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Hydro Tasmania - Edgar and Scotts Peak Dams

Dr Rosalie Woodruff MP

Dr Rosalie Woodruff MP  -  Friday, 2 December 2022

Tags: Hydro Tasmania

Dr WOODRUFF - This is a change of question, minister. This is in relation to Hydro's risk assessment of the Edgar and Scotts Peak dams. Can you please tell me when the most recent risk assessment was commissioned? When was it received by Hydro? Which company or regulatory authority conducted the risk assessment?

Mr BARNETT - Thanks very much for the question, clearly related to operational matters which I will pass to the CEO. As a government, I want to indicate that Hydro Tasmania delivers a rolling program of strategic maintenance work to ensure that all their assets abide by safety and operational requirements and responsibilities. Hydro needs to strengthen Edgar Dam to ensure it continues to meet its safety obligations. New downstream gravel filters and rock fill will be installed to support the existing dam wall and improve the seismic resistance of the dam. That's the advice I've received. Likewise, there's consultation with project stakeholders, Hydro undertakes that, and the wider community. I understand that started in July this year. It's expected that the project will be submitted for environmental assessment later this year. I will refer to the CEO to assist the member.

Mr BROOKSBANK - Thank you, minister, and through you. As the minister has outlined, the Edgar Dam upgrade works are in relation to an ongoing risk assessment that we conduct of all of our assets, dams and otherwise. The Australian National Committee of Large Dams (ANCOLD) in 1988 incorporated new earthquake guidelines. They were released and in 2019; the seismic risk was reviewed of our assets. I will take on notice the specific date of the last review and answer before the end of this hearing. Suffice to say we undertake that work to ensure that all our assets are safe and reliable to operate.

Dr WOODRUFF - Thank you. Through you, minister, so, it was based on the ANCOLD guidelines, updated in 2019. You said, CEO, that we undertook the assessment, so Hydro did the assessment based on the guidelines? What was the risk rating for both of those dams? Could you please explain what 'high', 'tolerable', 'medium' means?

Mr BROOKSBANK - Thank you, minister. By Hydro undertaking the risk assessment, I mean we, obviously, have consultants who help us and we worked within the ANCOLD guidelines.

Dr WOODRUFF - Okay, that was my first question. Which company did the assessment.

Mr BROOKSBANK - In terms of the dams themselves - the Edgar Dam, as we know, is situated near the Lake Edgar fault line. That was a known feature back at construction time in 1971 72. At that time it was thought to be inactive. Over the intervening decades, the understanding of seismic risk has evolved, and that has influenced the dam's rating. This is a question of the method of seismic assessment having evolved. While the likelihood of the fault moving is extremely low, we obviously take a very conservative approach to the risk management of our dams.

In relation to the level of risk, the likelihood of a major earthquake occurring in the area impacting on Edgar Dam is calculated to be extremely remote, a 1 in 10 000 chance of it happening in any given year.

Dr WOODRUFF - Sorry, what is the risk rating of both the Edgar Dam and the Scott's Peak Dam? High? Tolerable?

Mr BROOKSBANK - Sorry, through the minister. No, there are no dams in the ANCOLD high rating level, but there are dams that are higher than others in our portfolio and therefore they get the attention first.

Dr WOODRUFF - Thank you. Minister, Edgar Dam and Scotts Peak Dam questions again, perhaps for the CEO. The works that are proposed to be carried out on Edgar Dam are designed to shore up the dam against the effects of an earthquake. Is it fair to say that the proposed solution is really to provide additional protection against the type of earthquake that results in tremors and shaking of the ground, and the associated waves generated in the dam by such tremors?

Mr BARNETT - Thanks for the question. CEO?

Mr BROOKSBANK - Thank you, minister, and through you. As I stated earlier, we have a rolling program of dam safety. Edgar Dam is our highest-priority dam due to the proximity to the fault. The consequences of a large earthquake - greater than magnitude 6 - would be significant, but the likelihood of the event occurring would be extremely low.

For the sake of explaining how we would manage the risk currently, and then to the answer. We have remote monitoring and regular physical inspections of our dams, and of course we have emergency preparedness plans in place. We manage the footprint around the dams and ensure that they are fit for purpose. The work itself on Edgar Dam is a level of strengthening that we will be doing to the dam - work we have undertaken in numerous other situations, dams, over the years.

The planning for that work started several years ago, and we are looking at starting that shortly, at a cost of $24 million. The work is around strengthening the dam itself, to ensure that it would not fail if that event occurred, and that the dam did not overtop and spill.

Dr WOODRUFF - Thank you. Could you please answer the question, which was specifically about the type of earthquake that the solution is trying to protect against. Is it a tremor and shaking of the ground type of earthquake?

Mr EVERY-BURNS - A big one, or a small one?

Dr WOODRUFF - Not the magnitude. Just the type. There are different sorts of earthquakes. Do you understand the question.

Mr EVERY-BURNS - No I don't. I have quite a lot of experience with dams and I do not understand what you asking.

Dr WOODRUFF - There are displacement types - surface displacement - and shaking and tremor types of earthquakes.

Mr BARNETT - Through you, Chair. The Chairman of Hydro Tasmania was just responding, if we could allow the Chair of Hydro Tasmania to respond to that question, then to the CEO.

Mr EVERY-BURNS - Dr Woodruff, your question is highly technical in the engineering sense. The integrity of the dam has not changed since it was constructed.

