Most Likely Cause

The unexpected effects of the 2020 drought is the best explanation for the leaks found at the Dannevirke impounded water supply.  The one in fifty plus year drought caused the ground to dry and contract which, when paired with the lowering reservoir level, exposed the plastic liner on the reservoir causing it to vibrate and wear until it finally failed from fatigue.  The splits found in the plastic liner allowed for water to start removing the protective clay layer below, exposing the liner to underlying stones which then increased the damage. Once the reservoir began refilling, only then did the leak become obvious, and water moving over the liner was able to be detected.

We still cannot say with 100% confidence the precise sequence of events leading to the major leak discovered to date, however we are taking the opportunity to assess various possibilities and prepare mitigation strategies to prevent a repeat situation.

Work Done to Repair this Issue

“With over 22,000 square metres of surface area, there has been a lot of area to assess. Initially it was like looking for a needle in a haystack.” said Council’s Group Manager – Infrastructure Chris Chapman.

Given that leaks of reservoirs with liners is rare, experts have had to come in from across the Country to investigate the source and points of the leak. We can now confidently say that our team has inspected all seams and points of suspicion.

Council also brought in an underwater drone on 2 separate occasions to assess the liner in the reservoir. With the first drone focusing on the bottom of the reservoir and assessing the “plug hole”, the point at the bottom which the water outlet is taken and fed to the treatment plant. There was no visible leak from this point. The second drone focused on the walls and liner, when the drone pointed up, we saw a tear in the cover and some small holes. We have now been able to expose this area for later repair as the weather again closed in for another fortnight.

Up to four tonnes of protective lime has been replaced and compacted below the liner covering the hillside metal that was exposed during the event.  In addition, an extra thick layer of geomembrane material has also been installed to mitigate this from reoccurring. In recent years, this extra layer has been considered best practice and was installed in the new Woodville water reservoir.

Next Steps

Since July, progress has been continually stalled due to poor weather. However, the impounded supply is now being filled and will be monitored closely for any further leakage. Improvements to monitoring devices on the cover (measuring the water depth) and at the drain discharge (measuring the flow of subsoil drain and possible leakage) are being completed.

We would like to thank everyone for their understanding and patience during this unprecedented event. We are still exploring ways to reduce pressure on our water supply without increasing restrictions for urban users. We are unsure what this looks like yet but will provide you with an update when we can.