Manawatū-Tararua Highway: Stream diversions key to preserving highway landscape

A stream diversion under construction in Zone 4 near the Woodville end of the project.

When most people think of road construction, they might picture diggers, dump trucks and rollers, but there’s another, equally important, aspect of the work that often goes unseen on Te Ahu a Turanga: Manawatū Tararua Highway.

“Treading lightly” is one of our project’s principles, and our environmental, ecological and kaitiaki teams play a key role alongside our constructions team towards achieving our intention to leave the environment in a better condition than when we found it.

There are natural streams in the landscape surrounding Te Ahu a Turanga, which are habitats for fish, tuna (eels) and kōura (freshwater crayfish).

Whenever the highway crosses through a stream, the waterway must be diverted to preserve the habitat for wildlife.

A stream diversion involves recreating a stream impacted by construction and diverting it away from the highway.

A total of 11.4km of stream diversions will have been made on Te Ahu a Turanga by the end of the project.

Many stream diversions have habitats constructed within them that cater to the wildlife that will be living in them.

For the tuna, deep pools with slow-flowing water are created and large pieces of native wood are placed in and around the streams to ensure they replicate what might be found naturally in the area.

Rock cascades and riffles – shallow, fast-running water over rocks – are used to slow water down and create pools of water for fish to rest.

Where the highway interrupts the path of streams, culverts are built under the road to connect streams and allow the passage of aquatic life.

These culverts – which are large underground concrete pipes – are fitted with extra add-ons to make it easy for wildlife to pass through them.

They have features that mimic natural streams, such as ebbs in the water flow to create rest pockets, helping the fauna find their way upstream.

The project’s iwi partners are heavily involved in constructing these new waterways. Iwi have also been engaged in the defishing process, which involves aquatic life being transferred from a waterway interrupted by the highway to a nearby location. This collaboration between iwi and the construction team ensures something positive and enduring is created for the future of New Zealand’s wildlife and infrastructure.

Construction update

Progress on pavements, drainage and earthworks has been slower than anticipated due to some extremely wet weather this winter, but good progress continues to be made on the major structures.

At Parahaki Bridge, across the Manawatū River, the first section of the superstructure (the part of the bridge the road goes on) is nearing completion on top of Pier 1. This is an exciting milestone and allows for the form traveller – a large steel structure which allows the bridge deck to be constructed out from the piers – to go up in September. Initial ground works on the north abutment are also underway.

Over the Manawatū River at the Eco-Viaduct construction of the 12th and final column has begun, while steel beams continue to be delivered to site.

At Mangamanaia Stream Bridge at the Woodville end of the project, the eastern abutment is nearing completion and work has started on the western abutment.

On the landscaping front, the team has planted well over 200,000 native shrubs and trees so far this season, and they’ll plant the millionth plant (since construction began) in coming weeks.

For more information about the project, head to www.nzta.govt.nz/projects/te-ahu-a-turanga/ or visit the Woodville Community Library and Information Centre for the latest flyover, project updates and the Drive the Highway simulator.