Dannevirke dam issues still ongoing

Dannevirke's dam is still functioning and is being monitored for any further deterioration.

Despite a decision made by Tararua District Council a year ago on measures to mitigate problems with Dannevirke’s impounded supply, the work has not proceeded.

Council three waters manager Roger Earp said at the September meeting he was responsible for the fact that the approved pre-treatment and installation of additional treated water storage had not occurred.

Dannevirke’s dam, completed in 2013, has been plagued by troubles since July 2021 when a leak was discovered as flows from the subsoil drain network that ran under the structure were abnormally high.

According to a report tabled for the meeting, it was noted that a remotely operated vehicle (ROV) was used to inspect the floor of the dam and a leak was eventually found below the inlet structure.

However, while the section of liner was repaired, the dam continued to lose water, albeit at a reduced rate and experts Tonkin & Taylor were commissioned to assess the damage.

Repairs were made in June last year, but in the process, it was identified that the liner and subsoil drains were in a worse condition than expected and the council was advised to do remedial works during the 2023/24 construction season.

In October last year, the council had resolved to delay remediation works until design work was complete, geotechnical investigations had been completed and pre-treatment and additional raw and treated water storage were in place.

Earp said there had been no significant deterioration noted in the floor of the impounded supply when the ROV was used last month.

“We have had a review from Tonkin & Taylor [and] their comment is that obviously the dam could deteriorate rapidly but there’s nothing that definitely indicates that at this point.”

He said the next stage was to get divers in to re-mark the depressions so they could continue to be monitored to ensure that if there was significant deterioration that could be recorded.

Earp appreciated there would be some frustration among councillors in relation to the approval last year to start pre-treatment.

He said given that they didn’t have all the information in relation to the status of the impounded supply, he felt it was important those decisions were delayed until “we were confident that we knew where we were going at that point”.

“The total cost was still unclear and for me there were too many unanswered questions to be committing significant amounts of money, between five and six million, with all that uncertainty.”

The previous decision had been to purchase a relocatable treatment plant, but that had changed to a permanent additional treatment plant.

“Given that the plant that we were looking at purchasing had never been constructed before, so it was the first one,” Earp said.

“Personally I’m quite pleased that we didn’t proceed with that at that time and now we can go out to market and get some designs for a permanent treatment plant.”

Chief executive Bryan Nicholson said at the time of making the decision last year, there were a lot of unknowns in terms of the impounded supply and it had been a collective decision-making process regarding the postponement.

There had also been an independent review since which endorsed the council’s decision.

Councillor Peter Johns said he was disappointed the council hadn’t proceeded with an additional treated water tank or storage and the pre-treatment.

“We’re back to where we were a year ago.”

Nicholson said the council had been monitoring the impounded supply and it had been functioning.

“The current supply that we’ve been providing to the Dannevirke community was the same as before the leaks were detected, so there’s a fully-functional dam.”

He said the risk had been analysed and it was felt there was enough time to delay engagement of any contractors to do the work while they understood the issue a lot more than they did at that point in time.

“So I think it was 12 months of doing further due diligence on that.

“The last thing that we would have wanted is put all that in place and then it wasn’t the right decisions either, so I think the 12 months have been beneficial for us to understand the whole system more than what we did 12 months ago.”

Mayor Tracey Collis asked what would happen if there was a dry summer and if there was a plan in place to ensure people had water.

“Are you confident in the way the dam is being run at the moment that we could meet that?”

Earp said if similar conditions to those of 2020 were faced again this summer there would be some challenges but a leak detection programme was being finalised.

He said the programme would look specifically at the rural areas where the water has been consumed and was confident it would help in terms of managing that demand.

“The other thing that I’d stress is the approach that was taken around the whole water management over the last summer appeared to work very successfully.

“We didn’t have any water restrictions in Dannevirke and it’ll just be that close and regular monitoring of how we’re going, monitoring our rivers, monitoring our reservoirs and we’re monitoring demand.

“We’re able to do that effectively and efficiently and that definitely helped in terms of us managing the situation last summer so I’d recommend that again this summer.”