The dam in Adelaide’s south – which opened its floodgates on Friday for the first time in five years – has long been mooted for an upgrade due to the impact on downstream residents when water is released from the 46.4 gigalitres reservoir.

Concept designs released today show a plan to remove the dam’s remotely controlled spillway gates and replace them with one primary spillway in the wall’s centre.

According to SA Water, the central spillway – three metres wide and 5.6 metres high – will allow the reservoir to retain more water during extreme weather events, including withstanding a “one in one hundred year” flood event when the dam is already full.

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It will also allow water releases from the reservoir to occur three times slower, SA Water said, minimising the potential flooding impact on downstream residents.

Once our Mount Bold project is complete, the dam will be able to safely pass up to a ‘one in five million year’ weather event.

The state and federal government along with Onkaparinga Council earlier this year committed an additional $12.7m to fund the upgrade.

InDaily asked SA Water for a full project cost estimate but was told a final figure would not be released until “we have worked through optimising our concept as part of the detailed design, planning and procurement process”. 

SA Water general manager of sustainable infrastructure Amanda Lewry said the upgrade “will be one of the largest capital projects in our history once completed”.

“To elevate the dam’s safety, we’ll be constructing significant concrete protection downstream of the dam to reduce the risk of erosion, while strengthening the wall with new concrete to increase its capacity to withstand earthquakes and extreme flood events,” she said.

“Our proposed new design for the dam will facilitate self-managed flows to enhance the current gated system, which is a series of eight gates that are remotely controlled to release water.

“This is more efficient, allowing for the retention of water during extreme weather events, due to the dam’s ability to temporarily store more water and release it at a rate that’s three times slower compared to having all the gates open.

“When the reservoir is already full, the dam’s primary spillway will play this role up until around a ‘one in one hundred year’ event, where the other two spillways will be activated to help release water and preserve the dam’s structural integrity.”

Lewry said the project was “the most complex upgrade in our portfolio” and has been planned since 2010.

“We’re now optimising our concept as part of the detailed design phase, with scope for further refinement to ensure we’re delivering a cost-effective project,” she said.

“Once our Mount Bold project is complete, the dam will be able to safely pass up to a ‘one in five million year’ weather event.

“Importantly, our ongoing inspections confirm the dam remains safe, with our upgrade keeping it in step with the water industry’s contemporary safety standards.”

It comes after the reservoir’s floodgates opened earlier this month for the first time in five years, as ongoing spring rains brought Mount Bold close to full capacity.

SA Water reported the reservoir had taken on 30 billion litres of water since August, taking it from 30 per cent to 94 per cent capacity.

The controlled release saw water flow down the Onkaparinga River, catching some residents off guard.

Clarendon resident Susan Jeanes said locals living near waterways downstream from Mount Bold Reservoir were not warned about the release of water that gushed past homes and flooded her garden, the local park and an outbuilding at a neighbour’s property further down.

She said it brought back fears of flooding similar to when the gates were opened in 2016 and properties were inundated.

Water flooded parts of Old Noarlunga at the time, sending residents’ insurance premiums soaring. A safety upgrade of the reservoir has since happened.

The water released last week also flooded a concrete walking bridge linking the town of Clarendon to nearby carparking, the bowling club, and netball club, according to Jeanes, causing issues for the Clarendon Spring Fair on the weekend.

“Our garden doesn’t really matter but it’s just the fact that you suddenly have this water gushing by with no warning, and it’s pushed down a lot of baby gum trees further down from us,” she said.

A view of Susan Jeanes’ garden on Friday. Photo: Susan Jeanes/supplied

Jeanes said she understood current and forecast rain events required some preparation and gradual releases had occurred, but this had “caused a little chaos”.

Mount Bold Reservoir was constructed in 1938 and remains South Australia’s biggest reservoir.

Its stored water can be directed to Happy Valley Reservoir, which supplies water to around 450,000 Adelaide customers, according to SA Water.