Dannevirke Impounded Water Supply
In July 2021, leakage was discovered in our impounded supply. An extraordinary Council meeting was held in October 2023 to discuss the next steps for the impounded supply, which included funding for raw-water treatment and extra storage for both raw and treated water. Council also decided to delay the decision whether to repair the impounded supply due to outstanding geotechnical investigations, supply challenges, and issues relating to the impounded supply liner system.
In September 2024, a report to Council included the following:
- Tonkin and Taylor geotechnical report and impact classification assessment
- Damwatch geotechnical interpretation report
- New Plymouth Underwater inspection – this showed 3 depressions in the liner that required further attention
- Rationale – assessment of options
These reports are available below under the 25 September 2024 update.
Since September 2024, several steps have been taken to improve the Dannevirke Impounded Water Supply. It was advised that the impounded supply is functioning as it was designed, and the focus has been on building resilience. Peer review was sought, and it was agreed that this was the best approach as opposed to replacement. It was noted that while there are risks, there are no immediate risks and monitoring is continuous.
Divers have patched 3 pinholes in the liner. At this stage, no additional holes have been identified. Water leakage of the Impounded Supply is currently the least it has been since monitoring has started.
A potential plot of land has been identified for the construction of an additional reservoir for treated water. Once this is installed and filled, we can undertake any necessary repairs to the current reservoir, and it will also serve as a buffer when needed.
Samples have been taken of the Tamaki River during turbid conditions (which occurs after heavy rainfall) to investigate the NTU (Nephelometric Turbidity Units) of the water. Currently, our water treatment plant can only treat water with an NTU level of 10 or lower. When the river is in a turbid state, we cannot take and treat the water, which means we have to rely on our reservoir and impounded supply. The results of this testing will inform the decision on whether a clarifier or membrane plant upgrade is required to treat water with an NTU level above 10.
Due to the original design of the impounded supply, we are currently unable to use the lower 6 metres of water without pumping, and a temporary pump is currently on site. To address this, a permanent solution with large submersible pumps is being designed to enable us to draw the remaining water so that it can be treated and used by the town.
We will continue to monitor the situation of the impounded supply, including conducting 6 monthly underwater drone inspections, to ensure the depressions and patched pinholes in the liner do not deteriorate.
Testing of turbid water from the Tamaki River took place in March 2025.
New Plymouth Underwater Report - February 2025
A copy of the Dive Works report completed by New Plymouth Underwater in February 2025 can be found here:
Council Meeting - Dannevirke Impounded Water Supply - 25 September 2024
The report to the Council meeting of 25 September 2024 includes reference to the following documents:
- Tonkin and Taylor - Stage 2 Geotechnical Interpretation Report and Tonkin and Taylor - Dannevirke Raw Water Reservoir Potential Impact Classification Assessment
- Damwatch - Peer Review Stage 2 Geotechnical Interpretation Report
- Rationale - Dannevirke Impounded Water Supply Assessment of Options
- New Plymouth Underwater - Remote Operated Vehicle Inspection 9 September 2024
The agenda for the 25 September 2024 Council meeting can be viewed here:
Agendas, Minutes & Meeting Videos | Tararua District Council (tararuadc.govt.nz)
Mayors Update - Dannevirke Impounded Water Supply - 8 July 2024
The peer review of the Tonkin & Taylor Geotechnical Interpretation Report is due for completion by early August which will provide us with the necessary advice of our four critical considerations: The stability of the eastern dam embankment and the western reservoir rim, the degree of urgency of remediating the existing liner and underlying subsoil network, along with understanding the potential for internal erosion of the eastern dam embankment, including the dam foundation and around the subsoil outlet pipe. Please watch the Mayors video update to find out more (below):
Public Water Meeting - Dannevirke Impounded Supply - 28 November 2023
On Tuesday, 28th of November, a public meeting was held to provide the community with an update on the Dannevirke Impounded Water Supply, and to answer your questions.
Please refer to the following documents:
Presentation 28 November 2023 - Dannevirke Impounded Supply - Public Water Meeting
Information Pack 28 November 2023 - Dannevirke Impounded Supply - Public Water Meeting
This meeting was recorded and can be viewed below.
Extraordinary Council Meeting - Dannevirke Impounded Supply - 31 October 2023
An extraordinary Council meeting was held on 31 October 2023 to discuss next steps for the Dannevirke Impounded Supply. Please watch the video of this meeting below.
