Operating Warragamba when at 100% capacity

Operating Warragamba when at 100% capacity

20 August 2020

Warragamba Dam is a water supply dam, supplying water to more than 5 million people in Sydney and the lower Blue Mountains. WaterNSW operates Warragamba Dam using set procedures and protocols, including H14 protocol which stipulates the gates operational procedure for Warragamba Dam during a flood event.

If you are in a flood zone, plan ahead and be prepared:


Is Warragamba dam operated to reduce flood risk in the Hawkesbury-Nepean Valley?

All dams are either managed as water storage dams or flood mitigation dams (or both). For water storage dams, the purpose is to store the maximum amount of water to ensure the longest possible time of water supply. Flood mitigation dams are the reverse where levels are kept low to provide the maximum amount of airspace for flood inflows and reduce impacts in downstream valleys. As Warragamba is the primary supply for the Greater Sydney region, the dam is operated to capture and store water. When water storage dams are below 100% that is by the fact of having airspace available, the attenuation of high inflow events until the storage capacity is reached.

How is Warragamba Dam operated when there are high inflows and the storage is almost full?

WaterNSW only operates the dam at the required -0.3 to -1m of Full Supply Level (FSL) when full. Small releases are made at 100% to maintain this level and avoid the main radial gates repeatedly opening and closing due to small increase in storage or wave action.

It is more practical and safer to deliver a controlled release to drop the level through the radial gates by a small amount (less than a metre) to maintain the level just below 100%. This avoids repeated opening and closing of the gates.

There is an operational procedure (H14 operational protocol) for operation of gates at Warragamba dam, which is designed to manage the inflows during a flood event to protect the community.

For WaterNSW to release any more water prior to a forecast rain event (noting it would then not have that water stored in the event the rain didn’t come) would be a breach of the key operation objective of the dam being the provision of water security for Greater Sydney.

What conditions are responsible for causing flood conditions in the Hawkesbury-Nepean Valley?

The major rainfall events that cause flooding in the Hawkesbury-Nepean valley are primarily East Coast Lows. These systems are dynamic and prove difficult to forecast and track along the NSW coast. Reliable forecasts from BOM come only 4 days in advance of an event but have high uncertainty on the location of the heaviest rain and how much rain will occur in the catchment, or not. Based on the size of the Warragamba catchment, large rainfall events in the southern highlands may take several days to reach the dam.

It is important to note that the spills from Warragamba dam are not the only source of flooding in the Hawkesbury-Nepean Valley. Other downstream tributaries including the Grose and Colo rivers along with the Upper Nepean catchment also contribute to flooding. For example, in the March 2021 floods, water from Warragamba Dam only comprised 60% of all flood waters that reached Windsor.

The SES is the combat agency in NSW and takes the lead in managing flood impacts.

Official warnings and flood advice are provided by The Bureau of Meteorology (the BoM) on their website and the NSW State Emergency Services (NSW SES) on radio stations and NSW SES social media.

The community are encouraged to understand the risk and obtain up to date information on preparing for floods by visiting: https://www.ses.nsw.gov.au/hawkesbury-nepean-floods/

Why doesn’t WaterNSW make releases to create room for predicted inflows from forecasted rain events?

First and foremost, WaterNSW manages water supply dams and not flood mitigation dams and must operate the dam within strict regulations.

The SES takes the lead in managing flood impacts and works closely with WaterNSW and the BoM during flood events.

Pre-releasing water can lead to a number of issues, including:

- Exacerbating downstream flooding

- Bringing forward impacts of downstream flooding, therefore reducing the critical period for evacuating communities

- Artificially inducing drought conditions, should the dam not re-fill – negatively impacting critical water supply for Greater Sydney

Pre-releasing may also have limited benefits. For example, to avoid a spill in the March 2021 rainfall event, WaterNSW would have had to lower the dam to levels lower than that experienced in the recent drought (approx. 60%). That equates to around 3 years of water supply to Greater Sydney.


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