Submersible drones set to work on the dam problem
Time for a deep dive into Tararua district’s leaking water supply
Tararua District Council will use submersible drones to look for and assess a leak in the Dannevirke dam to help them figure out what to do next. Group manager for infrastructure Chris Chapman said the drones, otherwise known as remotely operated underwater vehicles, would be operated by someone outside the dam.
He said the drones would take videos to 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.
Engineers would go in early this week .
Mayor Tracey Collis said there had been multiple failures at the dam.
“You can’t develop a plan without knowing what’s failed and what’s the cause.”
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.”
The dam was close to 10m full, 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,” he said.
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.”
The cause of 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.
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.
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,” he said.