A date with an old flume11 February 2016
A detoriating section of the fluming carrying water to Tarraleah power station has been repaired on time and in budget, improving the safety and condition of this important infrastructure
Generating energy from water over the long term requires significant ongoing investment in the safety and performance of more than dams and turbines. Water conveyance infrastructure also needs regular attention to keep the water flowing across the many, often rugged, kilometres from storages to power stations.
"Across a century of hydropower development, Hydro Tasmania has constructed 55 major dams, 30 power stations, and hundreds of kilometres of waterways," said Hydro Tasmania's civil portfolio manager, Neil Smith.
"Many of these assets were world firsts when they were built and continue to provide outstanding service to this day. But, like any valuable infrastructure, our hydroelectricity and water assets require ongoing maintenance to perform well into the future."
A timely and successful repair of a rapidly deteriorating section of the flume carrying water to Hydro Tasmania's Tarraleah Power Station has greatly improved the safety and condition of this important piece of water conveyance infrastructure, and ensured its reliable service in Tasmania's power system for at least another 50 years.
And if that isn't cause enough for celebration, this repair has also been achieved within schedule and budget, using safe and innovative design and construction methods, and carefully considering the flume's heritage values.
Tarraleah Canal No. 1
The section of flume that needed replacing forms part of Tarraleah Canal No. 1 which was commissioned in 1938 and supplies 70% of the water to the Tarraleah Power Station. Tarraleah Canal No. 1 consists of alternating sections of in-ground canal (totalling 12.2 kilometres) and above-ground flume (totalling 6.5 kilometres), carrying water from Lake King William in the central highlands of Tasmania via the Butlers Gorge Power Station to Tarraleah.
The 30 metre-long damaged section was one of 22 transitions between the in-ground canal sections and the elevated flume sections, which are carried across a gully by a concrete bridge structure. Over time, the rockfill embankment supporting this transition section had settled, and this gradual movement was forcing the flume to bend, causing it to buckle and crack.
"Over recent years, a number of attempts had been made to control the settlement of the embankment, to repair leaks, and to increase monitoring and surveillance. But by April 2015, it was clear we would have to take action on a larger scale," said Neil Smith.
Repairs that were planned for autumn 2016 were brought forward to early spring 2015.
"A safe and effective approach to the repair was needed that would minimise the loss of electrical power production but also allow preparations to get underway rapidly in case the situation deteriorated further," said Neil.
The canal was dewatered by closing the control gate at its upstream end. It took about twelve hours to empty the whole canal, but as the construction site was only 6 kilometres downstream of the Butlers Gorge dam, the deteriorating section drained after only four hours.
Replacement sections for the 4.5-metre-wide by 2.5-metre-deep rectangular concrete flume were manufactured offsite to speed up the project. It was a significant logistical challenge to transport the nine 15.5 tonne pre-cast concrete units from Electrona in the south-east of the state across more than 150 kilometres into Tasmania's remote interior. Most units needed their own semi-trailer, and needed to be delivered on a single day, without compromising the safety of other road users.
To rapidly remedy problems in the flume's supporting embankment, a 4.7-metre-high roller-compacted supporting structure was constructed from new, locally sourced rockfill material reinforced with geogrid (a polymer material used to stabilise terrain) and contained by vertical face-walls built from pre-cast concrete blocks.
"This solution was chosen as it would provide a solid, engineered support with a high degree of compaction to minimise settlement," said Neil.
"It would also be quicker and more reliable than processing and re-using the older rockfill material, which was stockpiled to be used for other purposes including constructing a crane pad for installing the concrete flume units."
A marriage of old and new
The damaged flume sections were regarded as having significant heritage value, so Hydro Tasmania wanted to repair them as sensitively as possible within the constraints of the tight schedule and budget.
"A 25% dose of black oxide pigment was added to the pre-cast concrete mix to reduce the visual distinction between the new and old sections," said Neil.
"Future opportunities are also being explored to further maintain consistent appearance by replacing the original hand-placed stone façade adjacent to the road to Lake King William, which is frequently used by anglers, campers and tourists."
Care was also taken not to discharge any waste water or cementitious materials into the natural waterway.
Sustainably managing its civil and dam portfolio for future generations is a priority for Hydro Tasmania.
"We undertake regular and planned risk assessments for all our infrastructure," said Neil. "As these assets age, we monitor their efficiency and safety and we invest to ensure they remain fit for purpose and able to provide ongoing service. Planned maintenance and upgrades of our assets help to retain our world-class standards.
"Our major-project capital works schedule not only ensures Tasmania's electricity supply is reliable, but also maximises the power we can generate and supply to the National Electricity Market so that Hydro Tasmania remains Australia's leading producer of renewable energy."
The successful repair of this transition section of Tarraleah Canal No. 1 continues Hydro Tasmania's reputation for excellent custodianship of power and water assets, and will ensure that this flume can continue to function safely and reliably far into the foreseeable future.
By closely investigating the deteriorated flume and embankment section during the demolition process, Hydro Tasmania hoped to learn valuable lessons about the practices used during the original construction. These lessons can now feed into the renewal strategy for the other 21 transition sections along the canal.