New spillway design for Mount Bold dam in Adelaide Hills allowing it to hold more water with a slower release
Water being released for the first time in five years from Mount Bold Reservoir in the Adelaide Hills after heavy rainfall in 2022. Inset: An SA Water 2022 concept design for the the dam wall.
Images courtesy SA Water
SA Water designs, released in 2022, for an upgrade of Mount Bold Reservoir, South Australia’s biggest, came with a prediction they would give the dam capacity to withstand a “one in five million years” weather event.
The upgrade of the dam in the Adelaide Hills was also aimed to reduce the impact on downstream residents when water was released from the 46.4 gigalitres reservoir. The 2022 concept designs involved removing the dam’s remotely-controlled eight spillway gates and replacing them with one primary spillway in the wall’s centre. The central spillway – three metres wide and 5.6 metres high – would allow the reservoir to retain more water during extreme weather, including withstanding a one-in-100-years flood event when the dam was already full.
The new design for the dam allowed self-managed flows. This was more efficient, allowing water to be retained during extreme weather events, due to the dam’s ability to temporarily store more water and release it three times slower compared to having all the gates open. When the reservoir was already full, the dam’s primary spillway would be activated up to around a one-in-100-years weather event when the other two spillways would be activated.
Water releases being three times slower would minimise potential flooding impact on downstream residents. SA Water said significant concrete protection would be built downstream of the dam to reduce the risk of erosion, while the dam wall would be strengthened with concrete to increase its capacity to withstand earthquakes and extreme flood events.
The South Australian government looked in 2007 to expand the reservoir fivefold but the plan was criticised by the opposition for not providing any new water and by ecologist David Paton as threatening surrounding wildlife. In 2010, Mount Bold Reservoir was opened for the first time since 2005 after large rainfall. Water flushed out the Onkaparinga River catchment caused several flooded roads but removed debris built up for many years.
Mount Bold Reservoir was constructed in 1938 and remained South Australia’s biggest reservoir. Its stored water can be directed to Happy Valley Reservoir, which supplies water to around 450,000 Adelaide customers. The state and federal government along with Onkaparinga Council in 2022 committed an additional $12.7m to fund the dam’s upgrade.
