NZ’s first in-river labyrinth weir

The labyrinth weir is a multi-generational asset that significantly increases resilience in flood management.

Since becoming operational, the Rakaia River intake has faced several flood events which have impacted our ability to deliver irrigation water. During these events, the fuse plug (a gravel embankment) is engineered to blow and release floodwater back into the river to protect the irrigation scheme’s infrastructure. However, this process has historically led to some extended disruptions in water delivery, as we are required to wait for floodwaters to recede before we can safely reinstate the fuse plug.

The first in-river labyrinth weir of its kind in New Zealand was constructed during the off-season of 2023/2024. The goal of the multi-generational labyrinth weir is to increase the resilience of the irrigation scheme by reducing the frequency and duration of flood events impacting operations. The labyrinth weir will increase the scheme’s ability to withstand floods from approximately 1200 cumecs up to 3000 cumecs in a controlled manner and return the water safely back into the river.

Labyrinth weirs are complex hydraulic structures widely used by water schemes around the world to handle high-flow events. The $1.7 million engineering project faced tight timelines from design to completion, with commissioning required before the start of the irrigation season in the first week of September 2024.

A key factor in the success of this project was the seamless collaboration between multidisciplinary teams. Designers, contractors and the CPWL team worked closely to ensure the project’s innovation potential, sustainability and constructability were fully optimised. By leveraging the significant experience and capability of our CPWL team in managing large construction projects, we ensured that all procurement, project risks, quality and construction issues were managed successfully, resulting in the project being delivered safely and ahead of schedule.

Engineers meticulously designed the labyrinth weir to meet the site’s specific requirements, determining the optimal size, shape and number of labyrinth cycles needed to effectively manage water flow. Continuous communication and knowledge-sharing among the teams facilitated problem-solving and adaptive strategies, crucial for overcoming challenges. Monitoring weather conditions and flood risk during construction was essential, as any flood could disrupt the site. Pre-casting 80% of the structural elements offsite minimised the risk and achieved higher quality control, allowing the concrete components to cure under ideal conditions. This strategy not only reduced on- site construction time but also reduced environmental impact.

Labyrinth weirs are complex hydraulic structures widely used by water schemes around the world to handle high-flow events. The $1.7 million engineering project faced tight timelines from design to completion, with commissioning required before the start of the irrigation season in the first week of September 2024.

A key factor in the success of this project was the seamless collaboration between multidisciplinary teams. Designers, contractors and the CPWL team worked closely to ensure the project’s innovation potential, sustainability and constructability were fully optimised. By leveraging the significant experience and capability of our CPWL team in managing large construction projects, we ensured that all procurement, project risks, quality and construction issues were managed successfully, resulting in the project being delivered safely and ahead of schedule.

Engineers meticulously designed the labyrinth weir to meet the site’s specific requirements, determining the optimal size, shape and number of labyrinth cycles needed to effectively manage water flow. Continuous communication and knowledge-sharing among the teams facilitated problem-solving and adaptive strategies, crucial for overcoming challenges. Monitoring weather conditions and flood risk during construction was essential, as any flood could disrupt the site. Pre-casting 80% of the structural elements offsite minimised the risk and achieved higher quality control, allowing the concrete components to cure under ideal conditions. This strategy not only reduced on- site construction time but also reduced environmental impact.

Sustainability was at the heart of construction, with environmental planning and consultation a core tenet of the project. Sustainable infrastructure provides low-carbon productivity gains by working to provide critical water, enabling the production of food and fibre. Through the integration of sustainable design, the labyrinth weir will contribute to achieving the United Nations Sustainable Development Goals, delivering long-term benefits to society and the environment.

Project Manager
Hamish Tait Projects Ltd.

Design Engineers
Riley Consultants Ltd. – www.riley.co.nz

Contractor
Taylors Contracting Co Ltd. – www.taylorconstruction.co.nz

Subcontractor, concrete prefabrication
Smith Crane & Construction Ltd. – www.smithcranes.co.nz