Five hundred megalitres of water will be released from Porter’s Creek Dam to allow for repairs to be made to the aging dam wall.
A trial release is expected to take place on Friday, with daily discharges of up to 60 megalitres expected to continue over the next two weeks to bring the water level down to three metres below the dam’s full storage level.
The dam is currently 900 metres below capacity and, according to Shoalhaven Water director Carmel Krogh, the water must be release to mitigate safety concerns due to instability of the dam caused by excavation downstream which could undermine the foundation.
The lower water level will also ensure adhesion between the new concrete and the existing wall.
Ms Krogh said roadwork has been carried out on a 3.6 kilometre section of Porters Creek Dam Road and a site office set up while the $6.5 million remediation works are carried out.
The project will see the preparation and placement of about 4,000 cubic metres of concrete along the downstream face of the dam along with associated outlet and access works.
Councillor Patricia White said concerns had been raised about the release of water from the dam which would feed into the Clyde River, effecting water levels downstream.
Ms Krogh said a Reservoir Water Level Management Strategy (RWLMS) had “gone into considerable detail” covering how water quality prior and during planned releases will be managed.
She said the objective of the strategy, in conjunction with National Parks and Wildlife, was to achieve nil effects, such as pollution, downstream.
“Porters Creek Dam has two destratification units which alleviates a significant key concern associated with dewatering dams, which is temperature pollution,” Mrs Krogh added.
“A common risk when releasing waters from dams is the variation in temperatures between dam’s water released and receiving bodies and associated risk to aquatic ecology.”
The discharge volume has been limited to 60 megalitres per day through the main scour and Friday’s trial will confirm rates of discharge.
“The trial release is nominated to be undertaken over a duration of six hours, during which rates associated duration and water quality will be verified,” Ms Krogh explained.
Foundations for the proposed works will include demolition of part of the existing dam stilling basin and establishment of a concrete batch plant onsite.
The dam was constructed in 1968 and provides about 30 per cent of Southern Shoalhaven’s drinking water supply.
