After cyclonic flooding destroyed Kuranda Weir, engineers had less than a year to design and construct two dams, stabilise the rail corridor, keep workers safe and reinstate power – all before the next big wet.
It was one of the highest rainfall events on record in parts of Cairns, according to GHD Dams Service Line Leader APAC Nick Thomas-Kinsella CPEng. The water levels reached the top of the embankment, which was not designed for overtopping. In fact, the river had never before overflowed the clay-core, rock-fill structure, which was constructed in 1962-63.
When that did finally occur in late 2023, the embankment failed. “Once it overtopped, it resulted in head-cutting erosion on the crest and eventual back erosion and breach of that embankment,” Thomas-Kinsella said.
The consequences were immediate and severe. “The power station could no longer supply water because there was not enough head to run through the penstocks and power the turbines.”
Barron Gorge Hydroelectric Power Station, capable of providing renewable energy to 50,000 homes every year, was suddenly unavailable. Water bypassed Kuranda Weir, which acted as a regulating pool to provide water to the power station. The reservoir was also vital for whitewater rafting tourism operators downstream, who lost the controlled releases they rely on.
The clock began ticking. The next wet season was around the corner. To reinstate the weir’s function and restore generation, engineers from GHD and energy developer, generator and retailer CleanCo Queensland had to achieve in less than 12 months what might usually take three or four years.
“This wasn’t a linear project,” said CleanCo Asset Engineering Manager Geoff Woodgate MIEAust CPEng. “A civil works project like this would typically have an active horizon of about four years, with up to 10 years of feasibility and financial investment work.”
Rail, rain, risk
The project lay within one of the most constrained sites conceivable. It was wedged between the Barron River, a live Queensland Rail (QR) tourist line, unstable geology and damaged access roads, all within a protected World Heritage rainforest.
“The first challenge was the brownfield nature of the site,” Thomas-Kinsella said. “We didn’t have the time and opportunity to plan, and we were nestled between Wet Tropics national park vegetation, the Kuranda rail and the power station.”
QR’s iconic Kuranda Scenic Railway had suffered landslide damage during the same storm.
“We had to work with their engineering team,” Woodgate said. “They had constraints on their issues as well. And we had heavy equipment crossing the railway every day, multiple times. We had to work within their rail schedule.”
Access was problematic. Every truck, excavator, crane and concrete load had to pass through sensitive residential streets.
“It was an active and very public site,” Thomas-Kinsella said. “Once it was back in operation, the train would go by two to four times a day and everyone would wave.”
Compounding these constraints was the river itself, flowing fast toward Barron Falls just 500 m downstream. “Building a cofferdam across a flowing river is a major hazard,” Woodgate said. “It was absolutely necessary, but it was a massive task.”
Phase A: Rock-fill cofferdam
To create a safe, dry work zone and restore generation, the team first had to design and place a temporary rock-fill cofferdam.
“We used conventional earth moving equipment … progressively loaded and placed in the river, working from the right bank towards the left,” Thomas-Kinsella said. “We initially placed a platform of very coarse rock, around 500 mm in diameter, or boulder-sized.”
Finer materials were then worked into the matrix to slow the water flow. This was followed by a bitumen geomembrane liner. “The liner wasn’t completely essential, but it was an improvement, allowing us to operate more confidently.”
CleanCo’s risk management focus also called for a 24/7 system of sensors – tilt meters, water level detectors, etc. – to provide automated, real-time warnings in case of any movement.
James Archer MIEAust CPEng, CleanCo’s Site Manager Barron Gorge, said risk was a deep focus.
“For me, the biggest key element to this was how we understood risk and managed risk,” he said. “At any point in time, if people said ‘What’s the risk?’, we were able to tell them.
“We used a risk management tool called CGR. A lot of the risk surrounding this project went into CGR, and we used that as a basis for risk assessments and managing controls.”
The risk analysis, monitoring system, emergency action plan, inspection regime and training program were developed over weeks rather than years. Still, the deadline was merciless. Phase A was successfully completed within six months, allowing the power station to return to operation by July 2024.
Phase B: Iced concrete and workers in cages
With the river partially diverted and power restored, the next step was a more robust structure, a mass-concrete gravity wall to be built as the initial weir reinstatement. But pouring mass concrete in warm and wet Cairns required innovation.
“We needed placement temperatures under 25°C,” Thomas-Kinsella said. As ambient temperatures rose throughout each day, target temperatures became difficult, and sometimes impossible, to achieve.
At first, chilled water was used in the mix, but that didn’t do enough to cool the concrete. The contractor then moved to adding bagged ice at a late stage of mixing, which also proved to be too little, too late. Finally, ice was added earlier in the process, which kept temperatures below the target range.
The team also deployed a highly specialised conveyor system. “We used a telebelt to deliver the concrete,” Woodgate said. “Because of our tight site and because speed was an issue, it helped a lot.”
Concrete pours were sheltered from rain where possible.
The eight-metre-high gravity wall was cast in blocks with an optimised mix including high fly-ash content, reduced cement and oversized 40 mm aggregate. The low-carbon mix, adapted from GHD’s recent Rookwood Weir experience, delivered both sustainability benefits and performance under thermal constraints. Embodied carbon emissions were reduced by
60 per cent.
A further challenge was to get workers onto the intake structure in the centre of the weir. After various designs, analyses and discussions about temporary suspension and cantilever bridges, including with specialist scaffolders in Germany, it was decided that a 200 t crane and a workbox was the only solution for the first month, until a more stable connection was made between the riverbank and the structure.
Phase B reached completion in December 2024, days before the river rose again. “It got to within about 50 mm of overtopping the Phase B works,” Thomas-Kinsella said. “The cofferdam was completely under water by then.”
Collaboration key
An impressive outcome of the project, Thomas-Kinsella said, was the transfer of skills and knowledge.
“We had a senior technical director in Brisbane who was a great mentor and support. That meant I could lead, and there was also skills transfer to two more junior engineers. Now, we have more people in the industry who have built both a dam and a weir. That has been a real highlight.”
None of it would have been possible without deep collaboration between CleanCo, GHD, QR and the local Cairns contractor.
“The risks were quite high, and the consequence of any failure potentially catastrophic,” Archer said. “But the engineering went very well, community sentiment has remained really positive throughout after a lot of communication, and the holistic, collaborative approach to this project made it a success.”
This story was originally published in the February 2026 edition of create with the headline “River race”.
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