Fears for city’s water network

A network of pipes carrying drinking water to Dunedin is "hanging by a thread" as the Dunedin City Council prepares for a nearly $1 million upgrade, it has been claimed.

• Water shortage warning for Southland 
• Oamaru mayor praises townsfolk for saving water

The concern was raised by a council staff member, speaking on condition of anonymity, who told the Otago Daily Times the four main pipes bringing drinking water to the city were all "severely compromised".

Two of them — carrying water from Deep Creek and Silverstream - had already failed and remained out of action, after sections were washed away by recent floods.

The southern pipe was also being held together by "strops and posts", having been damaged - and not permanently repaired - by a slip near Abbots Hill two years ago, he said.

Sections of the Deep Stream pipe, which used to run under the stream, had also been exposed by recent floodwater, leaving it at risk of damage as well, he said.

A tree and other debris had been found wedged up against it during last month’s floods, putting the pipe under pressure, he said.

The state of the pipes meant Dunedin could be just one significant flood away from being without a way to pipe drinking water to the city.

If that happened, the city would be reliant on water already stored in the Mt Grand and Southern reservoirs - enough for about 14 days based on existing consumption rates.

"You’ve got two pipelines inoperable, one that’s hanging by a thread and one that’s still undermined.

"If that rain had come last weekend [as predicted], it could have been quite serious," the staff member said.

Council 3 Waters group manager Tom Dyer confirmed the Deep Creek and Silverstream pipes were out of action, while the Deep Stream and Southern pipelines remained operational despite problems.

The council had been designing repairs to all four pipelines, and the work was expected to be completed by the end of summer, at a combined cost of about $810,000.

The risk of a complete failure was "tolerable" for now, and the 14 days’ backup supply could be extended, if necessary, by voluntary water conservation efforts, he said.

"While we have been carrying some risk for a little while ... we still have ample water supply and still have plenty of room should something further go wrong.

"It requires action, but not rushed action."

Damage to the Deep Creek, Deep Stream and Southern pipelines dated back to July 2017, while the Silverstream pipe was damaged in 2015, he said.

The damaged section of the Deep Creek pipeline ran over a pipe bridge damaged in 2017, which subsequently failed during last month’s flood.

The Deep Stream pipe - carrying the largest volume of water of any of the pipes, at 38,000cu m a day - had been exposed by July 2017 flooding.

The pipe was not damaged, but was being monitored "closely" while DCC staff worked with the Otago Regional Council to obtain consents.

Sections of the Silverstream pipe had also been exposed by erosion in 2015, which had increased in later storms, leading to the pipe’s failure last month, Mr Dyer said.

The repeated flooding had been "extraordinary", but the low volume of water carried by the pipe - at just 16,000cu m of water a day - meant it could be "run to fail", he said.

A fix was scheduled for later this week, he said.

The Southern pipeline ran through a private forestry block on Abbots Hill, which had been harvested a few years ago, making it more vulnerable to slips, he said.

Despite that, Mr Dyer remained "relatively comfortable" with the level of risk to the city’s supply for now.

The existing 14 days’ storage gave enough time for an emergency repair, and permanent repairs could be accelerated if necessary, he said.

"We’re managing not to get to that point.

"We’ve started acting with a little bit more urgency ... but still in a controlled fashion and still with a reasonable degree of rigour."

chris.morris@odt.co.nz