The Department of Energy, Environment and Climate Action’s, DEECA, technical assessment of Lake Eildon’s operating arrangements has been released.
Hydrology and Risk Consulting conducted the assessment, finding ways to reduce downstream flooding and particularly reference the October 2022 flood event.
The report stated the assessment did not identify any option at Lake Eildon warranting further consideration, with six options explored to potentially improve protection for downstream communities.
Option one would have changed target filing curves, so Lake Eildon was full later in the year and under less conservative inflow statistics.
Option two reduced target storage levels by holding the lake, where possible, at a maximum volume of 78, 85, 90 or 95 per cent of full supply level, FSL, all year round.
Option three reduced target storage based on climate signals by holding the lake, where possible at a lower FSL based on industry-standard climate indicators indicating a wet year.
Option four would pre-release based on forecast rainfall to create airspace within the lake two to three days prior to forecast rainfall while option five changed maximum surcharge by increasing the amount of water held in the lake above the FSL during a flood.
Option six restricted maximum outflows by restricting the outflows to match the minor, moderation or major flood class levels downstream of Lake Eildon and allowed the reservoir to surcharge more option.
“All options explored were found to be ineffective or unreliable as flood mitigation strategies, or created costs and impacts that outweigh the benefits of a change to how the storage is managed,” it read.
“The assessment report highlights that tributaries that flow into the Goulburn River downstream of Lake Eildon have a more significant impact on flooding in communities downstream, including Molesworth and Seymour, than releases from the lake.”
Options one and two progressed to the detail technical change but were not deemed viable.
“These options could increase the flood mitigation provided by Lake Eildon, however the cost of offsetting supply reliability impacts outweighed the avoided flood damage,” the report read.
“The main reason for the low benefit to cost ratio is that the flood mitigation benefits provided by the changes to [option one] and [option two] diminish the further downstream the flood frequencies are assessed.
“Namely, the degree of difference between the frequency estimates reduce by Molesworth, and the difference is then minor at Seymour.
“This is because the tributary flows downstream of Eildon from the Rubicon River, Acheron River, Yea River, King Parrot Creek, Sugarloaf Creek and Sunday Creek influences the peak flows at towns such as Seymour.
“This means that changes to operations at Eildon have less influence on reducing the overall avoided damages downstream. In contrast, the approximate initial capital cost of water shares to implement these options ranges from $7.5 million to $266 million.”
The report found that when looking at the 1993 and 2022 floods the only option to make a difference would have been holding the storage to a reduced level of 78 per cent FSL.
“However, this option still had a low cost to benefit ratio. If option one or any other target storage within option two was implemented, there would have been no material difference to the flows observed downstream of Lake Eildon, Molesworth and Seymour,” it read.
