Highway Constraints, Collaboration and Best Practice

This paper is intended as a case study to show how a collaborative approach on a highly constrained highway corridor has resulted in an effective best practice stormwater system that meets the objectives of all stakeholders.

The NZ Transport Agency (NZTA) is four-laning a flat length of State Highway One in Christchurch. This will result in a major increase in stormwater runoff from a highly constrained site. Stormwater consents are critical to the project and multi-stakeholder satisfaction is essential.

Design criteria and solutions were developed with the NZTA, Environment Canterbury (ECan), Christchurch City Council (CCC), Selwyn District Council (SDC) and Christchurch International Airport Limited (CIAL).

The constraints include:

• A highway corridor sited over unconfined drinking water aquifers
• Surface water courses that cannot accept more flow
• Only 3.5m width available at each boundary for stormwater systems
• This Road of National Significance (RoNS) must stay in service during extreme events
• Ponding of surface water would raise airport bird strike concerns
• SDC water races must cross the highway
• CCC occasionally discharge large volumes from their water supply network to the corridor
• Traffic loads must be carried across full width
• Traffic safety dictates surface cross sections
• Depths to permeable soils range from under 2m to over 5m.