Questions over future of West Coast's SH6 if Alpine Fault quake hits

Wednesday, 10 April 2019

Tim Gibb is ferrying up to 30 cars and 100 people a day across the Waiho River in a dump truck after the bridge was washed away in a recent storm. (First published April 2019)

The West Coast part of State Highway 6 may not be worth saving after the predicted large Alpine Fault earthquake, an engineering geologist warns.

But if a decision is made post-quake to keep the highway open, the New Zealand Transport Agency (NZTA) will need a much larger operating budget, University of Canterbury geologist David Bell says.

The highway - which runs from Blenheim to Queenstown and Invercargill via Nelson, Murchison, Greymouth, Franz Josef, Haast and Wanaka - is a vital and extremely fragile lifeline, vulnerable to slips, washouts and bridge damage in frequent heavy rainstorms.

State Highway 6 closed by a slip between Franz Josef and Fox Glacier.

Several sections of SH6 on the Coast were closed as a result of the northwesterly deluge on March 25 and 26.

**READ MORE:

A temporary fix to a washout on SH6, south of Paringa.

* Is another Bailey bridge the best option for Franz Josef's Waiho River?

* Scientists mark 300 years since last great Alpine Fault quake - preparing for the 'inevitable'

SH6 along the West Coast has been battered by two cyclones eroding the road in some places, including Punakaiki.

* Videos show devastating impact across South Island if Alpine Fault ruptures

* New Zealand's piles of historic rubbish - with a sea view**

SH6 near Punakaiki is under threat. One more storm and the West Coast could be cut in two.

On Wednesday afternoon, the highway was closed again, between Haast and Stewart Falls near the top of the pass, by a washout.

Seismologists estimate there is a 30 per cent chance of a magnitude 8.0-8.2 quake being generated by the Alpine Fault in the next 50 years. It will isolate parts of the Coast for months and frustrate maintenance efforts on the highway for many years after.

State Highway 6 rolls through rimu forest south of Fox Glacier in South Westland.

NZTA says there is a general fund for emergency work and that it can seek additional Crown money if necessary for the restoration of vital links.

But there is no special fund set aside for repairs to the road following such a massive shake.

The Alpine Fault running through Franz Josef situated on the hump of the road on the right.

Bell said the geomorphology of the Coast meant keeping SH6 viable was a constant struggle.

'Along the West Coast, virtually all the [highway's] river crossings involve going up to the range front, often close to the Alpine Fault, as the braid plain is at its narrowest there.

The old SH73 Otira Gorge road took the precarious route along the top of this shingle scree; the viaduct takes the safer option within the valley.

'It is a geologically dynamic environment, and the next Alpine Fault rupture will add massively to the problems for 100 years after the earthquake.

'Sediment in storage from landslides will be periodically released, aggradation will take place across broad flood plains, and highway maintenance will become very difficult – if not impossible.

The viaduct through the Otira Gorge. A major earthquake on the Alpine Fault would badly damage parts of SH73 across Arthur's Pass from Canterbury to the West Coast.

'Organisations such as NZTA will have to either have a much larger budget, or the state highway, which is the only through route, may no longer be a viable option.'

Stuff asked Minister of Transport Phil Twyford's office if there was an Alpine Fault quake contingency fund and was told to check with NZTA's experts or with Civil Defence.

'These are operational decisions for experts, not political decisions,' a spokeswoman said.

Civil Defence Minister Kris Faafoi's spokeswoman said it was not a question for that organisation but for NZTA.

NZTA Canterbury-West Coast systems manager Pete Connors said SH6 was one part of the South Island network which would be inaccessible for highway workers after an Alpine Fault rupture because of aftershocks and associated hazards over many months.

A fund for emergency works was available nationally through the National Land Transport Programme (NLTP) to respond to damage from severe weather events and natural disasters.

'Most parts of New Zealand make use of this in any one year to respond to storm events, slips and floods.

'For major events - such as the November 2016 Kaikōura earthquake - additional Crown funding would be made available by the Government.'

The cost of restoring SH1 through Kaikōura, as well as the rail link, the harbour and the inland route via Waiau, was now well over $1 billion, he said.

'That work is still under way with additional resilience works still happening well after the highway reopened a year after the quake, with its multiple sites of damage.'

NZTA was reducing the risk of future disruption from natural events through highway improvement and maintenance work, installing some cell coverage and live cameras in remote places, developing response plans and training staff.

Bell said aggradation of the Waiho River bed at Franz Josef in the past few months to years had been so severe there was a real risk of the river suddenly forming a new course through the township.

'Serious consideration must now be given to relocation of the Franz Josef township.'