The Rangitata Diversion Race is the virtual lifeblood of rural Mid Canterbury, providing water for irrigation for plains farmers and powering two small hydroelectric stations. The Guardian chats with Tony McCormick to find out more about the flood repairs underway and future resilience planning.

Back in the 1800s, the plains around Ashburton provided a harsh environment to the point it was considered an isolated wasteland and was one of the last districts in Canterbury to be settled.

When the Ashburton County Council was formed in 1876, a way to channel much-needed water became an important focus.

County engineer William Baxter started work on an irrigation scheme in 1884 but it would take almost 50 years before construction began.

Since then, the Rangitata Diversion Race (RDR) has played a vital role in the economy of the Ashburton District, enabling local farmers to become an important part of New Zealand's food-producing economy.

The original race has stood up well for more than 80 years but was damaged in the 2021 floods. Repairs from the June deluge are now underway.

RDR Management Limited (RDRML) chief executive Tony McCormick said the damage occurred at seven sites where the RDR canal is taken under streams and rivers flowing from the foothills down the plains in an inverted syphon.

The flood waters got in behind the original concrete protection work from 1938 and damaged seven syphons, tearing off some of the concrete armouring. One of the sites damaged in June 2021 suffered further storm damage in the August 2022 floods.

McCormick said the damage was caused by a once-in-200-year flood. But the effects of climate change meant these events were likely to become more frequent and highlighted the need to rebuild the siphons and bring them out of the rivers.

But the cost of an upgrade means it is a plan for the future.

McCormick said they would only be able to repair and upgrade one site with the insurance payout.

RDRML is repairing the original syphons, using better quality concrete and more robust structures with more steel - at a cost of about $6.5 million.

All but an anticipated $800,000 would be covered by insurance, McCormick said.

Repairs started in February. One syphon has been repaired and work on the second syphon is underway and ahead of schedule.

McCormick said the race would continue operating during the repairs, which he expected would be complete by the end of September

"It's very dependent on the weather," he said.

When asked why repairs from an event in 2021 had taken so long, McCormick said the process had been complicated. The design proposals had to be approved by the insurance company and it had been tricky to get insurance cover for the repair work.

"It's taken longer than we wanted... The riskiness of working in rivers like this has challenged our insurers."

The repairs would be adequate for a mean annual flood.

A couple of areas were very vulnerable and RDRML was working at a 10 - 20 year plan to rebuild the more exposed siphons, McCormick said.

Climate resilience

Supply-side climate resilience has been on RDRML's radar for a while.

It had planned to build large storage dams at Klondyke and got consent to build the Klondyke Ponds in 2018.

However, this has since been sold to the irrigation shareholders of RDRML, MHV Water and Ashburton Lyndhurst Irrigation Limited. They will operate the dams under Mid Canterbury Water Storage Limited.

McCormick said the ponds would work with the water race and provide a more secure water supply than from river abstraction alone.

Highbank refurbishment

Manawa Energy's hydroelectric power station at Highbank will shut down for a major refurbishment for about 18 months from September this year.

McCormick said it was an end-of-life replacement for the 78-year-old power station.

"They will literally gut the building," he said.

The first step would be to install an overhead gantry crane to disassemble and reassemble the equipment.

Manawa Energy planned to replace the main hydroelectric plant, including the turbine, generator and ancillary equipment.

McCormick said the upgrade was "quite exciting" from an engineering perspective and would not affect power prices or reliability while the power station was out of commission.

The 25 megawatts the station generated was quite small in the scheme of things, he said.

Locals could expect some heavy loads being trucked to Highbank. The generators and other parts would come pre-assembled for specialist engineering contractors to install.

Once the project was complete there would likely be efficiency and performance gains from using modern technology with better materials, he said.

The shutdown would affect the RDR during the May to August power generation season when all the water typically runs through the power station.

"It's an unusual event. It's the largest outage that the scheme has ever seen," McCormick said.

The biggest issue could be in costs for other users. The water scheme allocated costs based on the amount of water used and Highbank's outage would leave a lot of water unused.

However, payments would be calculated on a five-year average and smooth out the effect, McCormick said.

RDRML also hoped to be able to supply any excess water to the Hinds-Hekeao Managed Aquifer Recharge Project to replenish declining groundwater levels.

RDR- by the numbers

Mid Canterbury's diversion race runs for 67km from the Rangitata River below the gorge at Klondyke to the Rakaia River at Highbank.

It was built between 1937 and 1944 as part of a government Public Works scheme.

The canal meanders across the plains to take advantage of land contours to create a gravity-fed scheme with offtakes that divert water for irrigation and stock water.

The RDR runs year-round, carrying between 28 and 36 cumecs, depending on river levels, water demand, and time of the year. At low river levels, the RDR flow is reduced to ensure the required minimum flows remain in the rivers.

The RDR feeds two hydro-power schemes owned by Manawa Energy, one at Montalto and one at Highbank.

The Montalto Power Station was built in 1982 and runs all year provided there is water in the RDR.

Highbank started to generate power in 1945 and operates in winter and when water is not required for irrigation from September to May. Highbank also runs a pump station on the Rakaia River.

Construction of the RDR was completed in 1944. At the time, a Department of Agriculture report said the race had increased the carrying capacity of the land from one sheep per acre to six sheep per acre.

The RDR has over 40 resource consents and water levels have been managed electronically through a computer system since 2010.

It is the largest water supply scheme in New Zealand and irrigates 100,000 hectares in Mid Canterbury.

The RDR supplies between five to six times Auckland's total water supply, 365 days a year.

By Sharon Davis