Opuha Water Limited recently commissioned a new low-level outlet at Opuha Dam, one of the most significant safety upgrades made to the dam since it was built. We're proud to have played a part in that commissioning, using our clamp-on flow meter to help the project team confirm actual flow rates through the new valve across a range of opening positions, including with the dam's hydro turbine running at the same time.
A dam with a dramatic history
Opuha Dam sits on the Opuha River, a tributary of the Opihi, about 17km north-east of Fairlie in South Canterbury. It's a 50 metre high earth dam, roughly 300 metres across the gorge, and it wasn't a smooth build. In the early hours of Waitangi Day, 6 February 1997, three days of heavy rain in the upper catchment caused the still-under-construction dam to overtop and breach. Around 13 million cubic metres of water broke through a gap that widened to 20 metres across and 6 metres deep, sending a peak flow of 1,800 to 2,000 cubic metres a second down the Opuha and Opihi rivers and destroying the nearby Skipton Bridge. Remarkably, nobody was killed. The dam was rebuilt and completed, opening in November 1998, and has operated successfully ever since. It was also the first large dam consented under New Zealand's Resource Management Act.
More than just irrigation storage
Lake Opuha holds around 74 million cubic metres of water across up to 710 hectares, and the scheme does a lot more than store water for farms. Reliable irrigation reaches roughly 16,000 hectares of South Canterbury farmland, supporting dairying, horticulture, arable cropping, and more, with a single hydro turbine generating around 7.5MW of renewable electricity from water released through the dam regardless of what it's being released for. Stored water is also used to maintain minimum environmental flows down the Opuha and Opihi rivers, particularly valuable during drought when those rivers would otherwise run low or dry. On top of that, the lake supports potable water supply security for Timaru, is a popular recreational spot for boating, swimming, and fishing, and the area is recognised by BirdLife International as an Important Bird Area for nesting black-billed gulls. A 2024 economic study put the scheme's gross economic benefit to the region at around $1 billion.
Why a new low-level outlet
Large storage dams generally benefit from having a genuine ability to release big volumes of water quickly and safely when conditions call for it, rather than relying solely on spillways designed for a dam that's already full. Dams without that kind of controlled drawdown capability have historically struggled to respond when things move fast, whether that's an approaching storm or an unexpected structural concern. Opuha's new low-level outlet was built specifically to give the dam that capability, and it's a genuinely significant upgrade for a dam that's been operating successfully for nearly three decades.
What the upgrade actually does
The centrepiece of the $8 million project is a new 1.8 metre diameter Fixed Cone Discharge valve, installed on a branch of the original construction-era penstock that had sat buried inside the dam structure since the 1990s. Commissioned last month, it gives Opuha Water Limited the ability to make large, controlled releases from Lake Opuha before a storm arrives, deliberately lowering the lake to create flood storage buffer capacity while the rivers downstream are still running at normal levels. When heavy rain does hit, that extra buffer gives the dam more room to absorb the inflow and reduce peak flood impacts further down the catchment, protecting roads, rail, bridges, and farmland across South Canterbury. During testing, the new valve delivered a historic first for the dam: total discharge through the penstock of around 52 cubic metres per second, roughly 187,000 cubic metres of water an hour, equivalent to draining about 75 Olympic-sized swimming pools every hour.
Where we came in
Commissioning a valve capable of moving that much water isn't something you can fully plan on paper alone, the project team needed real, measured flow data at a range of valve openings to confirm the new outlet was performing as designed. We used our clamp-on ultrasonic flow meter to capture flow rates across different opening positions during testing, including the more complex scenario of the new outlet running concurrently with the hydro turbine, to understand how the two interact when both are releasing water at once. Because clamp-on measurement doesn't require cutting into the pipework or interrupting the dam's operation, it let the commissioning team get accurate, real-time flow data exactly when and where they needed it, without adding complexity to an already carefully sequenced testing programme.
It's a good example of where clamp-on flow measurement earns its keep well beyond routine verification work, large infrastructure commissioning, where the flow rates involved are significant, the conditions are genuinely variable, and installing a permanent primary meter simply isn't practical. If you're planning a commissioning programme of your own and need confidence in the numbers, it's exactly the kind of problem we're set up to help with.