Implementing the New Zealand Fish Passage Guidelines on a road safety improvement project containing over 50 existing culverts has highlighted additional barriers to fish passage, beyond the physical barriers outlined in the Guidelines. In a challenging landscape, balancing the ecological, engineering and budget requirements appeared at times more onerous than the difficulties faced by aquatic life.
A step increase in traffic loads on State Highway 58 north of Wellington is expected on completion of the Transmission Gully Motorway. A 9km section of the road is being upgraded and widened to improve road safety prior to this increase, following a significant number of serious accidents on the road in recent years. As part of this project, the stormwater infrastructure is also being upgraded to meet current design standards, including compliance with the Fish Passage Guidelines released by NIWA in 2018.
It was considered that despite the number of culverts requiring design, a generalist approach to fish passage design was not feasible on this project. Instead, a variety of measures were required to ensure an economical design. Identification of specific culverts requiring fish passage, including detection of natural and manmade barriers early on was critical to project success. This required the input from a specialist freshwater ecologist during the preliminary design phase.
This paper discusses the issues and complications associated with upgrading existing stormwater infrastructure to meet the requirements of the Fish Passage Guidelines on a constrained and challenging site, while balancing ecology, stormwater and client requirements on a project where the primary driver is road safety improvement.