Bush Telegraph - 2021-11-22


Progress made on water supply leak


Over the past four months Tararua District Council Council and various suppliers have been investigating a leak in the Dannevirke impounded supply and providing residents with updates when available. The unexpected effects of the 2020 drought is the best explanation for the leaks found at the Dannevirke impounded water supply. The one-in-50-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. Council cannot say with 100 per cent confidence the precise sequence of events leading to the major leak discovered to date, however it is taking the opportunity to assess various possibilities and prepare mitigation strategies to prevent a repeat situation. “With over 22,000 square metres of surface area, there has been a lot of area to assess,” said council’s group manager — infrastructure Chris Chapman. “Initially it was like looking for a needle in a haystack. “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 two 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, a tear was visible in the cover and some small holes. Council has 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. 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. Council is still exploring ways to reduce pressure on the water supply without increasing restrictions for urban users.


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