Crews give tour of storm-damaged Tākaka Hill road

What does it take to repair a state highway? Local Democracy Reporter Max Frethey toured the damage on SH60/Tākaka Hill to find out.

"Three things wreck roads," said Rob Service, the New Zealand Transport Agency’s system manager for the Top of the South.

"Water, water, water… Water’s our archenemy."

Tasman has suffered a lot of water over the last month, with half of the district’s annual rainfall occurring over just a two-week period, flooding communities and leaving the ground sodden.

The region has left its state of emergency and is now in recovery mode, leaving NZTA with three sites that need repair on SH60 over the Tākaka Hill.

There were two underslips, one of which was accompanied by an overslip, and a slump in the road caused by the shifting of a historic landslide.

Service describes himself as a "naturally impatient man" and said it was both the community’s and his expectation that all the repairs to the route were completed within the next five months.

"I’ve asked that we get the next jobs done before Christmas – we’re driving that for a solution," he said.

There may be some closures as the repairs are conducted, but Service aims to keep the road open for most of the works – though some sections will likely remain at one lane until the repairs were complete.

Repairing an underslip – where water has eroded the ground underneath a road – typically takes a year, with three months for investigation and another three for design and approval.

However, Service was pushing a new approach for the Tākaka Hill, where several underslips have been repaired over the years.

The agency’s consultants had been tasked with taking previous underslip repair plans and adapting them for the two, newly-damaged sites rather than creating bespoke designs.

"Let’s not reinvent the wheel every time we have one of these. Let’s take what we previously know, modify it, and deliver," Service said.

Despite cutting back on the time spent planning, he was confident the process was robust and delivered resilient results.

That shortened approach was taken for another underslip on Tākaka Hill where remedial works concluded in December last year and the site was left unaffected after the recent deluge.

"We’re not starting from a blank sheet of paper, we’re using previous improvement designs that are on this hill… You’ve already got the basic recipe, you just might need to tweak it a little bit for each site. It’s not major engineering."

Service expected the cost of repairing each underslip to be at least $1 million.

Further up the road, at a site called Drummonds Corner, the road has buckled after a historic landslide that the road was built on shifted in the rainfall.

That site could be at least $2 million to repair once some problematic trees were removed and some drainage works are also undertaken.

"It moves every couple of years," Service said.

"Will we be back again? Quite possibly. Why don’t we be a little bit bolder and think about long term solution, a permanent solution that benefits road users, cuts down on maintenance?"

NZTA was now investigating building a brand-new section of the road that would cut around the historic slip site, further away from the hillside.

Service expected that option to cost $2-3 million, potentially a cheaper option long-term if the new route wouldn’t need to be repaired as often as the section on the historic slip.

"[The road] will have a much better alignment, with less sharp corners, much smoother and safer driving… All we’ve got to do is remove the vegetation, which is not hard," he said.

"I think there’s a case worth investigating."

A decision would be made in the coming months, once a financial analysis was undertaken.

Though the region has now had two weeks of sun, the ground still remains wet, and an orange rain warning for Tuesday has concerned NZTA.

Service has ordered contractors to clear the drainage and culverts on the hill, and bunds have been installed around the current underslips to try and limit water from worsening the problems, but he acknowledged that more slips are a possibility.

"It’s all we can do practically in the meantime to try and make it as safe," he said.

"The hill has been quite resilient, a lot more resilient than in previous years, so we’re hopeful that it’ll still hold up. We’re always going to get bits fall off here, it’s just the nature of the terrain that we are in… We just need to respond as quick as we can."

Further south, disruption on SH6 is ongoing as NZTA responds to damage at Spooners Saddle, Kohatu, Norris Gully, and Coal Creek.

Some places, like Kohatu, will only receive temporary solutions until summer when conditions are dry enough for permanent repairs.

The week-long closure of SH6/Rocks Road in Nelson which congested the city while NZTA responded to slips on the road has also concluded, but contractors are returning to the road on Monday to prepare the cliff face for Tuesday's rain.

Planning was underway to develop a long-term solution that would involve new retaining works completed at the top of the cliff.

Much of SH63 in the Wairau Valley between St Arnaud and Renwick remains closed to most vehicles except residents and essential traffic.

NZTA’s current priorities on that route was addressing the washout at Andersons Bridge and flood scouring west of the Waihopai Bridge.

Local Democracy Reporting is local body journalism co-funded by RNZ and NZ On Air.