Next Manawatū Gorge rock fall could be 170 times larger than two previous slips
The Manawatū Gorge could experience a rock fall 170 times larger than the two slips that caused the state highway to close in April.
The NZ Transport Agency expects as much as 630,000 cubic metres to drop at Kerry's wall, near the Ashhurst end, which has prompted Horizons Regional Council to install webcam monitoring.
In July, the agency announced the key route, which connects Manawatū and Hawke's Bay, would close indefinitely after the 3500cum falls.
In September, transport agency regional relationships director Emma Speight said the rock face continued to move and the total potential area that could fall was somewhere between 495,000cum and 630,000cum.
"It's a huge volume of material. That's more than potentially twice the size of the 2011 slip."
A portion of unstable rock at Kerry's Wall was predicted to fall first, but the rest could all fall at once, Speight said.
Previously, movement was about 60 millimetres over 18 months, but in August alone it moved 80mm, she said.
"It is continuing to move at quite a pace."
The agency's regional transport systems manager Ross I'Anson said geotechnical reports showed the slip would potentially fall in four stages, but nobody knew when it would happen.
The agency monitored movement weekly, I'Anson said.
Horizons emergency management manager Ian Lowe said it was likely that the river could be partially blocked.
A plan of action is under development should a slip fall.
"The prime focus of the action plan is on public safety and any ongoing requirements once the size of the slip has been confirmed."
The state of the river levels at the time would have a bearing on the actions taken. Monitoring would help Horizons take action quickly, Lowe said.
Massey University physical geography senior lecturer Sam McColl said a large landslide could block the Manawatū River and cause flooding to parts of Ashhurst and Palmerston North if the dam breached.
Horizons chairman Bruce Gordon said in the perfect-storm scenario, something catastrophic could happen. But, based on the transport agency's prediction that the slip would fall in four stages, it was unlikely.
Gordon previously suggested moving the road out from the hillside in slip-prone areas to allow for material to fall into a ditch rather than on the road, but on Wednesday he did not want to comment on which alternative route was now most viable, based on the slip predictions. "I'm just going to wait for the preferred route rather than making a comment."
The decision on a preferred route will be made by December. The transport agency does not expect a slip as large as 630,000cum to reach the gorge walking tracks. The walking tracks remain open but fences are in place to stop people accessing the road.