[Beth Ashworth - DEECA]
Thank you for joining us here today. I'd like to start by acknowledging the Traditional Owners of the land that we're all on. For myself here in Melbourne, that's the land of the Wurundjeri people. But I'd also like to acknowledge the lands of Lake Eppalock and it's surrounds on the land of the Yorta Yorta, Taungurung and Dja Dja Wurrung peoples and like to pay my respects to their Elders past and present and extend that respect to any Traditional Owners and Elders that are here with us today.

So welcome and thank you for joining us today for the presentation on the Lake Eppalock technical assessment. Sincere apologies to those who tried to join last Tuesday when we had some technical difficulties and unfortunately had to postpone the webinar. But we've got Simon Lang from HARC here in the office with us today. 
So very much hoping that we won't have any technical difficulties today.

Hi, my name's Beth Ashworth. I'm the Director of Water Entitlements, Licensing and Modelling here at DEECA, and I've got with me, Simon Lang from Hydrology and Risk Consulting who prepared the technical assessment report. As you know, in February the Minister for Water announced that there would be a technical assessment of both the operating and infrastructure arrangements at Lake Eppalock to assess whether there were any options that could provide better flood mitigation and any of the potential costs and impacts of those options. So HARC are here today to walk us through, step by step, the process they've gone through and the findings they've got.

There will be a Q&A session at the end. You'll see there's a Q&A button at the top of your screen, so I encourage you to submit your questions throughout the presentation, and then we'll have a dedicated time at the end to work through those questions. We do have someone in the background moderating the questions, so they may take some time to appear, but they will definitely appear.

We'll move on now to some of the context around the technical assessment. It will be one of the key inputs to the Rochester Flood Management Plan, which is not the only input. Other inputs will be things like updating the flood modelling calibrated for the 2022 floods that happened last year. The idea of updating the flood management plan is that it will assess all of the flood mitigation options available - those already known about and the ones that are looked at through this assessment that we're here to talk about today. The flood management plan will benchmark all of those options.

The flood management plan is being led by the Campaspe Shire Council with support from North Central CMA and utilising funding made available by the Victorian Government.

But let's get into what we're really here to talk about, which is the most recent technical assessment that has been done. I'll hand over now to Simon Lang, who will take us through the details of that report.

[Simon Lang - HARC]

Thanks Beth, and hello to everyone watching this webinar. I will be going through a lot of technical detail in the next hour and appreciate that some of the concepts may be new and some of the concepts may be familiar, but we thought it was important to give the same presentation that others have received. This is the same set of slides that other stakeholders have previously been taken through. Feel free, as Beth said, to put your questions in the Q&A and we'll do our best to answer them.

We'll start with what the scope of our work was. Essentially, we have looked at five different options and some with some variations amongst them, and then came up with a ranking. The rankings are based on two things and their approximate values. The first is to what degree would the options potentially reduce flood damages, on the one hand, and on the other hand, what would be the approximate initial cost to design and construct them? We recognise this isn't a full cost benefit analysis. There are things that haven't been included given the time available to do this study. So we haven't yet looked at operational and maintenance costs for the options, and we haven't looked at the socioeconomic costs if the volume of water stored in the system was reduced.

We have done a lot over the last half a dozen months and looked at flood frequency changes at Lake Eppalock itself to see if the options were to be implemented. We've looked specifically at - if the 2011 and 2022 floods were repeated - how the spills from Eppalock would change if the options were in place. We've looked at how the options may change flood damages. We've had to do that both upstream and downstream, because some options will increase upstream flood damages, whereas all options will decrease the downstream flood damages. We've looked at water resource implications for some of the options. We've also looked at concept designs and construction cost estimates as well as the recreational impacts around Lake Eppalock, if the volume of water held in storage was reduced. I looked at daily flow changes and the potential environmental implications of those at a high level. And then also in the report, which you can find online, provided some commentary on potential climate change impacts.

So that's the scope of the work that we've been doing over the past period of time. We'll now move into a high-level description of each of the options.

Option one is using the existing infrastructure at Lake Eppalock to reduce the level that the lake is held at. At the moment the intention is for Lake Eppalock to be filled when possible, and we refer to the level at which it starts to spill over the crest of the spillway as full supply level, or FSL for short. Option one was looking at whether we can use the existing outlet capacity to hold the lake, to the degree possible, at either 50% full, 70% full or 90% full. The pros of this option are that it is relatively cheap to implement, but there would be water resource implications. But in terms of infrastructure, [there would be] no additional cost. The cons are - as we'll see in later slides - it would be difficult to hold the lake at those types of levels with the existing outlet capacity.

