Mount Bold Dam Safety Upgrade

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Mount Bold Reservoir

South Australia’s largest reservoir will be getting a major safety upgrade. Mount Bold is an important storage for both natural inflow and water pumped from the River Murray, with a total capacity of around 46 billion litres. It was constructed between 1932 and 1938 and is approximately 25 kilometres south-east of Adelaide, within the Onkaparinga catchment in the southern Mount Lofty Ranges.

This reservoir is not directly connected to the water reticulation system. Instead, water is released as needed to maintain an adequate level at the Clarendon Weir, and from there is diverted to Happy Valley Reservoir for treatment before being supplied as safe, clean drinking to our customers across metropolitan Adelaide.

The surrounding reservoir reserve comprises more than 5,500 hectares of land, including three Trees for Life sites, and much native vegetation of conservation significance. More than 160 native animals call the reserve home, including the Southern Brown Bandicoot. The reserve is predominantly Stringy-Bark woodland, plus a Manna Gum woodland, swamps and creeks.

In the 1930s Mount Bold was designed and built to maximise water storage for Adelaide, and not a flood control function. While some flood attenuation can be provided when the dam is drawn down, this is limited at higher storage levels due to the initial design.

About the Mount Bold safety upgrade project

This significant project will strengthen the dam structure against earthquake, and its ability to safely pass any large flood event, including protection to downstream communities. This investment will align Mount Bold with the Australian National Committee on Large Dams’ (ANCOLD) updated guidelines that we follow to ensure best practice in the safe operation of our dams and reservoirs.

In terms of scale and complexity, this project will be one of our single biggest in the coming decade, with works expected to take place from 2022 to 2028.

Currently we are investigating design options.

Involving the community

By investigating the extent and forecast frequency of downstream flooding we have developed spillway options for design and cost evaluation. We are working with the local community and other stakeholders as we plan this project so we make an informed decision about the level of protection desired, and the funding necessary to achieve this.

In meeting ANCOLD’s improved standards and guidelines that support the ongoing protection of downstream communities in the event of flood and earthquake, we have modelled that the Onkaparinga River downstream of Mount Bold can pass 170m3/s (metres cubed per second) without flooding properties (figure 1a). Flows above this are expected to have a one in 15-year recurrence, that is significantly reduced by installing flood mitigation on the spillway (figure 1b).

Design options to maximise the value to the community

Achieving as much value to the community through this investment is important to us, and we know it’s important to you. A number of options are being developed to make sure every opportunity is explored to increase the benefits of the very practical role Mount Bold plays in storing water for metropolitan Adelaide.

For example:

  • To achieve the level of flood mitigation outlined above, the dam crest would be raised and a slot created to enable low flows to pass. This enables the full supply level to be maintained and removes the gates (figure 2). Modelling shows the spillway slot would be between 11 and 18 metres wide, and up to three metres deep.
  • To offset energy costs, we’re investigating the use of mini-hydro at the dam.
  • Opening South Australia’s reservoirs for the public to visit is a priority of the South Australian government. Through this project, we’re also looking at ways to help create a great experience for visitors when the Mount Bold reservoir reserve opens in future for people to visit and enjoy.

Next steps

Further investigation about the impact of raising the spillway crest on upstream vegetation in the catchment is needed, as periodically this would be inundated. We will continue to engage with the community and our stakeholders about the level of flood mitigation contributed by these slot dimensions and refine cost estimates.

For more information

If you have any questions, please contact our Stakeholder Engagement Advisor Selene Seet at selene.seet@sawater.com.au or telehpone (08) 7424 1820.

Mount Bold Reservoir

South Australia’s largest reservoir will be getting a major safety upgrade. Mount Bold is an important storage for both natural inflow and water pumped from the River Murray, with a total capacity of around 46 billion litres. It was constructed between 1932 and 1938 and is approximately 25 kilometres south-east of Adelaide, within the Onkaparinga catchment in the southern Mount Lofty Ranges.

This reservoir is not directly connected to the water reticulation system. Instead, water is released as needed to maintain an adequate level at the Clarendon Weir, and from there is diverted to Happy Valley Reservoir for treatment before being supplied as safe, clean drinking to our customers across metropolitan Adelaide.

The surrounding reservoir reserve comprises more than 5,500 hectares of land, including three Trees for Life sites, and much native vegetation of conservation significance. More than 160 native animals call the reserve home, including the Southern Brown Bandicoot. The reserve is predominantly Stringy-Bark woodland, plus a Manna Gum woodland, swamps and creeks.

In the 1930s Mount Bold was designed and built to maximise water storage for Adelaide, and not a flood control function. While some flood attenuation can be provided when the dam is drawn down, this is limited at higher storage levels due to the initial design.

About the Mount Bold safety upgrade project

This significant project will strengthen the dam structure against earthquake, and its ability to safely pass any large flood event, including protection to downstream communities. This investment will align Mount Bold with the Australian National Committee on Large Dams’ (ANCOLD) updated guidelines that we follow to ensure best practice in the safe operation of our dams and reservoirs.

In terms of scale and complexity, this project will be one of our single biggest in the coming decade, with works expected to take place from 2022 to 2028.

Currently we are investigating design options.

Involving the community

By investigating the extent and forecast frequency of downstream flooding we have developed spillway options for design and cost evaluation. We are working with the local community and other stakeholders as we plan this project so we make an informed decision about the level of protection desired, and the funding necessary to achieve this.

In meeting ANCOLD’s improved standards and guidelines that support the ongoing protection of downstream communities in the event of flood and earthquake, we have modelled that the Onkaparinga River downstream of Mount Bold can pass 170m3/s (metres cubed per second) without flooding properties (figure 1a). Flows above this are expected to have a one in 15-year recurrence, that is significantly reduced by installing flood mitigation on the spillway (figure 1b).

Design options to maximise the value to the community

Achieving as much value to the community through this investment is important to us, and we know it’s important to you. A number of options are being developed to make sure every opportunity is explored to increase the benefits of the very practical role Mount Bold plays in storing water for metropolitan Adelaide.

For example:

  • To achieve the level of flood mitigation outlined above, the dam crest would be raised and a slot created to enable low flows to pass. This enables the full supply level to be maintained and removes the gates (figure 2). Modelling shows the spillway slot would be between 11 and 18 metres wide, and up to three metres deep.
  • To offset energy costs, we’re investigating the use of mini-hydro at the dam.
  • Opening South Australia’s reservoirs for the public to visit is a priority of the South Australian government. Through this project, we’re also looking at ways to help create a great experience for visitors when the Mount Bold reservoir reserve opens in future for people to visit and enjoy.

Next steps

Further investigation about the impact of raising the spillway crest on upstream vegetation in the catchment is needed, as periodically this would be inundated. We will continue to engage with the community and our stakeholders about the level of flood mitigation contributed by these slot dimensions and refine cost estimates.

For more information

If you have any questions, please contact our Stakeholder Engagement Advisor Selene Seet at selene.seet@sawater.com.au or telehpone (08) 7424 1820.