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.
Update: 5 September 2022
Last week there were a couple of articles in the media about the Dannevirke impounded supply, our untreated water reservoir. I would like to take this opportunity to give a short overview of where we stand at the moment. Following some misinformation which has been circling around in the community, 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.
Due to repairs and a variety of other measures, we have far more water stored in the reservoir now than compared to last year. However, depending on the scope of future repairs to the reservoir, as well as possible droughts or heavy rains, and what we are allowed to take from the river, it is likely some level of water restrictions will be required for Dannevirke over summer months, although we are doing what we can to mitigate the level.
We have contracted national experts in dam engineering, Tonkin + Taylor, to help us with the reservoir. It is clear that a leak (or leaks) in the reservoir liner remains, however inspections have not identified their exact location yet. This means it is possible, although unlikely, that the leak has affected the strength of the dam wall. The likelihood of the dam collapsing is very small. We are monitoring the dam wall, to manage and mitigate risks associated with the leakage and potential for this to negatively impact the
dam’s structural integrity.
We now know there is no quick and easy fix to this. Ideally, the initial inspections and repairs completed would have resolved the issue – unfortunately it hasn’t been that straightforward. We currently expect to have to remove and replace the reservoir’s liner (or at least significant sections of it) as part of a permanent repair. This could require taking the reservoir offline for several months. This needs to be managed carefully, so that a reliable supply of safe water can be provided to Dannevirke while the work is completed. Work has already been completed to investigate potential underground water sources (i.e. aquifers) and more work is underway.
Maintaining water supply is a key consideration when planning for the permanent works. When the Tamaki River is in low flows, Council’s consented water take is less than Dannevirke’s consumption. In which case, water stored in the reservoir is used to supplement water taken from the river. This means that if we bypass the reservoir while it is being repaired, and the Tamaki river is in low flow, increased water restrictions will be required. That is, if no alternative solutions are implemented.
Water quality is another key consideration, especially during periods of heavy rain. Rain can stir up a lot of mud, silt and sediments in rivers. Our water treatment plant can manage turbid (murky) water up to a certain degree, but not if it gets too much. So if the reservoir is offline for repairs, and we have to take water straight from the river during heavy rains, regular boil water notices would be required. Investigation and planning of additional water treatment processes, able to appropriately treat turbid
water, is underway.
Specifically for Dannevirke, various other water projects and initiatives are underway and recently completed.
We have installed bulk flow meters to better understand where treated water is going and how it is used. This will improve Council’s management of water, especially for historic connections on the extraordinary supply mains (those outside the urban boundaries), that are not recorded or metered. We have also been conducting condition assessments of our water pipes. Repairs and replacements will require a careful balance between affordability, community expectations, health and safety and technical feasibility.
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:
- Operating the reservoir at lower water levels
- Undertaking targeted monitoring
- Developing a contingency plan
- 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 firstname.lastname@example.org or 06 374 4080.