Option two is a variation of option one, in that it also involves holding the lake at lower levels 50%, 70% or 90% of full, but constructing a second outlet so that the storage operators have much greater control over that lake level. On screen we've got a representation of what it might look like. Hopefully you can see my laser pointer on the screen. It would involve building a second outlet on the right-hand side of the dam and that would be designed to be approximately 3,500 megalitre per day capacity. In combination with the existing outlet, it would allow storage operators to release up to 5,000 megalitres per day.

The reason that 5,000 megalitres per day threshold was chosen was for three reasons, essentially. The first one is that type of outlet capacity would have enabled storage operators to hold the lake at a defined level in the lead up to the 2011 and 2022 floods. It wouldn't have been enough to hold the lake at a given level during the floods, but it could have meant that the lake was held at a certain level before the floods arrived. The other reasons are it would provide environmental water managers with additional capacity to provide important environmental flow recommendations downstream of Lake Eppalock. But it's also not so large as to potentially provide nuisance flooding downstream of the storage. That 5,000 megalitre per day capacity that we've used is certainly something that could be optimised in future, but for the purpose of this study was a good number to go with.

Those first two options involve reducing the target storage or trying to hold the lake at a lower level using the outlet. Option three is somewhat similar, but it would involve a permanent reduction of full supply level. This would be done - instead of increasing the outlet capacity - by essentially creating a slot in the spillway. There's an example there of a spillway slot on the Hinze Dam, which is upstream of the Gold Coast, and it would be designed to pass flows up to about 12,000 megalitres per day through the slot. If such a feature was installed at Lake Eppalock and then larger floods would go over the full length of the existing spillway. The addition of the slot would mean that the lake would be at or below 70% of full supply level prior to floods arriving. The main disadvantage of this option compared to the first two is that it would be difficult to undo. The first two options mean that if you decide to reduce the target storage and find out that in 10 to 15 years down the track things have changed and we want to change what our target storage is, that would be doable. If a spillway slot is put in at Lake Eppalock that would be much harder to undo.

Those first three options all involve a reduction of the volume stored in Lake Eppalock. The next two options, in contrast, would maintain the existing full supply level, so there wouldn't be any change to the water resource availability in the Campaspe system, but rather these next two options courses, which are on screen, would hold more water back at the dam during floods.

So option four, up on screen is the addition of spillway gates to the primary spillway at Lake Eppalock. We've got a representation of what they might look like. So essentially, put these gates onto the existing spillway crest level and then they would be used to hold water back above what can be done at the moment during floods, thereby reducing the flow downstream. But this option would increase upstream water levels, and therefore increase flooding experienced by those who have recreational or other facilities around the lake edge. The addition of spillway gates would also mean increased operational costs and risks for Goulburn-Murray Water, because in contrast to being a fixed crest spillway - as is the case now, where the flows come in and then spill over without operational intervention - this option will require additional Goulburn-Murray Water staffing to do flood forecasting and then make gate release decisions during floods.

Option Five is one where it would also hold more water in the lake during floods, but it would be a passive spillway. By that we mean it wouldn't need to be operated by Goulburn-Murray Water during floods, and this will be done by the addition of what are called piano keys. I'm just tracing over them with the pointer, hopefully you can see on screen. But what these piano keys are designed to do is hold the water back. This option would be to put some on either side of the existing primary spillway and leave a slot of the existing spillway in place so small flows would go through that central portion as the lake rose. Then the water would continue to be throttled through that section, and then once it goes over the piano keys, they're designed to pass additional flow so that we don't have dam safety implications. By the time you are reaching really extreme floods you're still able to pass sufficient water, so that the embankment wouldn't be overtopped.

One other difference with this option compared with the spillway gates option is it would only require small changes to the embankments at Lake Eppalock. Whereas for the spillway gates option, we would need to raise the embankments and raise the secondary spillway. For this one, we would be putting piano keys on a portion of the primary spillway and then all of the secondary spillway.

So that's an introduction to the five options that were considered in detail. We did look at other options and these are covered in the online report, but we won't be going through them with a lot of time today. We did look at using filling curves. That is concept of - instead of allowing Lake Eppalock to fill as per inflows arriving - trying to use the outlet capacity to control that rate of infilling. The option was looked at about 10 years ago by SKM and concluded that that wouldn't make a significant difference to the downstream flood frequencies. And that's understandable. I mean, the last two floods have been in October and January, months where even if your filling curve was in place, you'd be aiming to have the storage full in preparation for summer demands. So, the use of filling curves is unlikely to significantly increase the flood mitigation provided by Lake Eppalock, and therefore wasn't assessed in detail as part of this study.

We did look at increasing the outlet capacity while maintaining the existing full supply level. So that would essentially involve trying to predict when floods are coming and drawing the lake down in anticipation of those inflows. But as you'll see, hopefully on subsequent slides, that's a very difficult thing to do. This concept of being able to know when floods are coming and manage the lake accordingly, 
is good in concept, but very difficult in practice.

