Mikany Dam upgrade

Client: TasWater
Location: Tasmania / Australia
Date: July 2014 - November 2023  

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 developing 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 Mikany Dam lies in our detailed understanding of every aspect of the dam and its appurtenant structures.

Mikany Dam is located on the Deep Creek approximately 6 km south-east of Smithton in the north-west of Tasmania. It impounds Lake Mikany, a 2,700 ML storage, with two earth and gravel fill embankments, namely Embankment A (left, 8 m high and 281 m long) and Embankment B (right, 6 m high and 202 m long). Another small Embankment C provides access to an island in the centre of the lake. The dam was constructed in 1972 as a primary source of municipal water supply for the townships of Smithton and Stanley. The dam is currently owned and operated by TasWater (previously Cradle Coast Water and Cradle Mountain Water).

Mikany Dam is assigned a ‘High B’ consequence category for both sunny day failure and flood failure scenarios in accordance with the ANCOLD Guidelines on the Consequence Categories for Dams.

Entura completed a Dam Safety Review (DSR) in 2015 and revealed that the original embankment design was not compliant with modern engineering design standards with respect to piping, slope stability and flood capacity. The dam posted an unacceptably high risk of piping through the embankment and liquefaction of foundation materials. 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:

  • constructing a downstream filter buttress against the embankment to mitigate the piping risk
  • constructing a downstream rockfill berm to improve the embankment stability under normal loading conditions and to ensure adequate factor of safety against instability due to seismically induced liquefaction of the foundation material
  • modifying the embankment with core and crest raising to improve the flood capacity
  • replacing the spillway to improve the flood capacity
  • adding erosion protection measures in the area between the new spillway and the embankment
  • upgrading instrumentation.

Services provided

  • Dam safety review 
  • Dam upgrade detailed design 
  • Geotechnical investigations 
  • Flood hydrology study and modelling 
  • Spillway hydraulic assessment 
  • Outlet and 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 embankment and spillway upgrade works
  • detailed design for the outlet upgrade works
  • site power supply and electrical design
  • construction tender support
  • construction supervision.

TasWater completed the upgrade works during 2022. Entura performed the role of Principal’s Engineer during the 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.

In delivering this project, Entura successfully demonstrated value for money to TasWater, which we attribute to our long history of working on Mikany Dam since the early 2010s. 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.

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