Dannevirke Impounded Water Supply – Leaks Found; Temporary Repair Mission Being Prepared

16 May 2023

During examinations with Remotely Operated Underwater Vehicles (ROV) two tears have been discovered in Dannevirke’s Impounded Water Supply. The two tears are near the base of the western dam embankment.

The examinations were conducted by New Plymouth Underwater, who have been commissioned by Council to complete further inspections with the objective of locating possible leakage points and identify possible further deterioration.

Planning for the temporary repairs started immediately and is expected to be finalised on 19 May. It will be a joint operation between Council, New Plymouth Underwater, and other partners.

Following the finalisation of this plan, New Plymouth Underwater will develop a specialised plan for divers to conduct the temporary repairs. The divers will do the repairs underneath the reservoir’s cover in complete darkness.

During the temporary repairs water will be drawn directly from the Tamaki River and not the impounded water supply. The river water will undergo the same treatment process to provide safe drinking water. Once the repairs are completed, Council will revert back to drawing from the impounded water supply. During the repair works Council does not anticipate water restrictions. The repair works are currently set to be conducted the first week of June.

This repair work is expected to reduce the amount of leakage but Tararua District Council is not expecting it to stop the leakage entirely. These are temporary repairs to reduce the risk of further damage, and of disruption to Dannevirke’s supply of water. Permanent repairs to the liner and other aspects of the impounded supply are still required.

A workshop with key stakeholders regarding options to permanently repair the impounded supply is being held mid-May. This will inform a report that will be presented to a full Council meeting before the end of June 2023. Once Council have identified the best approach on how to proceed with full repairs, Council will engage further with the Dannevirke community.

Council will be providing regular updates on the repair progress on social media, the Bush Telegraph and our website www.tararuadc.govt.nz.

Here are some Frequently Asked Questions (FAQs) on this topic:

1. Why weren’t they found before?

We don’t know.  An underwater inspection was conducted in August 2021 but no leaks were located after an extensive search over one week.

2. How money have we spent on the dam repairs so far?

We haven’t spent any money on dam repairs to date.  The money that has been spent so far has been to quantify the extent of the damage, this has proven very difficult with a permanent cover over the dam and to identify and select the best way to fix the dam.

3. Have the repairs been factored into the latest rates increase?

An additional $5M has been set aside in financial year 23/24 but we don’t yet know exactly how much the repairs will cost.

4. What are the costs of the repairs?

We don’t know yet – this is something that is being worked on right now.

5. Are there any more leaks?

Not that were found and not that we know of but this wont become clear until the two leaks that have been identified have been temporarily repaired and seepage rates from the dam have stabilised.

6. How were the leaks found?

The leak was found by an Underwater Remotely Operated vehicle.

7. When were the leaks found?

The leaks were located over the period 5 – 7 May 23.

8. How close to the side of the dam are the leaks?

The leaks are located next to each other at the base of the western embankment of the dam.

9. How close to the dam wall are the leaks?.

We don’t have any dam walls they are sloping embankments.

10.          Have all the leaks been fixed?.

No, not yet.

11.          When will the leaks be fixed?

The leaks are scheduled to be fixed over the week starting the 29 May 23.

12.          Who will fix the leaks?

A company called New Plymouth Underwater.  This is the company that located the leaks, and they also have a specialist dive team that is currently preparing to repair the leaks.

13.          How will the leaks be fixed?

A number of dam and material specialists are working on the repair options.  The concept is that a filter cloth will be placed over the soil and gravel underneath the liner, the hole will then be filled with either gravel or grout and a reinforced PVC cover will be sealed onto the existing liner.

14.          How long will it take to fix the leaks?

We have allowed one week but if it all goes to plan, we expect it will take two days.

15.          Is there a risk of dam collapse or breach?

Not at this stage but we are monitoring it on a daily basis.

16.          What options are council looking at with regard to the future of Dannevirke’s water supply?

On the 18 May 23 an Optioneering Workshop will be conducted in Dannevirke with specialists presenting to key stakeholders.  A workshop will then be prepared and presented to Council on the 31 May outliner the various repair options and recommending a way forward.

17.          Why is council spending money on repairing the dam of three waters is coming in to play.”

The 3 Waters entities now do not come into effect until 2026.  Until this time TDC remains responsible for the dam and from a safety and water security perspective it needs to be repaired as soon as possible.

18.          “I thought the dam had already been repaired over the summer?

No, unfortunately the Dam has not been repaired yet.  Work continues to be completed to investigate exactly what is wrong with the dam and the best approach to completing the repairs.