We did consider adding spillway gates and reducing the full supply level, so that the embankments at Lake Eppalock wouldn't need to be raised as much, and therefore reduce the cost of that option. But the saved costs from the infrastructure changes would be offset by the cost associated with the water resource implications. So we didn't carry that option forward.

We did look at trying to vary how full the lake is according to climate conditions. By that I mean, you know ENSO/ISO climate indices, but there's a very weak match between those indices and inflows in any single catchment. Therefore it's very difficult to use those as a predictor of what might be coming into storage. This year is a classic example where we've got El Nino conditions but are still experiencing significant inflows in some individual catchments.

We also looked at transferring water to Greens Lake or Lake Cooper. The challenge there is in 2011 both of those lakes did not have additional airspace to take additional flows from elsewhere. Likewise in 2022, once the flood had passed or at the time of the floods, they only had a small amount of capacity compared to the inflows that Lake Eppalock received. That would make a marginal difference to the outflows from Eppalock if such an event were to be repeated, even if you had transferred some of the water to those lakes. As I said, there's more detail on the degree to which we assessed those options in the online report.

We will now take you through what we did. But just a comment before we start to get into some of the findings is that we have used historical data and existing models. 

So models haven't been updated yet to include information from the 2022 flood and that in turn will happen, and that in turn will update or supersede some of the values in our report. But that's OK. We are aiming here to do a robust comparison between options, and we anticipate the comparison between the options will stand up even when those models are refined to include 2022. And likewise, the construction costs that were estimated have been done to a higher level. We refer to this as an AACE class 5 level, and that essentially means that the true cost is likely to be somewhere between half of what the estimate is or double. We are certainly not saying that the costs are precise, but they've been done to a level that helps us or enables us to make comparisons between options.

I'll now talk about some findings in general. The findings are not necessarily specific to an option, but then we'll start to go into some of the real detail after these next slides.

The first general finding is that trying to hold the lake below full supply using the existing infrastructure is not a robust option, and there's two main reasons for this. The first is that if the 2011 and 2022 floods were to be repeated, it would be impossible to hold the lake at levels lower than full supply with the existing outlet capacity.

I'll try to demonstrate that with these plots up on screen. I've got here the laser pointer tracing along a blue line, and the blue line is simply adding up all the inflows to Lake Eppalock from the 1st of September. The 1st of September date isn't special; it's just the start of spring, a period we had picked to demonstrate the concept here. But each day of inflow from the 1st of September, we've simply added together to create a running total. You can see there was a big inflow to storage early in September, not much during October, another big inflow at the end of November, early December, then the big flood in January 2011.

And then on this orange line, it's a similar concept. All we've had done there is add 1,600 megalitres per day to each day and kept a running total. That's how much water could be released from storage if the existing outlet was run at capacity every day from 1st of September onwards during that period. That is, holding the outlet fully open for that whole period of time. You can see that by the time we get to January 2011, there's still a 90,000 megalitre difference between the blue line and the orange line. Even if the outlet was running at full capacity, 90,000 megalitres of water would have accrued in storage.

And it's a similar story for 2022. Again, the blue line is simply adding up all the inflows into storage from 1st of September and the orange line is how much total water could have been let go from the outlet capacity to downstream, if it had been run at capacity. From the 1st of September to mid-October, there's 120,000 megalitre difference. So even if the outlet had been fully open that whole time, 120,000 megalitres, or nearly half the storage, would have accrued.

The grey line there is how much water could be let go or could have been let go if the outlet capacity was 5,000 megalitres per day. You can see that in both those cases, the lake could have been held at a defined target storage if the outlet capacity had been 5,000 megalitres per day. So that's the first option and [the] main reason why it's not a robust option to try and hold the lake lower with the existing outlet. The second reason is if you did try to hold the lake at a lower level with the existing outlet capacity, it would change the downstream flow.

There are a few boxes on here that we will zoom in on or talk about. But essentially on this option we've got in blue is a representation of how the daily flow downstream of Eppalock generally looks. The blue shows that most of our flow is in winter/spring and it tails off in summer.

Now if we try to hold the storage lower with the existing outlet capacity, and particularly if we've reduced the target storage from say 90% to 70% to 50%, what we see is the yellow portion of these plots, which is representing what the daily flows might look like with that option in place, starting to bulge out into summer. We're moving flow from the winter period into the summer period because we're accumulating water in storage during winter/ spring when generally a lot arrives, and then letting it go to the degree we can. But because of the small outlet capacity, [we are] having to do that into spring and even summer, and that change of daily flow from the winter/spring period into summer is unlikely to be good for the environment.

You can see if the outlet capacity is increased to 5,000 megalitres per day though, the blue and the yellow tend to lie on top of each other, so we're not making as big of a difference to the daily flow downstream if the target storage is reduced and the outlet capacity is increased.

