Transcript
Ivy Jensen
Welcome to Talking Water with GMW where we discuss all things water. In this episode we are joined by Northern Victoria Resource Manager Dr. Mark Bailey. Mark explains the differences in the lakes, dams and reservoirs GMW manages and how they are operated. He discusses the various factors that determine if and when water is released from GMW storages and the role storage is played during floods. We have episodes on a variety of topics on our website, so be sure to give them a listen. When you were finished with this one.
Chloe Warburton
So how many stories is GMW responsible for managing.
Mark Bailey
GMW operates and maintains around 24 storages across northern Victoria. Some of those are lakes and some of them are constructed dams. The size and nature of them varies quite widely. We've got Dartmouth Dam, which is the largest dam in Victoria and that is fully in Victoria. We've also got a number of ways that we operate on behalf of the Murray-Darling Basin Authority.
The largest dam that GMW operates in its own right is Lake Eildon, which is around 3.3 million megalitres of water.
Chloe Warburton
What are the different ways in how these storage is managed, like is, say Eppalock, managed a similar way to Eildon, or they completely different.
Mark Bailey
It really varies. The way in which GMW manages a storage depends on many things, but the key ones are the size of the storage, where the storage is actually located in the broader river catchment and the actual infrastructure at the storage or the dam itself. So as mentioned, storages do have a wide range of scale, Like Eildon is more than 3 million megalitres, as I said, but we go down to storage such as Laanecoorie, Newlyn Reservoir and Hepburn Lagoon, which is our Bullarook System. They're all less than 10,000 megalitres in the case of Newlyn and Hepburn, so only about 3000 megalitres, so quite small where they're located too, also has a significant factor.
You mentioned Lake Eppalock before, Chloe , it responds to rainfall far more quickly than some of our larger catchments, it's just the nature of the catchment that Lake Eppalock is in, in the Campaspe and also where the storage is located within the catchment, as the lake is sitting relatively high in the catchment in terms of the catchment area. There are a number of significant tributaries to the Goulburn River downstream of Lake Eildon, and so whenever Goulburn Murray Waters operating Lake Eildon, it also has to be very mindful of what is occurring in those downstream tributaries, as they're affecting the Goulburn River and towns such as Seymour, Nagambie and Shepparton.
The dams themselves and the structure of the dams is also quite significant. Most of the storages that are operated have fixed spillways. That means that once they reach their full supply level or that level does, they simply begin to spill over spillways. The other spillway that we have at three of our storages is called a gated spillway and that's where the operators determine how much water should be released at particular times.
Now Lake Eildon and Lake Buffalo and Cairn Curran Reservoir all have those gated spillways and that requires the GMW staff to plan what the inflows are doing and then determine what the appropriate outflows are for the releases through those spillways into the river systems downstream.
Chloe Warburton
Excellent. How does GMW make decisions around releases from Cairn Curran and Eildon at different times of the year? Does it depend on different conditions and things like that?
Mark Bailey
The conditions in the catchment and what the demands really determine how releases are made during the irrigation season, which runs between August and May each year. We operate on a financial year basis or we call it a water year rather than the financial year, let's face it, that's secondary for us. Our releases downstream are really based on what the demands are, whether it's for irrigation, stock use or whether it's for environmental water use.
The gates at Cairn Curran and Lake Eildon and generally provides a bit more flexibility in how we can manage those storages compared to the fixed spillways. We will look at what we may need to do to create a bit of room within the storages, depending on what inflows predicted and will use information from the Bureau of Meteorology with its rainfall forecasts.
But we also look at the catchment conditions and just what's happening in the storage itself. The key thing that we look at at those times is what the impact on downstream communities. Maybe we know that releasing water out of those dams can be quite significant and it can have a large impact.
Lake Eildon has a filling curve, so we do operate to try and fill the lake in 95 years out of 100, and that's set by legislation that we act to. So what it's trying to do is protect the reliability of the entitlements that are supplied by the dams, but also provide a degree of flood mitigation downstream. It won't provide flood prevention, but it will mitigate the flows.
In October 2022, we saw some very high inflows into Lake Eildon in excess of 150,000 megalitres a day, but the peak outflow downstream was 38,000. So that's a significant mitigation.
Compare that with Cairn Curran Reservoir. It's also a gated spillway, but it's a much smaller storage. It's only 174,000 megalitres. It is in the catchment that does harvest a lot of water. So we've got to look at that as well. So we maintain the storage with a bit of airspace to try and capture by inflow of into acre or if they occur and then we'll try and create space if those are coming in.
Chloe Warburton
Excellent. Why does GMW allow for storages like Eildon, Eppalock and Cairn Curran to fill when it could flood in wet years?
Mark Bailey
The dams themselves are water supply storages, and we do know that both droughts and floods are a reality of our climate. The storages can't stop either event, but they do lessen the impacts of both. And so we've got to manage against both situations, both Eildon, Eppalock and Cairn Curran are storages that are built to hold water over multiple years. What that means is they'll hold enough water for effectively two or more years of expected use, and that's significant use too. It's not some of the lower rates that we've seen in recent years just due to some of the weather conditions. It's the amount that will be used in really dry conditions when irrigation is going at its hardest.
