Operating Warragamba when at 100% capacity

Operating Warragamba when at 100% capacity

20 August 2020

Find out how WaterNSW operate Warragamba dam when storage capacity is at 100%.

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. 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 Great Sydney region, the dam is operated to capture and store water.   When water storage dams are below 100% there 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.

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 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 prima 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 serval days to reach the dam.

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

The SES takes the lead in managing flood impacts and works closely with WaterNSW and the BoM during flood events. Similar conditions were present in 2016, with Warragamba full for a period of 4 to 5 months without a major flood event occurring. Conditions post this period turned extremely dry with Warragamba dam dropping to low levels by early 2020. If releases were made in 2016 to create airspace the storage would have been at even lower levels by early 2020, negatively impacting critical water supply for Greater Sydney.

Is there a plan to improve the flood resilience of the Hawkesbury-Nepean Valley downstream of Warragamba dam?

The Warragamba Dam Raising project will provide flood mitigation to reduce the risk to life and property in the Hawkesbury-Nepean Valley. This is achieved through raising the level of the spillway crests above the existing full supply level to temporarily hold back inflows. The planned extra space in the dam will be solely for flood mitigation and not for increased supply.

While a range of other infrastructure and non-infrastructure outcomes are included in the government’s Hawkesbury-Nepean Valley Flood Risk Management Strategy and must be part of the solution for managing ongoing risk, no other mitigation measures were found to achieve the same risk reduction as the proposed Warragamba Dam Raising project.

WaterNSW, as owner and operator of the dam, has been tasked with leading the environmental assessment and detailed concept design for the project. WaterNSW is consulting widely about the effects and benefits of the proposal to inform the environmental assessment and concept design. During its public exhibition, the proposal will be assessed on its merits and all interested stakeholders will provide further comment. The final decision on the dam raising proposal will only be made after all environmental, cultural, financial and planning assessments are complete.


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Tuesday 19 October