Council told to keep troubled Dannevirke dam below capacity, or risk collapse of wall

The dam may need to be taken offline in order to make permanent repairs. Photo / NZME

Tararua District Council has been told to keep the water levels of its trouble-plagued impounded supply below capacity, or risk a wall of the dam collapsing.

Earlier this week, Dannevirke residents were warned of the possibility of water restrictions again this summer, although the council is hopeful they will not be to the extent that they were the previous year, after repairs on a leak at the dam left it well short of full capacity.

Council engaged dam experts Tonkin & Taylor earlier this year to inspect and then assess the dam following issues of seepage, which had remained "irregular and abnormal" despite repairs after the leak was discovered in July last year.

In June, Tonkin & Taylor recommended that the dam be operated at lower water levels to reduce the chance of this failure.

Craig Ellmers has had regular communication from concerned residents and has done extensive research into the dam. Photo / NZME

Chief executive Bryan Nicholson said the risk of the wall failing was low.

A press release sent out to the public in June said the council was increasing its on-site monitoring of the dam to inform the investigation and better define the leakage risk.

That monitoring was also so that Council could detect any changes that would indicate an escalation of the leakage risk to a more "serious situation", the release stated.

After Hawke's Bay Today made enquiries, Nicholson confirmed that one such serious situation could include a collapse of the northern wall.

Dannevirke Residents' Association chairman Craig Ellmers said the association had been receiving regular correspondence from those in the town concerned about the dam, including its risk of failure.

He said a number of meetings had been held with Mayor Tracey Collis, Nicholson and council executives, where he and a working group had presented submissions about how to prevent Dannevirke from having yet another summer where it would not have the water levels it needs.

Ellmers believed Council had not taken those submissions on board.

"The association has gone to the effort to put together and present sound and logical high-level plans, calling on experienced locals."

The association's plans included establishing a separate water source (bore) for Alliance freezing works, which he said accounted for up to 25 per cent of the daily treated water consumption in Dannevirke.

Quotes had also been provided as part of those submissions.

Ellmers said the association's working group also presented two plans as part of the submissions to Council – effectively a continuity risk plan to enable water storage to take place and assessment and repairs to be made on the dam.

Nicholson said Council management had advised that various options for an alternate or supplementary water source were being investigated.

Bryan Nicholson: It was deemed not an appropriate use of ratepayers' money to construct bores without a reliable alternative. Photo / NZME

He said it was not deemed an appropriate use of ratepayers' money to construct bores without some confidence that a reliable and safe supply of water would be found.

"Recent results from the aquifer mapping investigations, which sought to identify potential underground water sources of suitable volume, indicate that money spent on constructing bores would not have resulted in a reliable water source."

Council staff would continue to work with Alliance Group management to support improvements for their supply and consumption.

Council was investigating alternate or supplementary water sources. Photo / NZME

Nicholson said investigation and planning work continued for Dannevirke's impounded supply.

In order for Council to undertake permanent repairs, it was expected the dam would have to be taken offline for several months so the existing liner could be removed, he said.

Nicholson said this was planned to be done in 2023.

Council was assessing the impacts to the town's water supply, and appropriate mitigation measures.

This was likely to include additional storage, new treatment processes, and potentially implementing an alternate or supplementary water source, Nicholson said.

"The current key focus is preparation for the 2022/23 summer, which includes a temporary solution to the hydraulic nature of the impounded supply to ensure Dannevirke's demand can be taken from the impounded supply regardless of the water depth."

The dam was currently at 8.5 metres full and it was expected there would be far more water in the reservoir at the beginning of this summer, Nicholson said.