Pet Dam upgrade design
Client: TasWater Capital Delivery Office
Location: East Ridgley, near Burnie, north-west Tasmania, Australia
Dates: 2021 – ongoing (building on dam safety services provided since 2009)
Drawing on deep asset knowledge and multidisciplinary expertise to support a safe and resilient upgrade of a critical regional water supply
Services
- Dam safety and risk assessment
- Geotechnical investigations
- Hydrology and hydraulic modelling
- Dambreak and consequence assessment
- Design of updated scour outlet and raw water supply pipeline
- Dam upgrade concept and detailed design
- Construction supervision and technical support
Background
Pet Dam is located on the Pet River approximately 13 km south of Burnie and is a key component of the Burnie region’s water supply system. Constructed in 1956 and raised in 1966, it stores up to 4,210 ML of water behind a 340 m-long earthfill embankment.
Entura has supported the surveillance and safety management of Pet Dam since 2009. Our work has included inspections, comprehensive surveillance reviews, consequence and risk assessments, dam safety emergency planning, flood hydrology and dam safety investigations. In 2015, TasWater engaged Entura to undertake a 20-year dam safety review (DSR) to assess the dam against contemporary engineering standards and confirm its risk status.
Our review identified that Pet Dam no longer met industry best practice. Key concerns included the condition and capacity of the existing spillway and potential seepage risks along the outlet conduit and the spillway walls. These risks were highlighted during a flood in June 2016, which caused significant damage to the spillway. TasWater subsequently initiated an upgrade to meet legislative safety requirements and support the dam’s continued safe and reliable operation.
Our ongoing involvement with the dam, working closely with TasWater over the past decade, gave us a thorough understanding of the dam design and a clear perspective on where action needed to be taken to bring the dam in line with current industry standards and guidelines.
Solution
Entura led the concept and detailed design of the upgrade, working with TasWater and the Capital Delivery Office to investigate and refine potential solutions. The design includes a new spillway, raising the dam embankment to provide the required flood capacity and freeboard, a filter buttress around the outlet conduit, a foundation seepage interception trench, outlet upgrades and improved instrumentation.
The constrained site and need to maintain the reservoir’s operation made designing the spillway and outlet works particularly complex. Entura assessed alternative arrangements for the spillway through geotechnical investigations, hydraulic modelling, structural design, multicriteria assessment and value engineering workshops. This process identified a new spillway on the northern side of the reservoir as the preferred solution, reducing construction complexity on an operating reservoir, and reducing dam safety risk while retaining flexibility for future upgrades.
We also designed an upgrade of the scour outlet to remove a safety risk and operational constraint, allowing additional control of the reservoir level ahead of the embankment works. Replacing the aged raw water supply line to reduce risks of supply failure required installation of a temporary siphon arrangement to supply the water treatment plant.
Our hydraulics specialists used complementary one-, two- and three-dimensional computational models to understand the complex flow through the proposed spillway and its interaction with the reservoir, downstream infrastructure and partially completed works. In particular, our use of advanced 3D computational fluid dynamic (CFD) modelling was an innovative approach to solving complex spillway hydraulic problems and developing an optimised spillway design. The modelling helped refine the spillway geometry, determine dam crest requirements, assess downstream flood levels, and inform scour assessment and detailed design.
Entura also updated the design flood hydrology, dambreak modelling and consequence assessment for the issued-for-construction design, taking account of the latest projections of climate change impacts. This provided an updated basis for risk management and emergency planning.
Outcome
TasWater is delivering the upgrade in two main parts: embankment and outlet works within the Ridgley water treatment plant site, and construction of a new spillway. Both components are scheduled from November 2025 to December 2027.
Entura is continuing to provide construction supervision and technical support to the site project team to ensure that the construction works are completed to the design intent and that the safety of the dam is maintained throughout construction. The upgrade will address identified spillway, seepage, freeboard and outlet risks, helping maintain the safety, reliability and performance of this important regional water supply asset well into the future.
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