Experts engaged following ongoing seepage at Dannevirke water supply reservoir

Following the repair of a leak from Dannevirke’s water supply reservoir in 2021, Tararua District Council (TDC) has been closely monitoring seepage from the reservoir. The monitoring programme indicates that the rate of seepage remains irregular and abnormal despite the repairs completed to date.

As such, Tonkin + Taylor (T+T) continue to be engaged as external experts in water storage dams to support TDC staff with further investigation and planning.

The external experts completed an initial site inspection in June 2022 and are undertaking further review and analysis, however, they have made preliminary recommendations, which TDC has started implementing.

“First of all, I would like to reassure Dannevirke residents that their water supply currently remains stable and is, of course, safe to drink. However, we do want to be transparent and make our community aware of our challenges and any associated risks. With dam safety, it is necessary to err on the side of caution when there is uncertainty,” says Tararua District Council Chief Executive, Bryan Nicholson.

The preliminary recommendations comprise:

  1. Operating the reservoir at lower water levels
  2. Undertaking targeted monitoring
  3. Developing a contingency plan
  4. Preparing for repairs.

Further detail on each recommendation is provided below.

Operating the reservoir at lower water levels
Keeping the reservoir at lower water levels reduces the leakage risk while investigation is ongoing. The reservoir is now approximately one-third full and TDC staff are reviewing historic trends to confirm the optimal water level to balance risks for secure water supply I.E., keeping water levels as low as possible to minimise leakage while still storing adequate water to supply demand.

Undertaking targeted monitoring
Onsite monitoring has been increased for two purposes. Firstly, the monitoring will provide information to inform the investigation and better define the leakage risk. Secondly, the monitoring is intended to provide early detection of onsite changes that may indicate an escalation of the leakage risk to a more serious situation. Council staff are also investigating options for automatic monitoring and alerts.

Developing a contingency plan
As a precautionary approach, TDC is developing a contingency plan in case the situation deteriorates. As previously stated, with dam safety, it is necessary to err on the side of caution when there is uncertainty. Development of the plan involves defining the situations and conditions under which to activate the plan, identifying responsibilities and actions to mitigate risk if the situation worsens, and having contact details, communication systems, equipment and plant at the ready.

Preparing for repairs

Repairs will be necessary for ongoing long-term operation of the reservoir. It is increasingly likely that this will require an extensive programme of works, as well as improvements to the current system to minimise disruption to water supply while repairs are being completed.  Planning continues for this.

TDC will keep the community and any involved stakeholders updated on any developments. If you have questions, please contact us on info@tararuadc.govt.nz or 06 374 4080.