So that's finding number one; holding the lake below FSL with existing infrastructure is not a robust option. 

The second general finding is that trying to release water from storage in response to rainfall forecasts is also not a robust option. It's something that can be done. 
It's certainly an option available to storage operators, but it's really difficult to get right. The challenge is the release of water before floods needs to be done in a way that both doesn't reduce the volume of water held in storage for entitlement holders or exacerbate downstream flooding. Meeting both of those requirements is difficult given the rainfall forecasts available.

This slide here we have two examples from the nine models available to the Bureau of Meteorology of predicted rainfall in the lead up to the October 2022 flood. We've shown these two models on screen because they're the ones that have generally have most weight placed on them.

We've got an Australian one and we've also got the European model, so we can make a few observations based on these screenshots. Firstly, both models are predicting significant rainfalls, so the colours are quite similar between the two. 
We've got really high rainfall predicted to be occurring in the period of the forecast. 
And then secondly, the general region is about the same – both are predicting rainfall heavy rainfall in Victoria.

What we can start to see is the location of the heaviest rainfall is really uncertain. On the left, the heaviest rainfall is predicted to be pretty much in central Victoria and on the right, the heaviest rainfall there's more predicted to be North East Victoria. That makes it really difficult as a storage operator to know is the rainfall going to occur upstream or downstream of the dam? Let alone which catchment is likely to receive the heaviest rainfall. Therefore releasing water from storage based on these forecasts, you might win, it might be a good decision or it could be that you end up making things worse because the heaviest rainfall occurs downstream of your dam or in a catchment different to what you were expecting.

That's one of the reasons why it's really difficult to make pre-releases on the basis of forecast rainfall. That type of concept is also illustrated by some of the streamflow forecast skill scores available on the Bureau of Meteorology site. Here we've picked a site for the Campaspe River at Redesdale, which is upstream of Lake Eppalock, and you can go online and check this out for yourself if you're keen. We have a skill score for the seven-day streamflow forecasts. I’m not going to go into the detail of what the skill score is, but [to] more point out that it's really, really good for one day ahead and then drops off for subsequent days. And that's just reinforcing the point that it's really challenging to predict with precision and accuracy what the inflow to a particular storage or catchment is going to be. The Bureau is really good at predicting general regions of high rainfall, but being specific about whether it's going to be in the Campaspe or the Eildon or the Loddon or other catchments is a challenge that's ongoing.

The other aspect of the Campaspe River that makes it difficult to predicting flows to Lake Eppalock is just its nature. On here we have plotted in blue recorded flow [of] Campaspe River at Redesdale. We've just picked a recent period of record January 2017 through to November 2021. The blue is taken from the gauge. The black is Goulburn-Murray Water's estimate of total inflows into Lake Eppalock. You can see that it follows the blue quite closely, And then for comparison purposes, we've just taken a gauge on the Murray River upstream of Lake Hume - that's what's shown here in red.

What hopefully you'll notice is that the Campaspe is very much, for want of a better term, boom or bust. It's quite dry for periods and then quickly responds to rainfall, but then quickly dries off again in terms of inflows. That is in contrast to other parts of the state where you'll have this nice base flow recession or base flow increase, and then your peak flows happen on top of that. Those base flows are helpful for managing airspace in your storage because they respond a lot slower than rainfall runoff. So if you've got a larger component of that base flow upstream of the catchment or storage that you are managing, that does provide a useful hint or indication of what the minimum inflow’s likely to be coming into your storage. In contrast, with somewhere like the Campaspe - where once it stops raining the flow quickly disappears - you are never going to be quite sure whether this peak was the last one you were to receive in a given year or there might be subsequent events.

So there's some general findings. We're now going to talk to specifically how the options we've looked at would perform in a repeat of the 2011 and 2022 floods. 
But we can say up front that in general, the greater the degree of additional flood mitigation provided by an option, the higher the cost will be to implement that option. Now, that doesn't mean that we get the best ratio of avoided damages to initial cost the more we spend. It will be that we can avoid flood damages by spending more. But that ratio of avoided damages to initial cost is what we we're aiming to get at.

On screen here we have in black a modelled representation of the 2011 flood. That is represented in a model we call RORB, and that is aiming to replicate what was observed on the ground back in January 2011. Then in colours underneath that, we've got a modelled estimate of what would happen if that exact flood were repeated but the options were in place.

We’ve got here, for example, 90% start storage. That's used to represent the option where we do have an increased outlet capacity and we are trying to hold the lake at 90%. This is showing that if the lake was at 90% prior to the January 2011 flood, this is the predicted outflow in the blue. If the start storage was 70%, we're down at the purple. If the start storage was 50%, we're down at the red. But I must stress again, those start storages would have only been possible if the storage operators had access to greater outlet capacity. Then we can see how those outflows compared to flood classes at Eppalock. We'll talk later about how they translate down to Rochester.

