Long-term closure of State Highway 1 through Far North's Mangamuka Gorge likely

Waka Kotahi NZTA says the Mangamuka Gorge is likely to be closed for some time. Photo/Supplied.

The "worst-case scenario" has become a reality for the Far North's most direct route.

The Mangamuka Gorge remains closed until further notice after its closure last Thursday due to severe damage sustained on State Highway 1 as a result of last week's deluge.

Several slips were reported along the state highway.

Speculation over the state of the Mangamuka Gorge has been swirling for weeks, after the appearance of numerous small slips and cracks in recent months.

The stretch of SH1 through the Mangamuka Gorge reopened only last year after a 12-month closure caused by a one-in-500-year storm in July 2020.

The 2020 event identified eight slips, with almost 8000cu m of dirt falling down to the river below.

The upgrade cost about $13.8 million, which included five new retaining walls and 800cu m of concrete.

Okaihau resident and former Northland regional councillor Joe Carr wrote to the Northland Age just days before the gorge's most recent closure.

He was concerned about "the likely catastrophic failure of SH1 over the Mangamukas because of yet another slip".

Carr could not believe Waka Kotahi NZ Transport Agency had allegedly watched the slip worsen without immediate intervention.

"The reason for their lack of concern and total lack of urgency in restoring SH1 to full operation when the road dropped out north of Mangamuka Bridge two years ago still has not been answered," Carr alleged.

"Was the 12 months closure of our vital link because of the incapacity or the incompetence of NZTA, or worse?

"A bridge for half the cost of the protracted slip repair could have had our highway operational in under two months. Where does the accountability rest?"

Carr explained there were at least five considerably unstable areas of the Mangamuka Gorge road.

Trapped water was thought to be the cause of the slips found at the site.

He said these could have been easily and cheaply drained and stabilised using horizontal directional drilling while the highway was closed.

Furthermore, the installation of vertical column drains intercepted at their base by directionally drilled, horizontal drains could have been a potential solution.

"How far in the poo do we have to get? We now have a SH10 bridge [over the upper Waitangi River] unfit for heavy vehicles due to lack of maintenance," Carr said.

One of the five slips identified at the Mangamuka Gorge. Photo / Supplied

"Route security alarm bells should be ringing in Wellington with both SH1 and SH10 so compromised.

"We won't even go near the SH12 Taheke Bridge that is held up by its handrails and what its failure would mean."

It would appear Carr's dire predictions have been a death knell for the gorge.

Just a week before last week's severe weather, Far North mayor John Carter was also spotted taking photos of a crack on SH1 at Mangamuka Gorge.

Carter confirmed he had been taking photos to send to Waka Kotahi, over concerns the road was deteriorating.

Last week he visited one of three significant slips in the gorge. What he saw convinced him it could be a long time before the road fully reopened.

"This is going to take many months at least to clear and likely cost the Far North millions of dollars," Carter said.

"This is literally the worst-case scenario for the Far North."

This closure comes at a time when the state of Northland's roads continue to be on the decline.

Waka Kotahi confirmed the potential for SH1 through the Mangamuka Gorge to remain closed for some time.

Waka Kotahi regional transport systems manager Jacqui Hori-Hoult said a timeline for rebuilding and reopening the highway through the gorge would not be known until geotechnical engineers had carried out detailed assessments of all of the damage sites.

"Our crews have only been able to gain safe access to some areas in the past day, with conditions in the gorge still highly unstable.

"Even with the fine weather currently, parts of the road and surrounding hillsides are continuing to move," she said.

"We're dealing with more than a half dozen major slips and underslips following this rain event."

According to Hori-Hoult, the soil in the area was generally soft and unstable, making the land stability in the gorge particularly susceptible to the effects of severe weather.

The most recent heavy rain had caused five new underslips in the gorge (with movement of land below the road).

Three of the underslips were currently being investigated by geotechnical specialists to determine the most effective permanent solutions for stabilising the land.

The remaining two underslips had been identified as "slow movement sites", which would continue to be monitored to determine the rate of movement and potential need for longer-term remedial works.

"We closely monitor all known land movement and slips in the Mangamuka Gorge, as we do for all state highways," Hori-Hoult said.

"But the increasing frequency and intensity of severe weather events as a result of climate change means slips and closures are becoming more common, on state highways as well as on local roads.

"We understand the importance of this route to the people of Northland, and will provide an update on the extent of the damage and likely timeline for completing repairs as soon as our damage assessments are complete."

The detour route while SH1 through the gorge is closed is SH10 via Kerikeri, Kāeo to Awanui and to SH1 Kaitāia.

Te Taitokerau MP Kelvin Davis was approached for comment but did not respond in time for edition.