Public Water Meeting - Dannevirke Impounded Supply - 27 June 2023
On Tuesday, 27 June 2023 a public water meeting was held in Dannevirke. The purpose of the meeting was to update community members about the recent temporary repairs and to discuss the critical decisions that have to be made with regards to a permanent fix for the impounded water supply.
The meeting has been recorded and is available below:
In July of 2021, leakage was discovered in our impounded supply. Additionally, our water treatment plant is unable to treat water with an NTU (Nephelometric Turbidity Units) level of above 10. This happens when our river is turbid which occurs after heavy rainfall. Due to the original design of the impounded supply, we are also unable to use the lower 6 metres of water.
The following steps have been taken to improve our situation:
- Council passed resolutions that included funding of $3.2 million for raw water pre-treatment, $400,000 to purchase a raw water storage tank, and a further $2.5 million for an additional treated water storage tank
- Geotechnical investigations into the impounded supply
- Decision to not empty the impounded supply to repair the holes but do underwater assessments and repairs. It was advised that the impounded supply is functioning as it was designed, and the focus has been on building resilience. Peer review was sought, and it was agreed that this was the best approach as opposed to replacement. It was noted that while there are risks, there are no immediate risks and monitoring is continuous.
- Underwater assessment on depressions and holes in the liner. 3 pinholes have been patched and there are currently no additional holes that we are aware of
- Testing of turbid water from the Tamaki River to inform the decision on whether a clarifier or membrane plant upgrade is required to treat water with an NTU level above 10
- Continue monitoring the outflow of the impounded supply
- 6 monthly underwater drone assessments of the liner and as required
- Investigation into the construction of an additional reservoir for treated water. A potential plot of land has been identified and our project team are working towards next steps for this
- Cost analysis of the options of membrane vs clarifier solution for the pre-treatment of raw water have to be finalised before we make any final decisions to approach the market. Following the river testing, design specifications are being drafted on a clarifier solution.
- A submersible pump is being designed and installed to enable the treatment of the lower 6 metres of water in the impounded supply
Essentially, we need to ensure a continuity of supply to the town. In order for us to do this we need to get in place the appropriate resiliency measures to support this. It was advised that the impounded supply is functioning as it was designed, and the focus has been on building resilience. Peer review was sought, and it was agreed that this was the best approach as opposed to replacement. It was noted that while there are risks, there are no immediate risks and monitoring is continuous. Our main effort is to establish an option that treats directly off the river flow no matter the conditions of the river – see information above under ‘what has been done to remedy this’ and ‘what are the next steps’.
Our teams are monitoring the impounded supply to watch out for any further deterioration. We now have approximately 18 months of daily data to support decision making going forward. We have reduced our physical inspections to three times a week, and as a procedure, if there are any oddities that stand out we increase our monitoring to daily. We also carry out six monthly underwater drone inspections, with extra inspections as required, and should it be needed, we will get in expert advice for technical review. Divers have patched 3 pinholes in the liner. At this stage, no additional holes have been identified. Water leakage of the Impounded Supply is currently the least it has been since monitoring has started.
Yes, the water continues to be safe to drink. Samples are taken every week from different locations across Dannevirke to be tested. Installing raw water pre-treatment will provide greater resilience during extreme weather conditions.
The likelihood of long dry summers is one of the considerations leading to the recommendation not to dewater the Impounded Water Supply as was initially planned. This means that next summer, we can expect to have the stored water in the dam as a contingency backup. Work is also underway to investigate potential underground water sources. We will update the community in the near future about this.
Alternative investigations are still underway, bore sites need to be assessed for their feasibility.
The best way to prepare for any potential water shortages is to have a plan, harvest rainwater and think about water conservation in our daily lives. Council will also be providing regular updates and information on how to conserve water.
Overview of important documents
Overview of News articles about the Dannevirke Water Supply
Infographic about the temporary repairs carried out between 13-19 June 2023
Infographic which provides an overview of the Dannevirke Impounded Water Supply
This infographic provides an overview of Dannevirke's Water Supply system; from source to tap
A full page spread on the Dannevirke Water Supply appeared in the Bush Telegraph Newspaper on 20 June 2022, including this infographic. This page can be downloaded here.