We can see that [with] the 50% and 70% options, the outflows would have been below the minor flood level at Eppalock. With the 90% start storage, [it’d be] in between minor and moderate, and then our other options are up here. So the piano key spillways and the spillway slot, just by coincidence, giving you very similar peak outflow. [With] the gates here we can see how it would ramp up - the gates in this instance have been designed or optimised for repeat of the 2022 flood - but in the 2011 flood would be anticipated to peak at an outflow of about 40,000 megalitres. And then how that dropped off after the event would really depend on operational decisions about how long to keep the reservoir surcharged above full supply level. But for the purposes of this assessment, it's the peak that we're interested in.

If we repeat that exercise for 2022 - so in this case we've used a simplified spreadsheet rather than the rainfall runoff model - but we do get a good handle on what would happen if the options were in place. Again, the black is what was observed, and the colours are if the options had been in place. One thing we can immediately start to pick up on is the order of the colours have changed. In particular this 90% start storage option is now providing the least additional flood mitigation. It does reduce the flood peak noticeably, but it's now sitting above the other options. That just reflects how each of these options will behave differently according to the size, the timing and the volume associated with the floods. Again, the piano key spillways and the spillways slot by coincidence are providing a very similar peak outflow. We've got our gates, as I mentioned, really optimised for the 2022 repeat. So if that was an option carried forward, that design would need to be refined to cope with larger range of floods. Again, it's the 70% start storage and the 50% start storage that really do reduce the peak observed in 2022.

So that's all well and good at Eppalock, but what does it mean further downstream? 
To help answer that question, what we've done is looked at how have spills from Lake Eppalock over the historic record compared with flows observed in Rochester.

On this plot on the left, we have from the water years in the mid-70s up to now. We've plotted for each year the peak spill from Lake Eppalock and the peak flow at the Rochester Syphon, and you can say that in some years there's really good match between the two. They both go up and down at the same time. Sometimes the peak at Rochester's much larger than the peak spill from Eppalock, and sometimes it's about the same. If we plot those values on the left against each other as an XY plot - so we've got the peak spill from Eppalock on the X axis, the peak flow at Rochester Syphon on the Y axis - you can start to see if we get these clumps of dots. This one in the orange is generally a relationship of 1:2 between peak spill from Eppalock and peak flow at Rochester. By that, I mean the flow Rochester is about 20% higher than the spill from Eppalock. That's in the orange cloud, but that's not always the case. Sometimes there's significant rainfall downstream of Lake Eppalock. You can see that occurred in the 80s and to a lesser degree, early 90s. In this blue section, the peak flow Rochester is actually about slightly more than three times the peak spill from Eppalock.

But for the purpose of this assessment, we've used the relationship between the dots and the orange cloud to estimate how the flows at Rochester would change if the options we've examined were in place. In this next slide, what we've done is taking the peaks from those curves a few slides ago - what we refer to as hydrographs - we've just taken the peak of each of those curves and put them on this plot.

These blue dots and the blue line is simply the historic record; they are the values taken from the previous slide. You can see 2010/11 is up here. We had a peak spill from Lake Eppalock of about 70,000 megalitres per day, and the flooding at Rochester was above the major flood threshold. Then we've taken from prior slides - what would have the peak been terms of spill from Eppalock if that flood was repeated with the spillway gates in place, with the slot spillway and the piano key spillways in place? And then use this blue line to infer what it would mean at Rochester. In 2011 with spillway gates, peak spill from Lake Eppalock of about 40,000 megalitres per day still would have probably been on the cusp of major flooding at Rochester. Similarly with the slot spillway and the piano key spillways, [it would be] probably on or just above the major flood threshold at Rochester. It's with the reduced start storages that you start to see that the flow from Eppalock and hence also at Rochester starting to go below those - in the first instance major flood threshold, and then moderate flood threshold. But it's only really with the 50% start storage that you would have been below the minor flood threshold at Rochester.

For a repeat [of] that exercise for 2022 - so 2022/23 the dot up here showing what happened in the historic record, and then the purple squares are an estimate of what would happen if 2022 were repeated with the options in place - you can see that 2022 with a 90% start storage, the flooding through Rochester likely would’ve been still worse than what was experienced in 2011. Likewise with the slot spillway or the piano key spillways, there still would have been major flooding. With the spillway gates, it probably would have been on the threshold. 70% start storage, bit less than major flooding. 50%, again down below minor flooding. I’ll stress again, these are indicative options and indicative estimates, but the pretty strong correlation between peak spills from Eppalock and peak flows at Rochester does allow us to do this with some confidence. That is our estimate of what it means in terms of peak flow. 

