City's water mains struggling to handle the heat

Dunedin's water mains network is starting to crack under the pressure of a long, hot summer dry spell.

Dunedin City Council staff and contractors have been kept busy repairing cracked water mains caused by rapidly drying ground conditions, just days after it was confirmed Dunedin had experienced its driest December on record.

Council water and waste services acting manager Mike Ind said five of the city's large 100mm-150mm water distribution mains cracked open last week, as clay surrounding them dried out and shrank.

Two of the damaged pipes were on Otago Peninsula, and a further three in the central city, including one near the Speight's Brewery.

The "ring cracks" occurred because the drying and shrinking clay placed pressure on the underground pipes, he said.

"The clay dries out and it shrinks, then it starts pulling on the pipes in directions that the pipes are not designed to be pulled," he said.

The repair bill so far stood at about $4000 but could rise in the coming months, if dry conditions continued and more pipes cracked, he said.

"If it carries on and the ground dries out at the rate it's drying, we'll probably have a lot more." The cracked mains had cut water supplies to "dozens" of surrounding properties in each case, but repairs had been carried out the same day, in most cases, and were now complete, he said.

The supply interruptions occurred as pipes were shut down for repairs, but in some cases supplies were able to be re-routed to minimise the disruption, he said.

"As soon as we identify a failure we're on to it, and it's fixed either the same day, or as soon as we possibly can."

The damage to the pipes came as Niwa last week confirmed the city had had its driest December since records began at the Musselburgh site in 1918. Just 9mm of rain was recorded last month, compared with an average of 81mm.

Damage to the city's pipes caused by summer drying was not unusual, but had in previous years occurred later in summer, council staff said.

Despite the drying, the council had no immediate concern about water reservoir levels and there were no immediate plans for water restrictions, Mr Ind said.

"As far as the network goes, as we have been, we'll just be repairing [pipes] as they fail."

chris.morris@odt.co.nz