Expert reports have validated Council’s October 2023 decision to enhance the resilience of the Dannevirke water supply scheme, and to delay the decision to start the physical works on the Dannevirke Impounded Supply. At the time, Council decided to delay remediation of the Dam due to outstanding information and other uncertainties regarding the dam structure and liner.

The peer reviewed expert reports were discussed at the 25 September 2024 Council meeting and will help Council to choose the best way forward. All these expert reports are available online at: https://bit.ly/4gXxUkK

The additional resilience measures for the Dannevirke water supply scheme include installing additional water treatment capability, and installing extra treated water storage. The purpose of the additional treatment is so Council can continue to treat water from the Tamaki River, even when the water becomes murky during heavy rain events. The extra treated water tank will also provide additional storage and resilience in periods of high water turbidity and low river flows. As decided last October by Council, the resilience measures have been budgeted in the 2024-2034 Long Term Plan and will be funded through a loan that will be spread out over 20 years, and are expected to cost $6.1 million.

The resilience work of installing additional treatment and extra water storage is expected to be conducted in this financial year, once these are in place and functioning, engineers will be able to dewater the impounded supply, remove the liner and once and for all determine the extent of work required to fix the dam, at that point public consultation will be conducted with the community to determine the future of the impounded supply. It is predicted that the consultation options will be; to repair the impounded supply; or to decommission the impounded supply; or to continue enhanced monitoring and surveillance.

The expert reports have outlined the scale of the physical works needed to mitigate dam safety risks based on monitoring data, geotechnical interpretation and analysis, concept designs and preliminary cost estimates. The reports reiterate that it will not be until the cover and liner of the dam are removed before the full cost and scale of repair work can be identified. This can only be done once the resilience measures are in place. This approach ensures that the Dannevirke community remains protected from a potential loss or reduction in water supply.

The reports have also stated that since the emergency repairs to holes in the liner in June 2023, there has been no significant deterioration in the dam floor or batter slopes. However, according to Tonkin and Taylor dam experts: “The longer the repair of the liner, subgrade, and subsoil network is deferred, the greater the risk that an emergency could arise that could affect water supply to Dannevirke. Therefore, we recommend preparing to remove the risk as soon as practicable” – which will be when Council has the resilience measures in place for the water supply to the Dannevirke community.

In line with the recommendations of the expert reports, Council continues to closely monitor the dam and manage the impounded supply.