Hybrid tunnel through Manawatu Gorge best solution says Tararua deputy mayor

A hybrid (partial) tunnel through the existing Manawatu Gorge route is an option backed by Tararua District Council's deputy mayor Allen Benbow. Photo File

A hybrid or partial tunnel through the existing Manawatu Gorge route is a viable and safe option, Tararua District Council's deputy mayor Allen Benbow believes.

Mr Benbow said the option - one of three identified as solutions to the indefinitely closed Manuwatu Gorge - could deliver for the region, as well as nationally.

"At the recent public meeting in Woodville to discuss the future of the gorge and options for an alternative route, the New Zealand Transport Agency (NZTA) said they were working at pace to have a tendering process complete by late this month, with the preferred options and an indicative cost due from the successful tenderer by December," Mr Benbow said.

"This would then enable NZTA to undertake consultation with the public on the right solution so design and consultancy work could begin."

Both National MP Alastair Scott and the NZTA have said funding isn't an issue, rather it is about selecting the preferred alternative option and building a good, sustainable, long term route to serve the region.

Transport Minister Simon Bridges has also said money wasn't the issue, because with a growing economy the Government can afford the best option.

"Given these comments and undertakings, it would seem fair and reasonable the residents of our district, the wider region and New Zealand can expect the best long term option to overcome the disruptive and devastating impact the gorge closure is having on our lives, our businesses, our families, district and our region," Mr Benbow said.

"While all three options and perhaps others, all need to be carefully assessed and costed, it seems a shame the public's perception has already been influenced by numbers being thrown about indicating a tunnel option could cost an eye-watering $1.8 billion which probably makes it unaffordable and out of the mix of being seriously considered."

With the Manawatu Gorge a vital, strategic link between the east and the west of the southern half of the North Island, it's not only a vital route for Tararua residents accessing the Manawatu for commercial and critical health needs, it is also a vital freight route, Mr Benbow said.

"With many Tararua health services now transferred to Palmerston North it is vital to have a reliable, convenient and timely road. Without it our commercial sector will be disadvantaged and the district will become unattractive to new businesses and Tararua will become a less appealing place to live.

"The NZTA's own table of regional strategic routes has the link ticking all the freight boxes, for volume and access to ports, including the Port of Napier.

"With shipping services continually consolidating, the Port of Napier growing and Palmerston North continuing to develop as a distribution hub, growth in freight passing along the route can only increase.

"So far as options are concerned, there is only one option which provides a flat route so important to heavy transport. Also there is only one route which can retain part of the uniqueness of the Manawatu Gorge and its scenery, history, geology and cultural significance. It also needs to be one of the more direct routes available. The obvious option is a hybrid tunnel.

"When speaking to people about a tunnel, the idea is generally widely supported," Mr Benbow said. "However, a concern regularly raised is the issue of earthquakes and fault lines and the associated risk. It is true there are two active faults passing through the Woodville end of the gorge, the Ruahine Fault and the Mohaka Fault."

Earthquake faults are not unique to tunnelling in the Manawatu Gorge, they have had to be considered in other tunnelling projects too, Mr Benbow said.

Tunnelling costs:

"Tunnelling is an expensive option and apparently the variable greywacke geology in the Manawatu Gorge makes for challenging tunnelling conditions," Mr Benbow concedes.

"Also unknown underground water conditions could add to the challenges. However, these challenges are apparently not uncommon in tunnelling projects and have been encountered and addressed in other New Zealand projects."

Mr Benbow said he understood the cost of tunnelling the Waterview Tunnel in Auckland was between $50 and $60m a lane .

"This equates to approximately $165m a kilometre for that 15 metre, three lane tunnel," he said. "Using the Waterview Tunnel as an indicative cost, the Manawatu Gorge option 1 tunnel would cost $700m and option 2, $450m. While initial geo-technical advice has been obtained these costs are guides only and require extensive geotechnical assessment and evaluations."

Mr Benbow said based on these indicative costs, he would like to see these tunnel options be thoroughly evaluated during the upcoming process by the NZTA to develop a proposed option for public consultation.

"The preferred option should not be selected because it best fits current budget constraints, rather it should be the best option for our district, region and New Zealand for generations to come," he said.

Tourist attraction:

Tourism around the Manawatu Gorge has increased significantly, particularly over the past year or two, with the gorge walks enjoyed by more than 49,000. Ferry Reserve at the Woodville end of the gorge is becoming a favourite place for campers, especially over the summer period.

"The drive through the gorge is a tourist attraction itself," Mr Benbow said. "And a new tunnel, which would be the longest road tunnel in New Zealand, combined with the drive through part of the gorge, will be a significant tourist attraction.

"When these features are combined with the future potential of significant tourist development being considered by various parties, they have the potential to dramatically improve the Manawatu Gorge as a tourist destination. This will have a significant beneficial impact on the region and Woodville in particular.

"These benefits need to be factored into the economic analysis of the tunnel options."