ARAPUNI DAM REMEDIATION WORK
The Arapuni Hydro Dam
The Arapuni Hydro Power Station is one of nine hydro stations operated by Mercury along the Waikato River that forms an essential part of the Waikato Hydro System. This system provides around 10% of Aotearoa New Zealand’s renewable electricity to the grid.
The Arapuni station began operating in 1929 and is now protected under the Historic Places Trust. Its eight turbines can generate 197 MW, making it the second biggest electricity generator of the nine Mercury hydro stations. Maraetai has the highest generation, at 360MW.
Project overview
Mercury has a dam safety assurance programme and policy, this is upheld through regular investigation by technical experts, and maintenance activities.
A recent engineering study showed the Arapuni Hydro Dam is safe and fit for everyday use, but work needs to be done to improve the left abutment seepage cutoff of the dam, to protect it from the risk of water seepage and internal erosion.
An abutment is a concrete wall that provides structural support for the dam. A seepage cutoff wall is a concrete barrier that prevents seepage.
Seepage happens when very small amounts of water soak around the dam abutment. A controlled amount is expected and needs to be managed, but over time too much seepage will result in increased pressures and an increased risk of internal erosion. This has potential to affect the long-term performance of the dam.
We want to reduce the risk of further seepage occurring and ensure that the dam continues to be around for another century. To do this we are assessing options to see what remediation work is required. We need to complete further geotechnical investigations and technical assessment to inform the remediation work design and construction methodology.
We’ve begun discussions with local iwi, Raukawa and Ngāti Korokī Kahukura, South Waikato District Council, Waikato Regional Council and Waipā District Council to better understand the impacts of the remediation work, and we’ll keep the community in the loop as plans are formed.
Project background
The Arapuni Dam is the oldest Mercury dam (in operation). It was built between 1924 and 1929 and was the first government-built high dam on the Waikato River.
The dam has had remediation treatment for leakage, dating back to its construction. Following the first lake fill in 1930, significant seepage of water traveling through the left abutment from the headrace to the dam face was observed. The lake was lowered, and seepage controls were retrofitted.
The Arapuni Hydro Power Station was constructed between 1924 and 1929.
Over the life of the dam there have been several improvements made to the seepage controls, with the most recent in mid-2005, where seepage cut-off/concrete walls were constructed beneath the dam.
During the mid-2000s, work was done to improve the foundation of the Arapuni dam.
Throughout this project, our teams engaged local people and the groups we work with to inform them of the work, and how this may impact them. Ongoing monitoring indicates that the remediation work was extremely effective. However, seepage changes were identified in the left abutment following two distant earthquakes in 2016.
Mercury is now investigating options for a modern equivalent to manage the seepage.
Timeline and next steps
We’re assessing options to see what remediation work is required. As we start to develop plans, we will continue to engage with partners, stakeholders, and community to let them know what is planned.
We expect the business case to be submitted for approval in late 2024. This will determine the timeline for design and how we progress to the construction phase.
Once the business case is approved, design and construction are estimated to take 2-3 years.
Download map here.
Project milestones:
We began our ongoing engagement with partners, stakeholders and the wider community in mid-2024.
We’ve been working closely with local iwi, Raukawa and Ngāti Korokī Kahukura to better understand the impacts of the remediation work.
We met with Waikato Regional Council, South Waikato District Council and Waipā District Council staff and elected members to share information about the project, in September and October 2024.
We held a drop-in session at Rhubarb Cafe, Arapuni, to speak one-on-one with local people so they could learn more about the project, on 5 October 2024.
Some of our staff and contractors attended a cultural induction hui led by kaumātua at Pōhara Marae, on 16 October 2024.
The induction cleared the way for us to begin work on removing vegetation from the left abutment of the dam, from October-November 2024.
Clearing the vegetation allows us to undertake geomapping of the abutment so we can plan for relief drilling work in early 2025.
From mid-January to around August 2025, we will be working on relief drainage at the bottom of the dam. We will also be working at the top of the dam, using a rig to relocate and introduce more subsurface instruments. This will help us monitor what’s happening underneath the left abutment.
From approximately April to June 2025, we will be working on a jet grouting trial. The results of all of this work will help inform the main programme of work which we’re aiming to start in 2026.
In 2025, we may need to use temporary traffic management on Arapuni Road to move equipment, but the road will remain open throughout this stage of the project.
In 2026, Arapuni Road across the dam will be closed to allow us to complete the main work on the abutment. We will use a one-lane detour route, managed by traffic lights, along Power Station Road which reconnects to Arapuni Road, north of the village. More details for the closure timing and durantion will be provided once understood.
For safety, we will have to close the western end of the swing bridge which connects to Power Station Road, where people walking and cycling would normally join the road.
We are still developing options for the remediation work; we will complete technical assessments to better understand any impact on the local community. As we start to develop plans for the works, we will engage with the community to see how we can minimise disruptions.
While there will be no change to the electricity generation, doing the required works now ensures that future generations can continue to benefit from this integral hydro asset. The power station will continue to operate as normally during the work.
We want to better understand what’s important to local people and the groups we work with. As we start to develop plans for the remediation work, we will continue to engage with stakeholders and local community to let them know what is planned. If you would like to contact the project team, please email: project.arapuni@mercury.co.nz.
Our contractors, Avalon, began work on clearing vegetation from the left abutment in October 2024 and completed the work in November 2024. We needed to clear the vegetation so we could accurately geo-map the abutment to determine the best plan for relief drilling work, which we want to start in early 2025.