Dannevirke's dam still has leaks but where and why is still not known. Photo / Leanne Warr
Dannevirke's dam still has leaks but where and why is still not known. Photo / Leanne Warr
Tararua District Council will be using submersible drones to look for and assess a leak in the Dannevirke dam to help them figure out what to do next.
The remotely operated underwater vehicle, or submersible drone will be used to help engineers see what is happening in the dam. Photo / Supplied
Group manager for infrastructure Chris Chapman said the drones, otherwise known as remotely operated underwater vehicles, would be operated by someoneoutside the dam.
He said the drones would take videos which would help them locate the current leak.
Using a drone instead of divers was deemed the optimal option, mitigating any health and safety risk as the dam was fully enclosed and pitch black.
Council chief executive Bryan Nicholson said they needed to know how to fix the leaks.
"We don't know whether or not it's going to get worse. It could be like this for 10 years or not get any worse, but what we need to know is the plan to fix it and when to fix it and how to fix it. And if we can say actually it's caused by this.
"It could be quite simple. We just don't know that yet."
Chief Executive Bryan Nicholson - the fix for the dam could be something simple, but they don't know that without assessing the leak. Photo / Leanne Warr
The dam was filled at close to 10 metres, the highest it had been for about 18 months, Nicholson said.
"Most of the water coming out of the dam is getting put back in, so at this time the dam's running as it should be. "
He said fixing the leaks would not necessarily mean Dannevirke was never going to get water restrictions again.
Group manager of infrastructure Chris Chapman said he didn't think anyone could guarantee there would be no water restrictions.
"It just needs to go down. Power failure, a mains failure or something at the reservoir or water treatment plant – that's anywhere in the country.
"There's only a certain amount of resilience in the supply network."
For instance, the Horowhenua district's water treatment plant really struggled when it rained, he said.
"On the flip side, to put enough resilience that essentially the risk of a water restriction at any given point in time is zero, that costs way more than the ratepayers would ever sign off on."
However, there was far more to the issue than just repairing the leak.
"There's no silver bullet," Chapman said. "[We need to] make sure our existing infrastructure is performing well and there's resilience in it and it's not about to fail."
What caused the initial leak was unknown, but the ground shrinkage associated with the 2019-20 drought was thought to be a possible contributor.
"Until we identify the remaining leak(s), we will not be able to assess the possible causes," Chapman said.
Chris Chapman: The remaining leak(s) needed to be identified before they could begin to assess the cause. Photo / Leanne Warr
Another option the council was looking at was finding alternate water sources.
Chapman said onsite aquifer mapping had been done and the results were being assessed, with a report due soon.
Tararua district mayor Tracey Collis believes there is a need to find alternate water sources. Photo / NZME
Collis said some people had said there was no need for another water source and that the rivers supplied plenty.
"Actually, we're seeing that they don't."
Another issue was the fact that in the last few months the region had seen intense periods of rain followed by dry spells.
There were huge amounts of rainfall in February and March but much less in April and May thus far.
Collis said it appeared that the district was getting the same amount of rainfall but at different times.
"Is that a climate change, is that a seasonal change, is that a La Nina?"
Nicholson said rainfall seemed to happening in shorter periods and all at once.
"Which is no good to us. That's why I think it's important for us to start exploring those other alternate water sources as well."
