Dannevirke Impounded Water Supply

Summary

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 patched three small pinholes in the liner. No new issues have been found and the impound has stabilised. On-going daily monitoring is required and we have plans and procedures in place should anything change.

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.

Project history

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.