Dannevirke dam operating with ‘higher risk of uncontrolled release’

Roger Earp detailed the work going on to keep Dannevirke's dam going until permanent repairs can be done. Photo / Leanne Warr

It could cost tens of millions of dollars to fix Dannevirke’s impounded water supply, Tararua District councillors were told at a meeting this week.

Group manager of infrastructure Chris Chapman, in an update on the dam, said it was a complex matter.

He said he appreciated the concerns of the community who had questions on why the council couldn’t just “whip in and fix it”.

Repairing the dam, which has had ongoing issues since leaks were discovered in July 2021, would cause “significant disruption” to Dannevirke’s water.

While work was to get under way soon to mitigate repairs to leaks discovered last month, there was still work to do before options could be recommended to the council, Chapman said.

However, it was estimated the cost to sort out the water storage issue as well as repairs could total in the tens of millions of dollars.

Acting Three Waters manager Roger Earp said the temporary repairs had been planned and in the meantime monitoring of the impounded supply had been increased.

“There’s a daily inspection occurring, daily reviews of the seepage rates and the depth of the dam.”

He told councillors there was particular concern for the north-eastern berm and they were keeping a close watch on it.

The next key task was a geotechnical investigation which would inform what action would need to be taken around the northeastern berm.

“Possibly we won’t have to do anything,” Earp said.

He said once that had been clarified, they would be in a position to design for permanent repairs and understand exactly what would be involved.

Temporary repairs on leaks at Dannevirke dam will get under way this month, but options for permanent repairs still need to be considered. Photo / Paul Taylor

There were further investigations around the water treatment plant and the ability to treat higher turbidity water as well as additional storage and what that might look like.

Earp said they were also engaging with Alliance Group meatworks in terms of looking for an alternate source for them, which would significantly reduce the demand on the impounded supply.

Once the repairs were done, those working on the dam would need to review in terms of being reasonably confident the dam would remain intact until September next year, which was the target date for construction.

“If, for example, we conduct the repairs and the seepage continues at an elevated level, then obviously the repairs haven’t been successful and we’re going to have to look at emptying the impounded supply with urgency,” Earp said.

He said it was hoped the temporary repairs would stabilise it and buy them time to do the work needed so they could complete the remediation in a deliberate and very planned fashion.

“Because we probably won’t get the best result if we’ve got to do it under pressure. That’s what we’re trying to avoid at this stage.”

Councillor Peter Johns asked about the work already done to find an alternate water source.

Chapman said that work was for a supplementary water source for Dannevirke but investigations had been somewhat unsuccessful in terms of underground water sources.

He said ideally they were looking for something around 20 litres per second, but what their investigations found were between two and 10 litres per second.

An alternate water source for Dannevirke was a long-term thing and there was a need to focus specifically on a supplementary source that didn’t have to be hooked into the water treatment plant that could be provided solely to the Alliance Group.

Chapman said if they could get that to 10 to 20 litres per second, and with some co-investment from Alliance Group, that would take some significant pressure off the town’s water supply.

According to the report, the demand would be reduced by up to 35 per cent.

Alliance Group also required a lower level of water quality than the town required under the Drinking Water Standards and already had on site treatment and storage.

Councillor Sharon Wards asked about the eastern wall and the risk of it collapsing.

Dewi Knappstein, a senior water resources and geotechnical engineer at Tonkin & Taylor explained there were steps that would need to occur before an uncontrolled release of the impoundment.

She said they’d seen the first steps had occurred and while it was not definite, the risk was there.

“I think there’s no question that we are dealing with a confirmed dam safety deficiency, adverse behaviour has been observed and that deterioration is continuing.”

The dam was currently operating with a higher risk of uncontrolled release than was accepted in modern practice.

Knappstein said even if mitigation measures were successful, there was no guarantee the measures would remain effective for a specific length of time.

“At any time the situation could deteriorate and lead to a situation with an excessive risk of failure and require you to empty the impoundment.”

She said the risk would remain present until a permanent repair was completed.

Councillor Scott Gilmore asked what contingency plans were there to supply the town with drinking water.

Chapman said drinking water would be drawn from the Tamaki through the water treatment plant.