NZ Transport Agency Waka Kotahi – Christchurch Northern Corridor nature-based design

Summary

• The Christchurch Northern Corridor (CNC) is the primary motorway link north of the city and opened in 2020.
• This project concerns an approximately 10km section of the motorway, delivered via a large-scale alliance between NZ Transport Agency Waka Kotahi, Christchurch City Council, and Fulton Hogan at a cost of $290m.
• Before joining Awa Environmental as Principal Landscape Architect, Mike Pentecost worked as a Senior Landscape Architect on the project – from alliance bid in 2015 to project practical completion in 2021.
• Mike played a lead role in in developing the planting strategy and design of a connected ecological corridor along the new motorway and contributed to designs for an innovative shared user path, important structures – such as illuminated bridges and cycle underpasses – and nature-based stormwater solutions.
• The overall project received top honours at the Civil Contractors New Zealand – Canterbury Westland Awards 2021, winning both innovation and environment awards for its design and sustainable management practices.

Challenges

• Building major transport infrastructure in challenging geotechnical conditions, including marsh soil, and in a low-lying area that had elevated flood risk.
• Delivering a cohesive landscape outcome across a large, linear infrastructure corridor.
• Integrating transport infrastructure with a strong ecological and visual identity.
• Coordinating design within a complex, multi-party alliance environment.
• Providing safe, accessible, and high-quality outcomes for multiple user groups, including recreational pedestrians and cyclists, and daily cycle commuters.
• Providing an ecologically sound outcome on time and within budget.

Solutions

• Mike collaborated closely with all members of the alliance to ensure on time and in budget delivery of his contributions, and design consistency from concept to construction.
• He helped design a connected ecological corridor along the new section of motorway that supported native species and linked other significant sites, such as Deans Bush and Ōtukaikino.
• Central to this was a planting strategy based on pre-existing natural ecosystems – a successional Canterbury plains forest – which re-introduced plant communities and habitat once lost to the area and earned the corridor the nickname “Tōtara Highway”.
• He integrated this with landscape and structural elements, such as bridges and cycle underpasses, to help create a unified corridor experience aligned with the ecological theme.
• Mike also contributed to the design of a shared user path and incorporated multi-modal transport options to ensure the corridor provides more than just road transport.
• The path utilised innovative design features, such as inclined subway walls to give the sense of more space, and safe areas for pedestrians to move to when cyclists pass.
• Mike helped enable holistic stormwater management by championing nature-based solutions in his designs, including extensive natural planted areas to reduce road flooding risk, improve surface runoff quality, and trap sediments within natural planted swales.
• This involved proving to senior project leaders that natural planted areas were more cost effective over the long-term than traditional mown turf grass.

Benefits

• New ecological connections and creation of habitat.
• Cost-effective flood resilience through nature-based solutions.
• Improved transport efficiency while providing ecologically sound outcomes.
• High-quality walking and cycling infrastructure along the corridor.
• A distinctive landscape identity for the route established.
• Effective and efficient project delivery within a complex, multi-party alliance.