Te Ahu a Turanga: Manawatū Tararua Highway Stormwater Design

Te Ahu a Turanga: Manawatū Tararua Highway is a complex, large-scale roading infrastructure project that will provide a key regional connection between Tararua and Manawatū. WSP have incorporated best practice stormwater design principles, design integration with iwi partners, and digital technology communication tools, to deliver stormwater solutions that tread lightly on the surrounding landscape. The proposed solution is a system of 40 culverts, 10 bridges, 12 wetlands, 5km of constructed streams, and 20km of open channels, along the 11.5km of new highway, which implement Te Ao Māori values and water-sensitive urban design practices.

The Solution

The Manawatū Gorge is a river valley that runs between the Ruahine and Tararua Ranges, in the southern region of the North Island. The stretch of State Highway 3 that passes through the Manawatū gorge provided an important east-west transport link between Hawke’s Bay and Wairarapa on the east of the ranges, and Horowhenua and Manawatū to the West, and has played an important role in regional development since its construction in 1872. The SH3 Manawatū Gorge route was permanently closed in 2017 due to frequent slips and ongoing geological movement.

Waka Kotahi have selected a 11.5km replacement route which is due to be completed in 2024. The road will also have a separated shared path for walkers and cyclists. The 4-lane highway will contain 10 bridge structures across tributaries of the Manawatu River and could involve significant environmental impacts. It is a challenging environment with a steep topography, high rainfall, existing flooding and erosion issues and the presence of numerous steep streams.

WSP were engaged to undertake stormwater and hydraulic design services for the new road alignment. Key design challenges included the requirement for a parallel design and consenting process for a highly complex project, the proximity of the Te Āpiti wind farm and the steep topography. Innovative design, associated with the nature of the steep terrain and high fill embankments, was required for a number of design elements, including induced trenching and energy dissipation for some culverts.

WSP, as part of the wider Alliance, has implemented best practice design to enhance environmental, cultural and social benefits. Water sensitive urban design and Te Ao Māori values and principles have been incorporated throughout the project, with the stormwater design developed in partnership with iwi. The project is a step change for Waka Kotahi, through its recognition of Te Tiriti o Waitangi and integration of Te Ao Māori values. Representatives from each iwi group sat on the Project Alliance Board governing the project.

The key Māori values that have influenced the stormwater design include Ki Uta Ki Tai (mountains to sea approach), recognition of the whakapapa of the stream tributaries of the Manawatū River (adopting a whole of catchment approach), and kaitiakitanga, realised through the protection and enhancement of the mauri of sensitive environments.

The stormwater design has incorporated the cultural considerations by protecting highly valued streams and wetlands, integrating with the surrounding landscape, separating clean (waimāori) and dirty water (waikino), promoting fish passage, minimising construction disturbance, native planting and naturally treating road runoff. Cultural indicators of environmental health have been developed and will be monitored throughout construction.

WSP’s approach will allow the project to tread lightly on the surrounding environment and respect and incorporate cultural considerations. 40 culverts, 12 wetlands, 8 wetland swales, 5km of constructed streams, 9km of network drainage pipes and 20km of open channels and drains are proposed to support the road construction, while maintaining and enhancing te Mauri o te Wai.

To support the project’s significant community engagement and consultation, the project team have used innovative digital tools to assist with visualisation and communication, including BIM, 3D modelling, hydraulic modelling and virtual reality. These tools improve the communication of the highway design and its challenges to stakeholders and help engage the community with this significant project in their region. The use of innovative digital technology has facilitated the management and design of a highly complex and multi-faceted project.