Project update

27 May 2022

Welcome to the latest newsletter on Te Ahu a Turanga: Manawatū Tarararua Highway. In this special edition, we are providing you with an update on the old road through Te Āpiti - Manawatū Gorge.

Image of boulders from a slip at the west end of the old road through the Manawatū Gorge.

New geotechnical assessment results in

Waka Kotahi has received draft results from a new geotechnical assessment of the old road though Te Āpiti - Manawatū Gorge.

The results have reconfirmed the decision to close the road to vehicles. Engineering geologists surveyed a 11km stretch of Te Āpiti - Manawatū Gorge, from the Ashhurst Bridge in the west to the intersection with Woodlands Road to the east.

The assessment looked at the risks of landslides and rockfalls now and in the future and considered how the road and existing infrastructure such as bridges, retaining walls and rock fall mesh might deteriorate over time.

Different timeframes from the present through to 2041 were considered and the various sections of the gorge were scored, depending on the risks.

The assessment found that much of the gorge, including an almost four-kilometre-long section in the middle, created a level of risk for people in vehicles that exceeded risk tolerability levels.

Risks to people walking or riding a horse or bike were found to be lower, if work was done to support safe public access. Risk levels for cyclists are the lowest, as they are generally travelling faster than people walking or on horseback.

How do you decide what is a tolerable risk?
New Zealanders are no strangers to taking risks while we are doing things we enjoy. From white water rafting or climbing Aoraki/Mount Cook parachuting or jetboating, we do little things every day that carry an element of risk to personal safety. We assess the risks against the benefits and decide what we’re happy to tolerate.

While New Zealand has no national guidelines for life risk tolerability, guidelines from Australia have been applied in our country several times and have set a precedent. This published information helped guide thinking. 

For pedestrians, cyclists and horse riders, the annual life risk tolerable threshold is 1 in 10,000, which means if you visited the gorge once a week for your entire life (80 years), you would have roughly a 1 in 100 chance of dying from the slope stability hazards.

What assessment methods were used?
Risk to people in cars was assessed using the NSW Slope Risk Analysis (NSW) method while the Australian Geomechanics Society (AGS) method was used to calculate risk to people in the open.

The NSW method has been adopted by Waka Kotahi as its preferred risk assessment methodology for vehicle occupants, which has been applied on a number of New Zealand locations to date, including the Kaikoura coastline following the 2016 earthquake, and the proposed Mt Messenger Bypass (SH3).

The AGS quantitative life risk assessment methodology, or variations of it, has also been applied extensively throughout New Zealand.

The Kerry’s Wall slip in the Manawatū Gorge, taken in May 2001.

Next steps

Te Āpiti is a special area and there is a shared vision within the community to protect, preserve and enhance the area.

Utilising the draft Geotech results, our focus now is working with iwi, LINZ and the parties responsible for managing parts of the Gorge to discuss next steps. We expect the final peer reviewed geotechnical assessment report in June.

The Manawatū Gorge old road is no longer required as part of the transport system and Waka Kotahi is unable to fund any future work on the road outside the scope of revocation and to meet our legal obligations.

Working with our community partners, we will develop a management plan on how to manage risks in the future under different ownership. Appropriate public access could be provided at this point if funding is secured.

Waka Kotahi thanks people for their patience while this work is underway and reminds people to not proceed beyond the locked gates at the entrances to the Gorge.

What is revocation and what is involved?
Revocation is the cancellation or removal of a status or offer. In this case, revocation is the removal of the status of ‘state highway’ from a stretch of road.

Revocation is undertaken when a road is no longer required as a state highway. This could be because an alternative route has been constructed or because the function of the route has changed over time and it now makes sense for the road to be managed by the relevant local authority.

If a state highway is identified for revocation, Waka Kotahi must consult with affected iwi, councils, stakeholders and with local communities who may be affected by or have an interest in the highway in question. Following this process, Waka Kotahi will make a recommendation to the Ministry of Transport to uplift the state highway status and change the road to a local road or to have the road legally “stopped”.

The road is normally then handed over to the territorial authority or other relevant organisation in the area to manage. If the road is stopped, this process involves Land Information New Zealand (LINZ).

Revocation of the road through Te Apiti is not simple because of the known geotechnical hazards and the challenging environment.

More information

For more information or inquiries, please email project communications and engagement lead Jean Archer on jean.archer@nzta.govt.nz