Wet weather part of bigger picture

Tuesday 13 February, 2024

Only a few years ago the sound of rainfall on our roofs was met with relief and excitement, but in recent months the sound has triggered a very different response in some parts of our region.

The first nine days of January 2024 marked the wettest start to a year Victoria has experienced since records began in 1900.

Similar to October 2023 and October 2022, parts of our region received well above average rainfall. Despite this similarity, the differences between the three rain events are even more notable.

The two October events were very distinct in themselves. In 2022, it was prolonged and widespread rain that fell across our region. In 2023, the deluge was more sudden and localised.

During 7-8 January 2024, the rainfall was widespread, but the amount of rain different regions received varied greatly, as it had with two downpours which occurred beforehand on Christmas Day and 3 January.

During the January 2024 rainfall event, the highest rainfall totals were downstream of our major water storages.

The degree of flood mitigation major storages such as Lake Eildon and Lake Eppalock can offer subsequently varies with each rainfall event, however, the way they mitigate floods remains largely the same.

When there is significant rainfall, Lake Eildon is managed to ensure releases have minimal impact on downstream peaks.

Releases from Lake Eildon take approximately 48 hours to reach Seymour, and the storage also has the ability to temporarily surcharge (go beyond full capacity). This enables us to hold off increasing releases until we know they will not be adding to peak flows downstream.

In January, for example, we lowered releases from Lake Eildon to 1000 ML per day before the rainfall and held releases at this level until the peak flows downstream began to recede.

While storages can often lessen the impacts of floods, they cannot prevent them, as they are only part of the broader catchment.

Unfortunately, there was still significant flooding in townships downstream of Eildon such as Yea and Seymour due to high inflows from tributaries, including the Sugarloaf and Sunday creeks and the Yea River.

This is why the Goulburn River at Seymour peaked at 66,000 ML per day, and Yea was inundated despite Lake Eildon’s mitigation.

Management of Lake Eppalock is quite different to Lake Eildon, as the storage only has a small valve and no spillway gates that releases can be made through. But Lake Eppalock is similarly consistent with the flood mitigation it offers.

Like many ungated storages, Lake Eppalock is designed to attenuate flows, even when it is full. This is due to the spillway acting like a funnel and allowing water to temporarily build up behind it so that outflows from the storage are more gradual than inflows.

Inflows to Lake Eppalock peaked at over 100,000 ML per day in January, yet outflows peaked at just over 6500 ML per day. When flows at the Campaspe River at Barnadown, downstream of the storage, peaked at about 45,300 ML per day, outflows from Eppalock were approximately 3400 ML per day.

In all three of the recent floods, the peak outflows from Lake Eppalock and Lake Eildon were considerably less than the peak inflows; despite the flood mitigation offered by these storages, communities were affected by the very high downstream river flows and rainfall.

These events reaffirm the importance of the technical assessments the Department of Energy, Environment and Climate Action is undertaking on the infrastructure and operating arrangements at Lake Eppalock and the operating arrangements at Lake Eildon. There is much for these assessments to consider and the information they provide will be important for local councils and CMAs when reviewing flood mitigation options in their local area.  

The three recent rainfall events demonstrate how no two floods are the same. Furthermore, the fact water orders have remained steady during the past 15 months emphasises what a crucial resource water is - that our storages have a significant role in reducing the devastation of droughts as well as floods, and that our storages are crucial assets to our primary producers.

With each new event and each passing month, we are gathering new information we can use to refine our processes. A considered, data-driven approach will put us in the best position to deliver for our region and our future.

Charmaine Quick,

GMW Managing Director