The recent wet weather has understandably caused concerns within communities and brought back memories of last year’s and previous floods.
Although longer term forecasts are predicting below average rainfall, they only provide so much comfort when people can still see a lot of water around them.
At Goulburn-Murray Water, we use forecasts provided by the Bureau of Meteorology to inform how we manage our storages but also make decisions with the knowledge that forecasts are estimates rather than guarantees.
The climate we live in makes it impossible to provide certainties regarding future flood events. While we cannot provide certainty, we are taking steps to ensure people are as informed as possible.
In recent weeks, we have been communicating with communities downstream of Lake Eppalock and Lake Eildon through various mediums, including media releases, social posts, face-to-face and online meetings.
The two storages are managed in very different ways and likewise, play very different roles in mitigating floods.
Lake Eildon is more than 10 times the size of Lake Eppalock. Most of the water released from Lake Eildon occurs via the power station. When higher releases are necessary, water can be released via the spillway gates.
Lake Eppalock meanwhile has a fixed crest spillway with no gates and a small valve that can release up to 1600 Ml/day. The embankment walls are nearly 5.2 metres higher than that of the spillway. This means that water is still held back even when water is going over the spillway.
The ability to release water is therefore different at both storages, but so is the rationale behind any releases.
Any decisions on pre-releasing water must take into account numerous factors.
Our storages are part of a complex system of waterways, meaning releases can often benefit landholders in one location while creating issues for landholders in another.
Catchment conditions, both upstream and downstream of storages, forecast rainfall, current storage levels and the potential impact on landholders, all factor into decisions to release water.
While it is understandable that the current focus is on the risk of floods, we must also balance any releases with the knowledge that droughts are an unavoidable part of our climate.
The water we are able to store now will be crucial when the next dry period arrives, so we must be careful not to release water in anticipation of rainfall events that do not eventuate.
At Lake Eildon, we look to manage to a target-filling curve through winter and spring that sets out the level we aim to be at each month to meet our obligations.
The framework under the Water Act 1989 requires us to manage Lake Eildon so that it reaches capacity at the end of the winter/spring filling period.
The filling targets seek to provide a balance between protecting customers’ water entitlements while providing a degree of flood mitigation.
In this sense, we must manage Lake Eildon to mitigate both droughts and floods.
Lake Eppalock is different due to its location within the catchment and the smaller size of the storage.
Pre-releases are only made through the outlet valve at Lake Eppalock when the storage is expected to fill and forecast inflows will replenish the water released.
As the Lake Eppalock catchment has highly variable inflows, holding the storage below full supply volume for flood mitigation purposes would mean water may not be available during dry times.
This is not to say that Lake Eppalock does not mitigate floods, as the dam itself lessens the peak of downstream flows.
This was also seen during the October 2022 floods, as outflows peaked at 103,000 Ml/day, while inflows peaked at 235,000 Ml/day. Without the structure, the 235,000 Ml/day inflows would continue downstream unmitigated.
Similarly, at Lake Eildon, the peak outflows during the October 2022 floods were 38,000 Ml/day, while inflows peaked at 145,000 Ml/day.
It is also worth noting that in major rainfall events, the water from storages is only one factor that affects downstream conditions.
The catchments have many different rivers and tributaries that run into the major waterways and increase river levels.
While water rushing over a spillway may be a striking visual, this water may only account for a small portion of what is seen downstream.
In the case of Eildon, only 23 per cent of the Goulburn catchment is upstream of the storage. The remaining 77 per cent is downstream of the storage and has a number of rivers and creeks that contribute to water levels along the Goulburn River.
We are fortunate to have forecast rainfall data available, this assists us in managing our storages but it is still a complex balancing act, between lessening flood impacts as much as possible while ensuring we have enough water in our storages to provide secure water supplies, including to manage any future droughts.
Nevertheless, we are constantly continuing to work in partnership with the Bureau of Meteorology, the SES, local council, and various catchment management authorities to ensure our decision-making process is sound and everyone is well-informed.
People can stay up-to-date with how we are managing the storages on our websites: www.gmwater.com.au/managing-eppalock and www.gmwater.com.au/eildon-management
People can also stay up-to-date with the latest flood warnings by downloading the VicEmergency app or heading to: https://www.emergency.vic.gov.au/respond/
By Charmaine Quick, G-MW managing director