A number of issues have been identified with the Laws Rd impounded supply. Photo / NZME
A number of issues have been identified with the Laws Rd impounded supply. Photo / NZME
Tararua District Council has voted unanimously to spend $1 million to plan repairs to Dannevirke's trouble-plagued dam.
Councillors were asked to approve a recommendation that would cost $1,085,000, but that was only a conservative estimate and didn't include permanent repairs, they were told at a council meeting last week.
Bryan Nicholson outlined the background to the construction of the dam and the recent issues. Photo / Leanne Warr
Chief executive Bryan Nicholson detailed some of the background that had led up to the discovery of issues in the impounded supply in Laws Rd in July last year.
Extensive investigations and repairs had been made, but a number of issues were also identified including apparent stress cracking of the membrane in multiple locations, scouring of the clay layer and subgrade resulting in unsupported liner, rocks the size of tennis balls in the subgrade and several others, he said.
However, the dam continued to leak and a possible risk to the dam wall was identified.
The council engaged the services of Tonkin & Taylor, a recognised expert on dams, to complete an on-site assessment and prepare a report.
"Their report focused on dam safety, which in the dam engineering industry refers specifically to safety from a dam failure. The dam safety focus considers the worst that could happen in a hypothetical situation."
That did not mean failure was likely to occur, Nicholson said.
Councillors were told the dam's maximum working level was reduced to 9.5m from 12.5m.
"It will remain at this level and staff will continue to monitor the impacts. If the risk of having it at this level remains low, then we may take a cautious approach to increase it incrementally."
So far, the dam had cost the council more than $286,000, but this amount excluded staff time.
Councillors were presented with three options - one where the council did nothing but continued to monitor the situation; to progress planning for permanent repairs but exclude planning for functional improvements; and progress planning of permanent repairs and functional improvements as well as planning for water pre-treatment and supplementary water sources.
The third was the recommended one and prompted several questions and comments.
Councillor Sharon Wards asked if the estimate was for monitoring and design costs only.
Nicholson said investigations would be started into the solution and that would include looking at consultants to assess the design of that solution.
"It won't be the pre-treatment plant, it won't be the actual repairs. We'll be definitely coming back to the council once we have all the information and costs associated with that."
Group manager of infrastructure Chris Chapman said there would be nothing in terms of delivery of physical works for permanent repairs or physical construction of pre-treatment and an alternate water source.
"But certainly a lot in terms of flow monitoring systems that we are going to need regardless under new dam safety legislation."
He said some of the big costs were in investigation, design and engineering to make sure there was a robust, long-term solution.
What the final cost of permanent repairs would be was something Chapman was reluctant to even make a guess at.
Corporate manager Raj Suppiah said when pre-treatment, alternate water supply and functional improvements were added to repairs, it became quite complicated.
"Until we've done the investigation to get clarity as to what options we choose, we can't say we have enough."
Alison Franklin wants to ensure the public know the situation about the dam so they understand the seriousness of the issue. Photo / NZME
Councillor Alison Franklin said she wanted to shut down rumour and speculation over the dam and to ensure people were aware of the risk to the community.
"They need to take ownership of their usage because that's vital, but they're not going to do that unless they realise what we realise, and this is how serious the situation is."
She said she felt strongly that people in the community needed to know what had happened.
"The public has a right to know that this is an absolute, for want of a better expression, cock-up."
Franklin said she had spoken with an expert in the industry for HDPE (high-density polyethylene) liners and the man had brought up several concerns.
"The one thing he did say was not putting protective textile under that lining was the biggest mistake ever made, because when you get infiltration of water into clay, we all know what happens."
Nicholson said there were a lot of concerns about the original design and some faults had been picked up.
He said he understood there was a lot of speculation around the liner, but none of what had been speculated had been proven.
Franklin said the man she spoke to had been in the industry for years and in his opinion, there should have been a liner under the membrane liner, instead of the clay.
"You say it's not proven, but the failure exists."
Chapman said when the dam was first designed, council had taken advice that a geo-synthetic, geo-textile liner between the subgrade and the membrane liner "wasn't the go-to standard at the time".
He said the council then had made the decision to "go above and beyond by doubling the depth of the protective clay layer between the subgrade which contained the stones and the liner".
The costs would be debt funded through capital expenditure and a portion would come from reserves.
