Huge rain event driving major Warragamba Dam spill

Huge rain event driving major Warragamba Dam spill

21 March 2021

Heavy rain is generating a major spill event at Warragamba Dam, with inflows from the catchment likely to sustain large-volume spill activity for days.

Current projections indicate that this weather event could result in flooding similar in scale to the 1961 flood.

Warragamba Dam spillway is currently releasing water at a rate of 450 gigalitres per day (GL/day) and that rate could increase as inflows to the dam storage continue to rise. (By comparison Sydney harbour is estimated to hold 500 GL).

Modelling indicates that approximately 1500 GL of water will flow into the dam in the seven days since the extreme weather event commenced, a figure that represents 75% of the dam’s storage capacity of 2000 GL.

Flow data up to this morning (Sunday, 21 March) indicates that half of the floodwaters in the Hawkesbury-Nepean river system downstream of the dam were from tributary flow, not the dam.

A spokesperson for WaterNSW said in order to have had enough capacity in the dam to capture the inflows from this event, Warragamba would have needed to be taken down to 25% of storage capacity, prior to the rain event.

The lowest level Warragamba Dam has ever been was 38.8% in 2004 during the Millenium drought.

While WaterNSW is not authorised to lower the storage based on weather forecasts, significant pre-releases prior to a flood event could make flooding events more dangerous. Downstream tributaries could be inundated in a big rain event (as they are now), and the addition of extra water released from the dam could increase downstream impacts.

WaterNSW confirms that 130 GL of water has been released in recent months to hold the storage at around 1m below full supply level. This was done in accordance with strict operating protocols and resulted in the need for dam releases during this event being delayed by a full day, allowing greater time for preparedness.

WaterNSW has established a dedicated 24-hour incident management team that is working closely with the Bureau and the NSW State Emergency Service to monitor weather and inflows to dams.

WaterNSW will continue to work closely with Sydney Water and NSW Health throughout the rain event, to watch for potential water quality impacts on untreated supply.

For weather forecasts and flood warnings visit http://www.bom.gov.au/

For emergency information visit https://www.ses.nsw.gov.au/ or contact 132 500

For information on WaterNSW dams visit www.waternsw.com.au or contact 1300 662 077

Media contact – Tony Webber - 0428613478

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