Bradys Dam Upgrade - detailed design

Client: Hydro Tasmania
Location: Tasmania, Australia
Date: 2022-25

Services: dam upgrade design, dam risk assessment, geotechnical investigations, flood studies, environmental risk assessment

Supporting a safe, efficient and cost-effective dam upgrade design

Backgroud

Bradys Dam is an existing earth and rockfill embankment dam that impounds Bradys Lake, which is one of the storages powering Hydro Tasmania’s Tungatinah Power Station. Entura was commissioned to undertake the Bradys Dam detailed design, for which Hydro Tasmania intends to do the implementation in the future.

Findings from a Dam Safety Review (DSR) of Bradys Dam indicated that work is required on both the upper and deeper parts of the core and on the filter materials, which led to Hydro Tasmania considering upgrade works for the dam.

The preferred option to progress to detailed design was construction of crest filters in the upper embankment and a partial-height coarse rockfill berm. This combination would eliminate the need to excavate as deeply to install compatible filters over the full height of the dam, removing the need to take the dam out of service and reducing the excavation volumes, material supply requirements and the risk of instability during construction.

A key constraint, however, was that the dam would need to remain operational throughout any upgrade works. This would be a key consideration for managing construction and dam safety risks. The presence of Aboriginal heritage around the site was another consideration in developing the design and in any arrangements for future construction works. The presence of privately owned shacks and powerlines in the vicinity also needed to be taken into account.

Solution

Entura’s work on the detailed design included preparation of tender and permit application documents such as detailed design drawings, technical specifications, design report, dam safety emergency plan for construction, geotechnical investigation reports, preliminary sediment control and erosion protection plans, and safety in design. The project also included a range of supporting tasks and studies, including:

  • environmental studies and surveys (aquatic values assessment, fauna and flora assessment, sediment mobilisation potential)
  • constructability assessment, including construction risk assessments (practical solutions needed to be identified to install the crest filters given the limited construction width on the crest, the potential risk of overtopping and instability during construction, and filter trench excavations and backfilling within the core)
  • hydrology and hydraulic assessments to:
    • determine the maximum lake operating level during construction to mitigate overtopping risk, including a dam failure assessment (dam break modelling, consequence category assessments, and mapping flood extents)
    • determine the rainfall of flood inflow forecasting trigger levels and subsequent operating rules to further mitigate overtopping risk during construction
  • development of construction cost estimates and schedule
  • geotechnical investigations and reporting, including cone penetration testing and boreholes to confirm the condition of the core of the dam, test pitting to confirm the geometry of the embankment and also to get a better understanding of the foundation conditions for the coarse rockfill berm (this information was used to better define the proposed foundation level, ultimately reducing the potential footprint, estimated construction quantities, schedules and costs)
  • development of a draft Dam Safety Emergency Plan (DSEP) Addendum for Construction
  • design for decommissioning the existing dewatering conduit.

Outcome

This project demonstrates Entura’s extensive multidisciplinary expertise in managing complex water systems and providing practical solutions to minimise dam safety risks during major constructions and upgrades. Our detailed studies have provided Hydro Tasmania with reliable information and specialist advice to guide robust, risk-informed decision-making for the proposed raised dam upgrade.