SH10 Culvert Replacement is a finalist in the Excellence in Environment & Sustainability Category of the 2022 IPWEA NZ Asset Management Excellence Awards. The Excellence in Environment & Sustainability Category is sponsored by Morphum Environmental.

This project was driven by the objective to rectify wrongs done decades before to the local community when the construction of a causeway with a small, undersized culvert was constructed, blocking of access to the water for local Iwi and the community. The bridge and the associated channel have restored water flow to the environment, regenerating the wetland that existed before.

The project site is located on SH10, approximately 2km south of Mangonui in the Far North.

The goal of the project was to reinstate the waterway that had existed at the location prior to the construction of a causeway in the 1960’s in place of a bridge. The causeway stopped the flow of water in the area as the only allowance for water passage under the new causeway was a single, small culvert, this action turned what once was a free-flowing waterway into a mangrove swamp and took land away from local Iwi, subsequently blocking easy access for their people to the water.

Discussions between Waka Kotahi and local Iwi identified this as a significant concern for the locals and funding was allocated through the Provisional Growth Fund to attempt to restore the waterway to what it once was. Waka Kotahi commissioned a Cultural Impact Assessment and engaged a Hydrologist (KRJ Consultants) to assess what culvert size would be sufficient to re-establish the ecosystem.

In May 2021, WSP was approached by Waka Kotahi to complete the detailed design for the 4×2.5m box culvert in the proposed location. The short timeframes associated with the funding necessitated procuring the Physical Works through the NOC Maintenance Contract. This allowed for an ECI model to be adopted and early on in the design phase discussions with the team revealed that the proposed culvert and channel clearing was not a practical solution and that a bridge would be a more effective.

Construction of the bridge went smoothly, with collaboration between the Client, Contractor and Consultant teams facilitating easy and honest communication. Any risks encountered during construction were approached as a team, and with a solutions-focused mindset. Kaitiaki from the local Iwi were onsite throughout the construction and were very positive about the project, and especially pleased to see increased channel flow and the environmental impact that this had. 

Asset Management Principles

Optimisation of Design

The original scope for the project was for the construction of a large box culvert to reinstate the flow of water in the area to restore the wetland eco-system that was there prior to the construction of the causeway. This evolved into the construction of a bridge instead.

The design team came up with a structure that would be easily maintained, as well as one that was appropriate for the region being appropriately sized for the location whilst achieving the project goal of re-establishing water flow in the area.

Additional considerations for the design team were to; simplify future maintenance requirements, improve environmental outcomes, and allow construction to occur with minimal disruption to the local community in order to maintain the level of service expected for a State Highway.

Maintaining Service to the Community

It was critical to the Project team that the construction of the bridge cause as little disruption to the servicing of the Far North community as possible. This section of SH10 has an ADT of 3,500 vpd. To maintain traffic flow throughout the construction period, a temporary road was built on the eastern side of the project extents, sealed, line marked and maintained for the full duration. This Temporary Road was greatly appreciated by the local community and maintained the flow of traffic in both directions on State Highway 10 for the duration of the project. This was especially appreciated during periods of high traffic volumes i.e. in the lead up to Christmas 2021.

Resilience in Design

Resilience of both the asset and the area around the asset was considered in the design and construction phases. The embankments around the structure were protected through the inclusion of rock spalling along from the abutments on both the western and eastern side of the structures. The bridge was designed as an integral structure to allow for flooding over the top of the bridge and road without any damage.

The rock spalling is constructed primarily using locally won material and with a deep toe embedment in consideration for the tidal marine environment. Rock Spalling is a solution with demonstrable effectiveness in marine environments.

Preserving the Future – Moving Forward

The alignment of the channel, as well as the extent of the new excavation was determined with consideration to the remains of the historic bridge that was constructed at some time prior to the 1940s. If the channel is observed from a bird’s eye view, there is a slight kink from east to west, this was in order to avoid excavating too close to the remains of the abutments. The current alignment cuts through the historic piers, however, these have a ‘buffer’ zone around them that was left untouched during construction to maintain the integrity of the piers and preserve them in their present location for the community.