But what we're trying to do is compare avoided damages with the initial costs. So we now need to translate these estimates into an approximate value of avoided flood damage. By that I mean if 2022 flood observed up here was repeated with a 90% start storage and therefore the peak was about here, what does that mean in terms of avoided damage to houses, to non-residential buildings, to other infrastructure like roads and rail and agriculture? To do that, we've made use of prior flood modelling done in 2013 and 2018. On here we have available from those flood studies an estimate of where the water would be in and around Rochester if there was a 74,000 megalitre per day flow at the syphon, with the colours representing the depth of water. And then we've overlaid on that databases of building locations and infrastructure locations to come up with estimates of how much damage there would be versus a given flow at Rochester. Using that relationship between the Rochester flow and the lake Eppalock flow, we can translate that back to an avoided damage with a reduced spill from Lake Eppalock.

The other thing we need to keep in mind is that some of the options will increase flooding or the water level upstream of the storage. For example, the spillway gates and the piano key spillway options would both increase the water level if October 2022 flood were repeated and the pink dots here around the lake show the additional buildings - primarily these are holiday and caravan park buildings - that would be inundated if an event of that size was repeated with the piano key spillways in place.

To try to take that information presented over the prior approximately half a dozen slides, we have this table. This table is simply a repeat of all the information presented on prior slides to try and again demonstrate how we've use the information at hand and the modelling tools available to try and get at this estimate of how much avoided flood damage there’d be with the options in place. We've got here the 2011 flood or the 2022 flood. Base case means so under current conditions or what was observed historically. And then we've got the options listed under that base case. We have our estimated peak spill from Lake Eppalock. We use the historic relationship to translate that down to Rochester Syphon.

We know for some of the options, the flood damages upstream of Eppalock will be increased, so that's represented in this third column. We know for each of the options - based on our relationship between flow at the syphon and flood damage - what the approximate damage would be in the reach between Eppalock and Rochester. In brackets here we've also got an estimate of the number of houses 
at which the property would have some water. We haven't differentiated between above/below floor flooding at this point in time. But those two columns together give a total, and then we compare that to the base case.

So we have approximate, very approximate, flood damage in 2011 of 200 million. If it was a 50% start storage, approximate flood damage at 0. So, the difference is 205 million. With the piano key, spillways are going right to the bottom, you would still have proximately $115 million worth of flood damage, so $90 million was flood damage avoided.

Repeat that exercise for 2022 - much larger flood, and therefore much larger flood damages than the base case, and you can see the degree to which flood damages are avoided as the options go down the page.

So when we get to comparing that avoided flood damage to the initial capital cost, we then need to also account for, how often are these floods likely to occur? That has been factored into the comparison. Now we know our estimates of that at the moment are undercooked, but that's OK. We're doing comparisons between options here. So provided we do that frequency analysis consistently between the options, the comparisons will be robust. But we do know that our estimates of avoided flood damages, will go up in future.

And that's one aspect of our comparison. So we're comparing avoided flood damages to initial capital cost. We've talked about the engineering involved at a high level.

The other aspect of the initial capital cost though, is that there will be water resource implications for the first three options.

The first three options all involve holding the lake at a level lower than the full supply level to the degree possible, either using the existing outlet and increased outlet capacity, or by putting a slot in the spillway. And we’re showing on this slide what the storage trace would be. By storage trace I mean the modelled water level in Lake Eppalock overtime. So we're showing here from 1975 to current day a modelled storage trace for Lake Eppalock. 
Again, the black is the base case; that's - if the historic climate conditions were repeated with the existing infrastructure in place - existing demands and rules for the system. And then in the colours -  what would that storage taste potentially look like if we tried to hold the lake at 90% in the blue, at 70% in the purple, or the 50% in the red. You can see that during wet periods you're, often bumping up against that target storage - drawing down in summer and refilling back up to the target storage in winter/spring.

But then during this Millennium drought period, all options are well below the target storage and to the degree to which all of them - given the extremely dry conditions that were experienced - to like essentially becomes empty. We can also see that if the target storage is reduced, that period where the lake is essentially empty is brought forward. Particularly for the 50% option, we can see that we're bringing that period forward with the lake essentially being empty through to 2002. That water resource implication would definitely have costs, socioeconomic costs, which haven't yet been taken on account, but also reliability of supply and costs for all entitlement holders.

As part of the initial capital cost, we've estimated how much water would need to be recovered from entitlement holders and water shareholders, so that the reliability of supply would be the same if the target storage was reduced. We'll show what those costs are in subsequent slides. As I mentioned, we haven't accounted yet for socioeconomic costs of that reduced volume of water in storage. The other thing we're conscious of - but haven't yet put a dollar value on - is there would be recreational impacts if the lake was held at a lower target storage. 

