Upper Reservoir Dam upgrade
Client: TasWater
Location: Tasmania / Australia
Date: August 2018 - June 2022
Providing detailed design and construction supervision for a dam upgrade
Background
Over the last decade, Entura has provided ongoing services to TasWater to assess the risk of TasWater’s portfolio of dams and to assist TasWater in the development of its dam safety improvement program. Clear identification of dam risks informs TasWater’s ability to prioritise the resources, finances and time required to undertake necessary remedial measures.
Entura has successfully assisted TasWater with dam safety surveillance – from identification, investigation and analysis of dam safety issues, through to design, implementation and performance review of mitigation measures.
Entura’s strength in undertaking the upgrade of Upper Reservoir Dam lies in our detailed understanding of every aspect of the dam and its appurtenant structures.
Upper Reservoir Dam is an 18m-high puddle-clay-core earthfill embankment located on the Sandy Bay Rivulet, approximately 4 km southwest of Hobart. It is one of three dams that comprise the Waterworks System (Lower Reservoir Dam, Upper Reservoir Dam and Ridgeway Dam). The dam was constructed between 1885 and 1888, with upgrade works undertaken in 2000, consisting of works on the bypass channel and construction of a small filter buttress on the downstream toe of the embankment. The dam is currently owned and operated by TasWater (previously Hobart Water and Southern Water).
Upper Reservoir Dam is assigned a ‘High A’ sunny day failure consequence category and an ‘Extreme’ flood failure consequence category in accordance with the ANCOLD Guidelines on the Consequence Categories for Dams.
Entura completed a Dam Safety Review (DSR) in 2017 and revealed that the original embankment design was not compliant with modern engineering design standards, with an unacceptably high risk of piping failure mainly associated with the outlet tunnel. The DSR also identified a high risk of flood overtopping failure. This resulted in an unacceptable societal risk and individual risk when assessed against the ANCOLD Guidelines on Risk Assessment.
In response to Entura’s assessment and recommendations, TasWater initiated a business case to upgrade the dam to comply with the ANCOLD Guidelines on Dam Safety Management.
Solution
The scope of the upgrade was developed based on the recommendations in the DSR and subsequent investigations, and included:
- a localised filter around the outlet tunnel adjacent to the existing toe filter to mitigate the risk of piping along the outlet tunnel
- works on the embankment crest, including crest raising to mitigate the flood overtopping risk, and incorporation of a crest filter trench to mitigate the risk of piping through the crest
- stability and drainage improvement works around the reservoir rim to divert stormwater away from the dam reservoir
- removal of redundant pipelines in the embankment
- stability and vegetation clearance works on the bypass channel and spillway
- intake and outlet works refurbishment, including replacement of deteriorated valves and pipework
- instrumentation upgrade, including installation of additional piezometers in the embankment.
Services provided
- Dam safety review
- Dam upgrade detailed design
- Geotechnical investigations
- Flood hydrology study and modelling
- Hydraulic assessment
- Pipework design
- Dambreak modelling and inundation mapping
- Dam safety risk assessment
- Construction supervision
Outcome
Entura completed the scope, which included:
- geotechnical investigations to obtain additional design data
- historical and Aboriginal heritage studies and impact assessment
- flora and fauna studies and environmental impact assessment
- stakeholder and community consultation
- flood hydrology and spillway hydraulic assessment
- upgrade options assessment including workshop with client and stakeholders
- dambreak and consequence assessment
- dam safety risk assessment update
- detailed design for the upgrade works
- intake and outlet improvement works
- reservoir and site drainage work 3D modelling and improvement
- construction tender support.
TasWater undertook the necessary upgrades to the dam during 2021, with Entura performing the role of Principal’s Engineer during construction, providing an expert team responsible for the ongoing construction supervision and sign-off in accordance with the Water Management (Safety of Dams) Regulations 2015.
Entura successfully demonstrated our value for money to TasWater in delivering this project, which we attribute to our long history of working on Upper Reservoir Dam since the early 1990s. In addition, through our long-term good working relationship with TasWater, we have built a collaboration based on trust, with a comprehensive understanding of TasWater’s needs and expectations. Along with our previous experience in similar dam upgrade works, we brought a unique value proposition to TasWater both in terms of quality and content for the projects.
