Warragamba Dam spill update

The Warragamba catchment received a significant amount of rain from midnight at an average of 100 mm over 6 hours, with the sub-catchment, close to the dam wall, receiving ~130mm. Up to 200mm was received over 24 hours in some areas.

This created flash flooding in the catchment and into the dam, causing it to spill.

Warragamba Dam is spilling at a rate of around 200 gigalitres per day (GL/day) with that rate continuing to rise.

With the flooding in the Hawkesbury-Nepean Valley the spill will contribute to the flood waters already in the system.

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.

Greater Sydney water supply dams currently spilling are Warragamba, Tallowa, Nepean and the Blue Mountains dams. Woronora and Cataract dams are also expected to spill today.

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 – 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.

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

Background – operating rules

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.