On this map we have an aerial photo of Lake Eppalock. In yellow we've got where the water's edge would be if the storage was held at 90% of FSL. Green is 70% of FSL. This pinkish colour is 50% of FSL. And you can start to get a sense of how the water body would reduce if those targets storages were in place. So especially in the shallow areas, you can start to see the yellow to the green to the pink, and start to see that there's real reduction of the water body area. There would be recreational impacts, and you can start to envisage what they'd be. For example around boat ramps, some of these might need to be extended so that the recreational use of the lake can continue with that reduced target storage in place.

So this table then brings together some of the costs that we've mentioned along the way. This is the other side of the equation we've looked at to rank the options. We've talked about the avoided flood damages; this is now the cost of implementation - not ongoing costs, but rather cost of implementation. The first column here is our AACE Class 5 estimates of design and construction. You can see that the slots spillway by far the cheapest. Putting on an increased outlet capacity up to 5000 megalitres today would be approximately $30 million, piano key spillways at approximately $60 million and spillway gates approximately $200 million. Then in this column here we have an approximate estimate of how much it would cost to buy back the entitlements required to offset the water resource implications using existing market prices in the Campaspe system. So, a 90% target storage would be 
a $15 million dollar cost to offset that water resource implication for existing entitlement holders. The first two columns give us our approximate total and in this table the options have been ranked from least costly to most costly.

So we now have our two main bits of information - the avoided flood damages and also the approximate implementation or initial capital cost. Now we compare one to the other, and we've done this over a 50 year horizon. That's where this estimate of flood frequency comes into, and for meeting the caveats that will appear on subsequent slides. These numbers will definitely be superseded in future, but we feel that the comparisons or ranking between them are pretty good.

The colours represent our bandings or rankings of options. We've got in the blue two options that provide the highest ratio of avoided damages to initial capital cost. They're the options with the slot spillway and also a 70% target storage with increased outlet capacity. You'll see that they give very similar ratios. A key difference though is the spillway slot would avoid less damage - let me get my words a bit better there - the avoided damages would be less. If it’s with the 70% target storage and 5,000 megalitre-per-day outlet in place, the avoided damages would be higher, but the cost to do it would be higher. That's why the ratios turned out quite similar.

In yellow, we've got our options to hold the lake at 50% or 90% with increased outlet capacity. Those ratios are less than for 70%. So we know that if that option was implemented, 70% given the metrics we've used will be better than 90 or 50. But that doesn't mean necessarily that 70% would be the most optimal value; it might be a bit to the left or a might be a bit to the right. But we know it's not as far out as 90 or as far in as 50.

Then with the two options where we’re holding the lake at the current full supply level and implementing infrastructure-only ways of reducing the downstream flood peaks, they had the lowest ratio of the avoided damages to initial capital cost. Down in white, we've got the option that involved only reducing the target storage using the existing infrastructure. For reasons that we go to in detail in the report, we've essentially ignored those. For the reasons we discussed upfront in this presentation, we don't think it's a robust way to go, and it's just some of the technical details of the modelling that means that they come out with higher ratios. We've presented them here for completeness, but we don't think it's worth including in the comparison.

So you might see that there's a few asterisks in that table and we’ll go through some of the caveats.

As I said, we think the comparisons between the options is helpful, but the estimates of avoided damages are approximate. We have developed the relationship between spills at Lake Eppalock and flood damages. To do that, we've had to interpolate between Rochester and Lake Eppalock in terms of flow at one location and the other. But we've also had to interpolate between the information we've had on hand and then also had to extrapolate that to cover flood such as 2022. So that relationship between spills from Lake Eppalock and flood damages definitely approximate. Our study boundary ended downstream of Rochester. We haven't gone further downstream to Echuca for example. As I mentioned, we think the damages avoided by reducing the target storage with the existing outlet capacity are overstated. That's why those three options in the white box were held down the bottom in the prior slide. And we know that our estimates of avoided damages will increase once the flood modelling is updated to the count for records available from 2022. By that I mean the avoided damages in this table will go up, and some of those ratios will go up. But the relative difference between the options we think will stay pretty similar.

Now on the cost, I’m stressing again that the cost estimates are approximate. So AACE Class 5 is typically within minus 50% to a plus 100% of the true costs. The cost associated with recovering entitlements to offset the reliability impacts are approximate and would definitely vary depending on how or if that was done. We haven't looked at the socioeconomic costs of reducing the volume of water stored in the Campaspe system. And we haven’t include additional operation and maintenance costs. Though to be fair, they would be highest for the spillway gate option,  and that one is already coming out on at the bottom of the rankings.

So in summary - hopefully leaving enough time for questions - we do find that reducing the target storage or full supply level to about 70%, either using an increased outlet capacity or spillway slot, provides the best ratio of avoided flood damages to initial costs if, and I stress the if, ignoring the socio-economic costs of reducing the water storage in the Campaspe system - i.e. holding the lake lower - and if the additional maintenance costs are minimal. 

