Warragamba Dam spill update

At 5am Sunday, Warragamba dam was spilling at a rate of 122 Gigalitres (GL)/day and decreasing.

Water continues to flow into the dam from streams in the catchment but this is slowing.

The radial gates of the dam are likely to be closed later on Sunday. However, smaller releases of water will continue to be made from the dam over the next few days as the storage is drawn down to between 0.3m and 1m below full supply level for operational reasons. Other Sydney dams currently spilling are Tallowa, Nepean and Woronora dams and the Blue Mountains storages, except Upper Cascades.

• Woronora began spilling at 11pm on Saturday and is currently spilling at a rate of 0.6 GL/day and stable.
• Nepean Dam is currently spilling at 4 GL/day and decreasing.
• Tallowa Dam is currently spilling at 29.6 GL/day and decreasing.

Warragamba Dam remains closed to the public. Subject to an operational and public safety review, it will likely reopen on Monday 8 April.

WaterNSW continues to work with the Bureau of Meteorology and SES to monitor the weather event impacting the state.

Follow the Bureau of Meteorology for flood advice and warnings (http://www.bom.gov.au/), and the NSW State Emergency Service as the lead agency for emergency response (https://www.ses.nsw.gov.au/)

Background

Warragamba Dam reached full capacity and started spilling at 5.45am Saturday 6 April following heavy rainfall overnight Friday at the upper end of Bureau forecasts.

The Warragamba catchment received an average of 100 mm over 6 hours from midnight Friday to 6am Saturday, and up to 200mm over 24 hours in some areas. Rainfall overnight Friday in the sub-catchment, close to the dam wall, was about 130mm.

The Warragamba Dam catchment area covers 9000 sq/km and stretches across much of the Blue Mountains to Goulburn in the south, and to Lithgow in the north-west.

As manager of the state’s major supply dams, WaterNSW operates Warragamba Dam in line with NSW Government operating rules and protocols.

These rules do not permit water being released to reduce the storage level ahead of predicted rainfall for flood mitigation purposes. This reflects the dam’s critical role in supplying 80% of Sydney’s drinking water.

Background – dam operations

During times of flood, the gates on Warragamba Dam automatically open and close based on water levels.

After rainfall has occurred and when the water rises above full supply level, the gates progressively open in sequence. As water levels begin to drop, the gates will progressively close in reverse sequence.

The gates can be manually operated to reduce the level of water in the lake for operational or maintenance reasons only outside of flood operations.

The dam gates are designed to safely pass extreme floods, much larger than the worst flood on record for the Hawkesbury-Nepean Valley.