Call for new highway around slip-plagued Mangamukas

One of 18 slips - three overslips, like this one, and 15 underslips - that wrecked SH1 through Mangamuka Gorge during last week's torrential rain. Photo / Waka Kotahi

A Far North councillor is calling for a road west of the Mangamukas to be upgraded to a state highway, as an alternative to the storm-wrecked SH1 through Mangamuka Gorge.

A series of major slips and slumps have made State Highway 1, the main freight route to Kaitaia, impassable since the torrential rain of August 18-19.

At last count, there were 15 new underslips and three overslips on the roughly 12 kilometre-long gorge road.

No reopening date has been given, but the last time Mangamuka Gorge was hit by major slips, in July 2020, repairs took almost a year and cost $13.8 million. The highway only re-opened 13 months ago.

The official detour is SH10, around the east coast.

District councillor Felicity Foy, of Kaitaia, said it was the second time in two years the Far North had been dramatically affected by slips in Mangamuka Gorge.

"Last time it was for a whole year. I wouldn't be surprised if it's another whole year, if not longer. It's happened twice now; why wouldn't it happen again?"

Given the vulnerability of SH1, Foy said the western route around the Mangamukas — through Broadwood, Herekino and Ahipara — needed to be upgraded to handle extra traffic and serve as a detour any time the gorge was closed.

That was beyond the means of Far North ratepayers, but if it was designated a state highway, the cost would fall on Central Government.

The Broadwood-Herekino-Ahipara route was already being used as a detour, including by heavy trucks and milk tankers.

"With this route being used more, we need to ensure it can stand up to weather events like this, especially if they're going to be more frequent. They've had quite a few slips there too, and the road needs attention, especially through Herekino Gorge."

Far North District councillor Felicity Foy. Photo / Nicole Jecentho

The Council received government subsidies of up to 67 per cent to upgrade district roads, but the funding was limited, and the Northern Transport Alliance, the region's land transport organisation, decided which projects got the money.

"So without changing the classification, we won't change the funding," she said.

Foy said Kaitaia residents were "sad and extremely frustrated" about the closure of SH1.

"Transport links are essential to business. If we want to keep the businesses we have in Kaitaia, and make sure they don't just survive, but thrive, we need good links."

The Broadwood-Herekino-Ahipara route is part of the Twin Coast Discovery Highway tourist drive.

Foy also called for it to be made a state highway when the gorge was closed in 2020.

SH10 also vulnerable

While SH1 remains closed through Mangamuka Gorge, the official detour follows SH10 around the east coast.

From Whangārei or the Bay of Islands, there's little difference in terms of driving time to Kaitaia — but for traffic originating from the west of the district, it's a major detour.

SH10, however, has its own issues.

A stretch of highway just north of Kāeo Bridge is vulnerable to flooding, and until last week, the Waitangi River Bridge near Puketona was closed to over-weight vehicles due to a structural defect found during a routine inspection. Heavy trucks had been ordered to detour via SH1.

One of many slips in Mangamuka Gorge triggered by the torrential rain of August 18-19. Photo / Waka Kotahi

Now, with SH1 closed, Waka Kotahi NZ Transport Agency has reopened the bridge to heavy traffic, but it is checked daily for further deterioration.

Drivers of overweight vehicles are instructed to keep to the centre of the bridge and travel at 10km/h.

Heavy trucks also require a special permit to use Parapara Bridge, northwest of Taipā on SH10.

Another concern is the extra traffic on the more built-up SH10.

The last time SH1 was closed, in 2020-21, firefighters in Kerikeri and Kaitaia reported a significant increase in traffic and crashes on SH10.

Businesses worried, frustrated

Kaitaia Business Association chairwoman Andrea Panther said any idea for resolving the situation was a good one, but she was concerned it could take just as long to upgrade the route through Herekino as it would to fix the gorge.

"So it's a bit of a catch-22, but at least we'd be future-proofing."

In the meantime, with SH10 also vulnerable, it wouldn't take much to cut off the entire Far North.

"That would have significant detrimental effects on our area, economically, socially, in terms of health services... the list goes on."

Panther also worried about the upcoming summer. The last time SH1 was shut, tourists were reluctant to visit the Far North because SH10 was "so clogged and slow", she said.

Slip debris covers SH1 after the torrential rain of August 18-19. Photo / Waka Kotahi

The Mangakahia Rd precedent

Mangakahia Rd, an inland route linking Whangārei with Kaikohe, offers a recent precedent to designating a local road as a state highway.

A major storm in 2014 closed SH1 south of Kawakawa for a week, leaving Mangakahia Rd the only freight route to the Far North.

It soon became clear Mangakahia Rd couldn't withstand the huge increase in heavy traffic — and nor could the Far North District Council afford to upgrade the 70km-long road.

As a result, Mangakahia Rd became the new SH15, which means government roading agency Waka Kotahi is now responsible for its maintenance.

Under or around?

The idea of drilling a road tunnel through the Mangamuka Ranges is revived any time the gorge is shut by slips.

The Advocate couldn't find a tunnelling expert willing to be interviewed by edition time, but the cost would likely be enormous.

Currently, New Zealand's longest road tunnel is the 2.4km Waterview Tunnel in Auckland, completed in 2017 at a cost of $1.4 billion.

The straight-line distance through the Mangamukas, from Makene Rd to the Raetea North Side Reserve, is 8.5km.

Multiple slumps and underslips have made SH1 impassable through Mangamuka Gorge. Photo / Waka Kotahi

Even if the tunnel started halfway up the south side, where the scenic reserve begins, and emerged halfway down the north side, it would still be about 4km long. That's almost doubling Waterview's length, and would be more than triple the length of the Homer Tunnel.

However, any Mangamuka tunnel would likely have a single lane each way, unlike Waterview's six-lane total.

In 2020, then-National leader Judith Collins announced a tunnel through the Brynderwyns was part of the party's 20-year infrastructure plan.

No cost estimate was provided, however, and it was not included in the plan's $31 billion price tag.

'Very significant investment'

Building a tunnel through the Mangamukas, or a new state highway route, would require "very significant investment", a Waka Kotahi spokesperson said.

Neither option would meet the criteria for the expenditure of such a large sum of money.

On average, SH1 through Mangamuka Gorge carried only about 1300 vehicles per day, compared to 10,000 a day on SH1 over the Brynderwyns.

"SH1 is a vital economic lifeline for Northland, and Waka Kotahi is committed to ensuring that the route is as resilient and dependable as possible," he said.

"The full extent of the damage sustained at several sites through the Mangamuka Gorge from this most recent weather event is still being assessed, and decisions on the nature of repairs and reconstruction will depend on the outcome of those assessments."