Whether the water sharing arrangements in the system can be changed to enable a lower target storage or FSL is not yet known. Further assessment is definitely needed, and that would involve assessing the socioeconomic consequences, but also the potential water recovery mechanisms, because the two are interlinked. For the purposes of this study, all entitlement water shareholders are being treated equally, 
but there is a wide variety of entitlement water shareholders in the system. The water recovery mechanism - if the lake were to be held lower - would influence in turn the socio-economic consequences. I suspect there will be further information on that or questions on that at the end of this.

If water sharing arrangements were to be changed, definitely additional work is required to optimise the trade-offs. By that I mean the trade-off between the target storage, the water resource implications, the avoided flood damages and the cost of the increased outlet capacity. Those four elements together would need to be optimised, and 70% might not be the best level. It might be to the left or right of that. Likewise, 5000 megalitres per day outlet might not be the best thing. It might be a bit more, might be a bit less.

If the water sharing arrangements cannot be changed, then only the spillway gates or the piano key options are plausible. Both have relatively low avoided damage to initial cost ratios, and adding spillway gates would definitely present increased operational costs and risks for Goulburn-Murray Water.

The options we've considered are also specific to Lake Eppalock, and will need to be benchmarked against other flood mitigation options that are being considered as part of the update for the Rochester Flood Management Plan.

As it's been mentioned a few times, we had done a lot in a short period of time, but there's definitely potential for further work. The flood hydrology modelling would ideally be recalibrated to the 2022 records. We've used a monthly water resource model to assess water resource implications. It would be good to do that with the updated daily source model that's being developed by DEECA. We've mentioned that socioeconomic consequences a few times. And a lot of the assessments we've done could be refined that meant the flood damage assessment or the initial assessments of recreational, environmental and cultural impacts, the concept designs and construction costs for the infrastructure options would definitely need to be done in more detail. 

And the operation and maintenance costs will need to be thrown into the mix at some stage. And then the trade-offs between that lake level, upstream flooding, downstream flooding, water resource implication, size of outlet, etc. would need to be optimised in future and stress tested using additional climate conditions. 
I think I've spoken nonstop for an hour now, so it might be time to pause for questions.

[Beth Ashworth- DEECA]

Thanks, Simon.

I think Simon did a really good job of summarising what has been quite a lot of work in quite a lot of complexity in assessing each of those options.

I can understand and appreciate that that was quite a lot of information and quite a lot of technical detail for you to take in. So really, I do encourage you to have a look at the summary report that HARC have prepared and also the explanatory note that DEECA have prepared to accompany that, that provides a bit of a context for the technical assessment, They're both available on the DEECA website.

But I just wanted to talk - before we move to the Q&A session - I just wanted to talk about what happens from here. This has obviously been a really important session in providing and sharing some of the findings from HARC from the technical assessment. But from February and March we’ll be out consulting on what some of those potential socioeconomic benefits and impacts might be across the five options. 

So we'll be providing some more information on the website about that. We’ll be engaging via a survey on the Engage Vic website and also be conducting some face-to-face sessions as well. From there, we'll provide our closing-the-loop report on what we've heard through that community consultation. That'll happen sometime from March to April, And then both of those - both the technical assessment itself and the closing-the-loop report - will be really important inputs to the Rochester Flood Management Plan, which will be conducted over sort of late 2024 and into 2025.

From there - depending on what comes out of that Rochester Flood Management Plan and what the preferred flood mitigation options might be from that - further work will need to be done around full cost-benefit analysis, broader community engagement, development of a business case, and if need be for major investments, environmental approvals and funding processes and design and construction and so on.

So I want to iterate that we are at sort of the start of a process. We're in a much better position than we were a few months ago. HARC have done a lot of work here, allowing us to do some initial prioritisation across those options. Some of those numbers, as Simon said, are not absolute. But they allow for a really good comparison. And then there’ll be further work from here as needed to firm up some of those numbers and progress. The Rochester Flood management plan will be the key process for that.

With that I think we're at that time for the Q&As. I actually can't see that there are any Q&As. If anyone does have any, this is a reminder that now is the time to add them into the Q&A function. There's a button at the top of your screen. But if none show up in the next minute or so, then we might wrap up.

I'd like to thank Simon again for his presentation.

Really thank all of you for making the time to participate and come along to hear what we've got to say. 

We'll be coming out, as I said in February and March, to consult on the broader socioeconomic benefits and impacts across the five options. But given we've not got any Q&As I think we'll wrap it up there and really hope all of you have a really safe and healthy festive season. I look forward to continuing these discussions in the new year.

Thanks very much.

Page last updated: 16/01/24