Warragamba Dam auxiliary spillway
Warragamba Dam auxiliary spillway
The auxiliary spillway was built to divert excess floodwaters.
Safety Management of Warragamba Dam
In New South Wales, Dams Safety NSW (DSNSW) is the state regulator for dam safety. It is responsible for developing and implementing regulation with enforceable safety standards for the effective dam safety management to protect life, property, and the environment from dam failures.
WaterNSW is obligated to meet the requirements of DSNSW under the Dam Safety Act 2015 and Dam Safety Regulation 2019. Hence WaterNSW dams are designed, constructed, operated, and maintained in a safe manner that complies with regulatory requirements and appropriate industry good practice.
WaterNSW has an established framework for dam safety management activities and decision making. A robust technical assurance and governance framework was established that includes independent review of dam safety decisions. The dam safety management practices are regularly reviewed and audited to ensure it remains appropriate and in line with good practice.
The safety of Warragamba Dam is important due to the significant consequences to persons, property and the environment downstream of the dam. The importance of the dam also lies in the fact that dam provides approximately 80% of Sydney’s drinking water supply. Hence, since commissioning in 1960, Warragamba Dam underwent 2 major upgrades in 1990 and 2002 to cater for changes to the Probable Maximum Flood (PMF). The 1990 upgrade involved raising the dam by 5.11 m and installing a series of post-tensioned anchors. In 2002 the auxiliary spillway was completed, which sits on the right abutment. Additional upgrade works on the spillway gates have been carried out subsequently between 2006 and 2011 to improve the reliability of gate operations to allow greater flow to safely pass through the gates during extreme flood events.
WaterNSW considers that dams must be sustained in the long term to secure societal value. To ensure sustainability of Warragamba Dam, all reasonably practicable efforts are made to prevent and mitigate potential failure scenarios and accidents. This includes optimising the operation and maintenance of the dam over the entire life cycle and ensuring that the desired levels of performance and dam safety are met.
How the auxiliary spillway works
Warragamba Dam’s existing crest gates and central spillway manage normal discharge of floodwaters up to around a 1:15,500 AEP flood event at which point the auxiliary spillway starts to support the dams discharge capabilities. .
In the event of an extreme flood, five erodible earth and rockfill embankments called 'fuse plugs', constructed across the upstream crest of the auxiliary spillway, would progressively wash away (or ‘trigger’) when overtopped by the rising floodwaters. The fuse plugs are approximately 14m high and the crests are set at different levels, so the embankments trigger at different flood levels. As each fuse plug progressively washes away, the capacity of the auxiliary spillway increases to pass the incoming floodwaters.
The downstream end of the auxiliary spillway chute has a ‘flip bucket’ structure which dissipates the energy from the flood discharges flowing through the spillway, prior to flow entering the river channel downstream of the dam. This minimises erosion of the riverbed and banks.