The biggest weir built in Australia since World War II is a masterclass in engineering innovation and sustainability, and services a catchment area larger than England.
This article was originally published in the November 2024 issue of create with the headline “Going with the flow”.
The Fitzroy River in central Queensland supports six species of freshwater turtle, two of them threatened species.
To forage for food and conduct breeding migrations, the reptiles must sometimes swim tens of kilometres up and downstream, leaving the safety of the water only to nest or, during dry conditions, move between isolated pools in different sections of the river.
So a $568.9 million weir stretching more than 350 m across the river that’s made from 131,000 m³ of concrete could present a challenge for a white-throated snapping turtle heading upstream in search of juicy tadpoles or water striders.
Luckily, the engineers and scientists behind Rookwood Weir (Managibei Gamu) – the biggest weir to be built in Australia since World War II – found an innovative solution that means the hungry turtles needn’t fret about their next meal.
The team deployed a series of ingenious solutions that have made the structure one of the most technically accomplished recent engineering projects, and won it the Australian Construction Association’s prestigious Australian Construction Achievement Award for 2024.
Engineers Australia CEO Romilly Madew AO FTSE HonFIEAust EngExec described it as an example of the power of creativity.
“By integrating sustainable practices and cutting-edge engineering, they’ve shown the construction industry can lead the way in environmental responsibility and innovation,” she said.
Completed in November 2023, Rookwood Weir is 55 km west of Rockhampton and a short drive from the small rural town of Gogango. Its sweeping spillway rises 16.2 m from the riverbed and, at full supply volume, holds 74,325 ML of water, 40 per cent of which will go to local farms and create 500 jobs. It is projected to also inject $3.5 billion into the area’s agricultural sector over the next 20 years.
One recipient of the new supply has already planted 800,000 macadamia trees, while another has announced plans to grow 4000 lychee trees. Water has also been allocated to the Traditional Custodians of the land, the Darumbal people, to support their economic development.
The weir was jointly funded by the Queensland and Australian Governments, with design and construction carried out by Sunwater in alliance with GHD, Acciona and McCosker Contracting.
“It will enhance long-term water security in central Queensland, creating sustainable employment and economic development opportunities for local businesses,” Sunwater Project Director Inaki Goni said.
The project was conceived and approved after a feasibility study by Sunwater and the Gladstone Area Water Board evaluated the opportunities and potential impacts of building a water storage asset on a river with a daily discharge of 16 GL into the Coral Sea and a catchment of 142,665 km² – an area larger than England.
By regulating the water flow, the weir ensures businesses and residents receive a consistent and reliable supply. And that the local turtles aren’t left high and dry.
Ecologically friendly design
Rookwood is home to the world’s first bespoke turtle passage, which consists of a 172 m textured ramp with resting pools every 15 m. It was built following the biggest-ever freshwater turtle telemetric tracking project to understand their behaviour, and allows the animals to climb up and past the weir.
“It’s a unique design feature, and we used the results of the tracking research to develop the design criteria for the ramp,” Rookwood Weir Alliance Lead Ecologist Dr Natalie Clark said. “It will be monitored to assess its success in maintaining turtle movements, and avoiding injury and mortality.”
Equally groundbreaking is the fish lock that allows barramundi, sooty grunters and other species to swim freely through the weir with flows of between 0 and 1.3 million L/s. It contains two lock chambers with multiple entrances and exits, which doubles the operating range of a more typical single-chamber design.
To reduce turbulence adjacent to the fish lock that might deter fish from entering, five castellated crest blocks were placed on the downstream face of the spillway crest. Flow guide slots concentrate the fast-moving water, thereby increasing the depth and reducing any churning.
Bank defences
Another implication of high water flows on the left bank is slope stability and seepage which could lead to the gradual erosion of the riverbanks.
“A high river flow is extremely powerful and was one of the main project risks, with potential failure of the alluvium riverbank, so we had to make sure we had robust defence mechanisms in place,” Alliance Construction Manager James Heenan told create.
The original concrete mix design had to be adjusted after it was discovered to cause cracking, while the mix of conventionally vibrated, unreinforced concrete used elsewhere on the build was also innovative. It contains an unusually high 61 per cent of fly ash, sourced from nearby Gladstone, and an aggregate size of 75 mm (three times the norm) to prevent cracking.
An inspection committee was established to ensure the structure’s foundations met design requirements. The variable geological conditions meant the foundations in some locations were up to 12 m deep. This was much deeper than anticipated.
“Our geological foundation was extremely complex, resulting in having to go down deeper than expected,” Heenan said. “We adjusted our approach accordingly to be completely confident the foundation is on competent rock.”
The importance of equipping the weir for severe weather events was brought home to the construction team when work had to be halted six times due to flooding. The first stoppage lasted 154 days, with the others totalling 117 days. For the 250 workers billeted in specially built cabins nearby, they were frustrating times.
To minimise the impact, contingency tasks such as preparing precast concrete, in-situ casting and work on the right abutment above inundation level were carried out.
“Rookwood Weir is a great example of the power of pioneering design, cutting-edge engineering and environmental responsibility,” Heenan said. “It will continue to benefit the local communities for decades to come.”
Less than a month after the building work wrapped up, the water level reached the top of the spillway. After three years of intensive earthworks, concreting and mechanical installation, the site fell strangely silent other than the roar of water cascading down the concrete slope. And the almost imperceptible pitter-patter of tiny, webbed feet navigating their way up and down their new ramp.
Sustainability focus
Minimising the carbon footprint of Rookwood Weir was central to its design and construction, and has resulted in it becoming the first weir in the country to be awarded an “excellent” rating by the Infrastructure Sustainability Council, for its focus on sustainability, habitat connectivity and local employment.
Alliance’s Environment and Sustainability Manager Jason Cork oversaw a swathe of initiatives that drastically reduced emissions.
“We replaced 98 per cent of potable water with non-potable water and reduced supplementary cementitious materials by 50-60 per cent,” he said. “Our design reduced the spillway length, which means we’ve used less concrete, steel and water, and caused less disturbance to the site.”
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