/Supplied.
The slip affected 25.8 hectares of land and created the lake, about 50 metres deep, which was rising at a rate of around 60cm per day.
Macdonald said there were two possibilities for the water to escape the lake; either by overtopping it and creating a new channel through the slip material or finding a way underneath the slip and slowly leaking over time.
"We just need to make sure that if the dam does unexpectedly burst then we know what the impacts are on those bridges and if we need to shut roads," he said.
"That's unlikely. It is possible but it is unlikely."
The owners of four properties in Wairoa downstream of the site had been made aware of the landslide but Mr Macdonald said they were private properties so any failure would not affect members of the public.
Authorities were monitoring the situation on a daily basis and working with landowners to keep them informed, he said.
Paparatu Rd would remain closed to the public for the foreseeable future and people were asked to keep away from the area.
