Dannevirke’s impounded water supply remediation planning

Planning for how to remediate Dannevirke’s impounded water supply has been extremely complex as the business world has become risk averse. We have experienced reluctance by companies to sign off without further reports and more information. We absolutely must get this right and are committed to that, however it reaches a point where a decision needs to be made and all involved have been made aware of this.

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.

This information will then be peer reviewed with the final report due at the end of August. The report will outline the likely scale and cost of the physical works to mitigate and dam safety risks based on monitoring data, geotechnical interpretation and analysis, concept designs and preliminary cost estimates.

Independently, Rationale Ltd have been commissioned to complete an independent options assessment of the cost, benefit and need of the Dannevirke Impounded Water Supply. The intention is that both the Tonkin & Taylor and Rationale reports with recommendations on remediation work will be presented to Council in August. Significant costs may require an amendment to our Long Term Plan.

These reports will be made public, and our intention is to hold a public meeting to share and discuss this information along with an opportunity to hear directly from the experts involved before the Council decision is made. Having the opportunity to hear local knowledge will assist the experts and council alike.

Key findings in my opinion are the dam originally was built with a low-risk classification, however given the volume of water held and a greater understanding of the earthquake and hazard profile, it should have had a higher risk classification. There is also now a much greater focus on dam hazards in New Zealand and this has been reflected in the new dam safety legislation of May 2024. Any repairs which take place will take this into account.

However, standards and design did change and the Woodville reservoir, which was completed a few years later than Dannevirke, was designed differently and did not suffer the same consequence/damage as the Dannevirke dam did following the 2020/21 drought.

All information will be placed on the website.

What we do differently to prevent anything like this happening again is important and every project is run through our project management team, follows a procurement policy and all aspects are reported transparently through council along with the information being made publicly available.