By Peter de Graaf of RNZ.

Ongoing heavy rain events and fresh slips are expected to delay the re-opening of a storm-damaged section of State Highway 1 until well into next year.

A 13-kilometre stretch of highway through Mangamuka Gorge in the Far North, just south of Kaitāia, was destroyed by slips during torrential rain in August 2022.

Project director Norman Collier said Waka Kotahi New Zealand Transport Agency’s $100 million project to rebuild the road was initially due to be completed in May 2024.

“That date is becoming more and more challenging. Indications are that we will probably overshoot that [by] a couple of months,” he said.

“The main reason for that is we’ve had more slips. After the cyclone, there’s been more heavy rain events. We’ve had seven in the first six months of this year, with more than 100 millimetres of rain in one night.”

The original 26 slips had now grown to 38. Of those, 15 were initially deemed to be critical, meaning they could have taken the entire road away.

After consulting the local community, Waka Kotahi had decided not to re-open one lane for holiday traffic in December.

Collier said locals’ priority was to get the job finished as soon as possible, and a partial opening would have created delays and disruptions.

At one point, Waka Kotahi considered closing the section of highway permanently because the damage was so severe.

In November last year, however, the board decided to set aside $100m to rebuild the gorge road.

Collier said that was because of the road’s importance to the local community, but also because of the effect a permanent closure would have on the rest of the Far North’s transport network, especially SH10.

Senior project manager Hendrik Postma said the project was hugely challenging.

As well as ongoing rain, construction crews had to deal with steep terrain, isolation, cold and land that was still on the move.

A series of warning lights throughout the gorge warned workers when any of the slips started moving again.

In places where the highway had disappeared almost entirely, a temporary road had to be built first so heavy machinery could be brought on-site.

Only then could permanent repairs begin.

Postma says the finished road would be supported by hundreds of reinforced concrete piles, each driven 12 to 21 metres into the ground.

The piles would be capped with concrete slabs which would then be anchored deep into the hillside with steel rods.

He dubbed the massive structure the “Mangamuka Wall”.

Despite the difficulties, Postma said it was the most rewarding project he’d ever worked on.

Much of that came down to the workers, 85 per cent of whom were drawn from the local area.

As well as ensuring good spirit and camaraderie, that increased productivity because workers didn’t have to travel long distances each day to reach the site.

They were also motivated to get the gorge open again.

One of those local workers is Mangamuka-born-and-bred Tomo Otene, the project’s Ngā Hapū o Mangamuka representative.

Otene worked on major tunnelling projects across the Tasman and on Auckland’s City Rail Link before being lured home to work on the gorge.

The closure of the highway had a serious effect on whānau on either side of the gorge, he said.

Mangamuka residents faced a long detour to buy supplies, get medical care such as dialysis treatment, or visit family in Kaitāia.

It was time they would never get back, he said.

The project had, however, given large numbers of locals work close to home in an area where employment was scarce.

“It’s put bread and butter on the table for a lot of people,” he said.

Between 100 and 150 people were working in the gorge at any one time, and more than 500 people had worked on the project at some point.

Despite the setbacks and challenges, Otene was confident the gorge road would re-open.

“It’s not going to be an easy fix, but we will get there.”

History repeats

The current closure of Mangamuka Gorge is the second Northlanders have had to endure in recent years.

In fact, by the time the road re-opens, it will have been closed for a total of three years since the beginning of 2020.

The first major closure came on July 19, 2020 when, after months of crippling drought, more than 200mm of rain fell across the region in one night.

It was described at the time as a one-in-500-year event.

Repairs took just under a year and cost $13.8m, with the highway re-opening on June 30, 2021.

That lasted little more than a year.

During a storm on August 18-19, 2022, the 13km stretch of road over the Mangamuka Ranges was destroyed by 26 slips, with 15 of those described as critical.

The damage was so severe there were fears the gorge might never re-open, as was the fate of some Northland roads after Cyclone Bola in 1988.

At the end of 2022, Waka Kotahi committed an initial $100m to repairs, with a construction contract awarded in February 2023.

The first stage involved shoring up what was left of the road and making it stable enough to support the heavy machinery required for reconstruction.

The work includes improving drainage to make the road less vulnerable to future extreme weather events.

State Highway 10 around the east coast has been the main detour while the gorge is closed.

For Hokianga residents travelling to Kaitāia or further north, the west coast route via Broadwood and Herekino is shorter, but it’s unsuitable for large volumes of traffic and is also vulnerable to slips and flooding.

For motorists travelling, say, from Kerikeri to Kaitāia, the SH10 detour adds only about an extra 20 minutes to their journey.

But for Mangamuka residents who need to head to Kaitāia, traditionally their main service town, travel times have more than doubled.

Waka Kotahi has previously said a tunnel through the ranges, avoiding Mangamuka Gorge, would cost an enormous amount of money and could not be justified by the roughly 1300 vehicles that used the gorge each day.

By comparison, SH1 over the Brynderwyns, another slip-plagued range of hills, is used by more than 10,000 vehicles a day.

State Highway 10, the main detour, has its own challenges.

It floods regularly north of Kāeo and a bridge across the Waitangi River, near Kerikeri, was closed to heavy traffic in March 2020 after cracks were found in the concrete structure.

Heavy trucks were instructed to use SH1 until the closure of Mangamuka Gorge made that impossible.

A 10km/h speed limit was imposed for the heaviest trucks instead, until bridge repairs were carried out earlier this year.

Increased traffic on SH10 has led to congestion and a reported increase in crashes.