Actually having the dams full doesn't remove their ability to mitigate floods. What the spillways at all storages do is actually reduce the amount of water that goes downstream. The spillways cause the water to effectively back up and rise and slow the rate at which water is moving downstream. That's a natural effect for all of those dams. It's just how the design of the spillway has been made.
The spillway gates at Eildon and Cairn Curran do give a bit more flexibility about how we can do that, so we can create a little bit more space more quickly. One thing we do have to be conscious of is what capacity we have to release. At Lake Eppalock, for instance, we can only release through the river valve. We don't have those gated spillways, so we've got to be conscious of what's occurring upstream and what's occurring downstream.
But even with that, for example, Lake Eppalock, inflows into the dam in October last year peaked at 235,000 megalitres a day, but the flow downstream was 103,000 megalitres a day. That's still a significant rate of flow and we certainly don't walk away from that. We do recognize that by halving it is actually had far less impact. Not to say that there wasn't significant impact downstream, but it had much less impact as what occurred, so we've got to be aware of that.
Chloe Warburton
So why does GMW not manage storages to below 100%. Why don't they manage it to maybe 90%, is it more risky to have them get 100% or close to?
Mark Bailey
It's a question that has been asked amongst a number of communities, and it's not just GMW that's affected by this. We do know that the State Government is running an assessment into two of our storages about what's the appropriate operating and infrastructure arrangements for Lake Eppalock and Lake Eildon, and GMW fully supports the assessment as we want to ensure that our storages are serving the communities and not just our customers as best they can.
We do recognize that storages are part of a broader network, so releasing water from one of those storages may have an impact further downstream or may actually impact on other storage downstream. In the case of Eildon, where we also have Goulburn Weir and Waranga Basin that we operate, the Eildon catchment itself is only 23% of the overall catchment for the Goulburn River system, but it still produces a significant volume of water.
But we've still got to recognize what is also happening downstream as well. So in October last year, the floods that affected Seymour were actually caused by local catchment runoff and tributaries between Lake Eildon and Seymour. It wasn't actually the flows from Lake Eildon that contributed to the high degree of flooding that occurred in Seymour. The other thing I would just like to say is when we do have rainfall events after several weeks of extremely high rainfall like we experienced across northern Victoria in October and November 2022, any dam, regardless of whether it's full or even 90% of capacity, will struggle to make a large impact on those floods.
Even if we'd had say, Lake Eppalock at 90%, it's highly likely that the dam would have filled very quickly, particularly with the amount of inflows that were coming in on the 13th, and they still would have been significant flows passing downstream. Once we get into a really significant event, there's very little that the dams can actually do there's just such a significant amount of water.
We keep working with the likes of the emergency services and the Bureau of Meteorology to obtain the flood intelligence and the flow intelligence that we can use and then work with local councils, catchment management authorities to ensure that we've got good decision making processes that can help communities when they're being affected by the high flows and to make sure that the communities are being kept well informed.
Chloe Warburton
There are times when there's more rainfall than forecast. Does GMW adapt how it manages its storages, like you know, you mentioned earlier that we had a bit more rain that was initially forecast. What did we do to adapt to that?
Mark Bailey
I guess one of the key things that I'd like to get across is that all storage operations are adaptive. We never rely on the Bureau of Meteorology forecasts or any other water corporations or weather corporation's forecasts as being a guarantee. We do look to have some adaptability built into our operations, so we'll be looking very closely at what the catchment conditions are doing, as well as observing what the actual rainfall is doing.
Then using that information to understand and estimate how much water will be entering a storage and what the releases that might ensue could be doing downstream. So we respond to the size of any rain event and look at it how that's influencing storage, whether it's how much water is within the storage at a particular time, but also what is happening on the tributaries downstream.
That's particularly important in the Goulburn system where we know we've got tributaries can provide significant flows and impact on what happens downstream. So we've got to be very conscious of what we do at Eildon, which in terms of our releases and what they might do and interact with those other tributary flows which aren't controlled by major dams.
Chloe Warburton
Yeah, if people want more information on how we manage our storages, is that publicly available information? Is there somewhere they can go to get that information?
Mark Bailey
Yes. GMW maintains a 'managing water storages' page on its website and in particular for Eppalock and Eildon. If you were to search GM water managing and the storage name you will find those sites.
If people want to find out what's happening on a daily basis at Lake Eildon, there is a storage and flow advisory recorded service and that can be reached on 03 5774 3928. And that's one because there can be some significant variation in the releases downstream from Lake Eildon.
But if people are also looking for some support after last year's floods, we certainly know that there are a large number of people within the community who are still looking for some assistance, we recommend that they go to their local council or they call the Victorian Government flood recovery line 1800 560 760. And those services are free for those people to use.
And if people are looking for up to date flood warnings, if we've had some significant rainfall and there's a chance of flooding. We recommend that people stay up to date by using the emergency app. But also listen for updates on ABC radio that will be coming through at various times.