Hayward Interchange - Innovative Drainage Design to Overcome Site Challenges

The SH2/SH58 Haywards Interchange Project, commissioned by NZTA, is part of the wider Hutt Corridor project for improving safety, reliability and efficiency for SH2 and SH58 users. The project has created a new grade-separated interchange for SH2 and SH58.

Dry Creek is a tributary of the Hutt River with a catchment area of approximately 92 hectares. The creek passes through a series of culverts beneath the existing SH2 and SH58, local roads, and railway line.

The revised road layout required modifications to the Dry Creek system including extension of the culvert beneath the realigned SH58 and new culverts beneath ramps for SH2. The scope of the project did not include for upgrading of the downstream network. This, and other restrictions at the site of the interchange, constrain the capacity of the Dry Creek system at the site of the interchange.

This paper explores the challenges of designing sections of the Dry Creek drainage within tight physical and hydraulic constraints, as well as providing a fish passage and ecological improvements. The challenges presented an opportunity for innovative design solutions. Features of the innovative design solution include:

• Designing for current downstream constraints as well as for future upgrade scenarios
• Providing fish baffles within new culverts and accounting for the impact of the baffles on pipe capacity. Integrating fish resting areas and adapting energy dissipation solutions for fish passage
• Providing in-pipe energy dissipation because of limited space available for conventional dissipation measures, and
• Managing hydraulic constraints within low headloss bellmouth type outlets in manholes and provision of inline flow retention to ease the flow into the downstream stormwater system.

Constructability and operational health and safety were key factors in the design and implementation of the project.