Filling the enlarged Cotter Reservoir

Using the best available assumptions about future weather, ACTEW projects that there is a 95% chance that the enlarged Cotter Reservoir will fill in under 5 years from its completion in 2012.

Relatively good flows

The Cotter River is the most reliable and productive catchment in the ACT. The best option for increasing our storage is a dam at the Cotter. While flows to our Cotter system have been down by around 40 per cent in the last ten years, they are still greater than the long term averages of the other catchments. Flows into Googong and the site identified for a Tennent Dam are down by over 70 per cent.

Operational improvements

As the new dam will be much bigger than the existing Cotter Dam, the chance of it overflowing will be much less. This means the Cotter Pump Station, which is now mainly pumping water from the existing Cotter Dam to prevent it from overflowing, can be dedicated to pumping water from the Murrumbidgee River. This allows inflows into the new dam to be stored while ACTEW supplies water using its other sources. This operating change is only possible because of the Enlarged Cotter Dam and related investments that enhance ACTEW’s ability to draw and treat water from the Murrumbidgee River. The operating change means that in a very low flow year we will be able to store around 15 gigalitres in the new dam - 20% of its capacity – and in years like those in the last decade we will be storing around 40 gigalitres – 50% of capacity.

Usefulness of diverse water supply system

With ACTEW’s new investments in water supply there will be less reliance on the Cotter River. For example, the advent of the Murrumbidgee to Googong Water Transfer and the Tantangara Transfer projects will on average increase water available in the Googong Dam, meaning Googong can be used more than it has been in the past. This eases demand on the Cotter River. With eased demand on the Cotter River there is also a greater chance that Bendora Dam will overflow, its spills going into the Cotter Dam. Both these factors will add to the speed with which the enlarged Cotter Reservoir will fill.

Expected time to fill

If we consider the range of weather we are likely to experience in the next few years, including very dry years like those seen since 2001, it is very likely – a 95% chance - that the enlarged Cotter Reservoir will fill in less than five years. This also means that there is a 5% chance the new dam will fill over a period longer than five years. However, in this context it is important to consider that the new dam does not have to be full to be useful as a supply source. Any storage in excess of the existing dam size will be useful as it can be captured and used for water supply.

How do we estimate filling time

ActewAGL, on behalf of ACTEW, use a computer model of the water supply system to understand how the system performs under different conditions and predict issues such as dam levels and water restrictions. The model balances system inflows: those pumped from the Murrumbidgee River and inflows into reservoirs; and outflows: such as the Canberra and Queanbeyan consumption, evaporation and environmental flows.

The magnitude of these factors can be determined for any given month, using predictions, required environmental flow releases, but most importantly assumptions around climate as this factor drives both supply and demand.

Three climate scenarios are investigated in assessing the time it will take for the enlarged Cotter Reservoir to fill:

• expected climate in 2030, which includes 200 possible samples of the next 50 years’ weather, all assumed to be equally possible. This also includes the assumption that the Canberra climate has already changed to that which the CSIRO predicted for Canberra for the year 2030;
• historic climate, including the last 138 years of historic weather information; and
• a repeat of the very dry climate we have had since 2001.

Incorporated into the model are also the assumptions that the date for completion of the Enlarged Cotter Dam and Murrumbidgee to Googong Water Transfer projects is 2012.

Related environmental issues

ACTEW will continue to adhere to the environmental requirements determined by the ACT and Australian Governments. The ACT Government imposes environmental flow requirements through the ACT Environmental Flow Guidelines. These dictate the amount of water that must be protected in the river on a day to day basis and released from our dams.

The ACT Government also provides ACTEW with Water Access Entitlements, which dictate how much water ACTEW can extract from various rivers to provide a drinking water supply.

As part of the approval process for the Enlarged Cotter Dam, ACTEW was also required to meet all ACT and Australian Government requirements in relation to threatened fish species in the Cotter Dam. ACTEW has done extensive research on fish habitat to help ensure the threatened fish species are protected in the context of the Enlarged Cotter Dam.