What has changed is an appreciation of the fault, and the likelihood or lack of likelihood of the fault moving at all. We are talking about a fairly significant earthquake. To be quite honest with you, it has not been put to me whether it is - well, it is an earthquake, that is what it is, and the mass of the dam will be increased, in which case the dam will not be susceptible to an earthquake, tremor, or call it what you will. It won't be susceptible to that risk.

Mr BARNETT - Did you want to answer that, CEO?

Dr WOODRUFF - I have a second question. A geological survey of that particular fault - which was done by Kevin McCue and others, and published in the 2003 Annals of Geophysics - has showed that, at that point, the two most recent earthquakes on that fault had involved surface displacement, in which one side of the fault rose by between two and three metres. Those earthquakes are rare, but the fact is, the last two that have been involved on that fault line involved significant vertical displacement. That appears to be concerning.

My question is, how could the proposed works possibly protect against a displacement of the surface of one side of the dam were it to be displaced by a number of metres on the other side of the dam?

Mr EVERY-BURNS - I am confident that our civil engineering group and the experts we have had advising us are well ahead of that issue. They totally understand it. I am not confident I understand it perfectly, but I am confident that the people who advise us do. I am confident that we are supported by other experts who I rely upon.

Mr BARNETT - I think the CEO wanted to add to that answer.

Mr BROOKSBANK - Thank you, minister. As the Chairman has said, we are managing the risk of the earthquake occurring, and you have described how that might look. There are two stages of work. The first stage involves draining and excavating the toe pond, which is at the base of the dam. Once that is prepared, the outer dam facing wall - which is concrete - will be removed in segments, and gravel filters and rock supports installed.

Dr WOODRUFF - Thank you. I have had a quite substantial briefing on this.

Dr WOODRUFF - Minister, you've confirmed that Hydro has mapped the flooding that's expected to occur downstream on the Huon River in the event of a worst-case dam failure of either Edgar or Scotts Peak dams or of both together. Why have you chosen not to release the mapping for public scrutiny by the community who would be affected in that event were it to happen?

Mr BARNETT - Thank you very much for the question. As I've shared previously and I'll confirm on the record, the CEO has outlined a whole range of answers to your questions on operational matters. It's clear that Hydro Tasmania provides relevant documentation, including maps, to all organisations involved in the coordination and delivery of incident responses. These documents are securely held on the Tasmanian Government's Common Operating Platform. It is important to note, as I said in my initial response, that Hydro Tasmania takes its management of dams' safety risk very seriously, consistent with relevant state legislation and it must continue to abide by that. Also, planning, coordinating and implementing emergency management responses, that is, likewise, very important.

Dr WOODRUFF - You didn't answer the question. I'll ask another one.

Given Hydro has provided the flood mapping, as you say, through the Tasmanian Government common operating system where it is stored. You said it had been provided to relevant agencies. I understand its Huon Valley Council and Emergency Services who have that information. Have those agencies/bodies been given an instruction not to release it to the public? If so, on whose authority?

Mr BARNETT - I can't speak for and on behalf of the Huon Valley Council. I can only speak on behalf of the Tasmanian Government. My answer remains as it was, that all those organisations involved in the coordination and delivery of incident responses and are held on the Tasmanian Government's Common Operating Platform. I will check if the CEO can add to that answer.

Mr BROOKSBANK - Thank you, minister, and through you. I can confirm that, yes, we have provided the relevant information to the appropriate bodies and that is held on the Tasmanian Government Common Operating Platform.

Dr WOODRUFF - Have they been instructed not to release them to the public?

Mr BROOKSBANK - Through the minister, I am not aware of any instructions they've received.

Dr WOODRUFF - So, they could be released to the public?

Mr BARNETT - In answer to the member for Franklin, as I said earlier, we can't answer for the Huon Valley Council and other entities. We can only answer for ourselves.


Mr BROOKSBANK - Thank you, minister. 

Before I answer the question, I would like to correct a statement I made earlier in relation to dams in the high-risk category of ANCOLD.  There are two dams, Edgar Dam and Scotts Peak.  The risk assessment for Edgar Dam was updated in 2020 following adjustments to the relevant assessment methods under the ANCOLD guidelines.  This work was completed by Entura. 

Edgar Dam and Scotts Peak exceed the limit of tolerability as defined by ANCOLD, and as such that risk level warrants taking action to lower that level.

In relation to the question, Hydro Tasmania does not know the final volumes Tas Irrigation requires for its Greater South East Irrigation Scheme and no agreements have been entered into.  Theoretically, there is water available, with an early indication figure of about 38 000 megalitres, which represents less than 2 per cent of the total average annual outflows from Lake Meadowbank. 

Hydro Tasmania will be completing a review of the impact any takes might have on Hydro Tasmania's availability to meet its existing obligations and commitments, and I will talk more to that in a second.  Hydro will continue to meet our existing environmental commitments in relation to that. 

With the contemplation of the Southern Midlands Irrigation Scheme also taking water into the Derwent catchment, Hydro Tasmania will be reviewing via long-term forward-looking modelling any potential impact that water taken will have on Hydro Tasmania's ability to continue to meet our special licence obligations as well as other downstream riparian and environmental flow-in requirements, water level commitments and constraints.  Hydro Tasmania would not enter into any agreements where these commitments were impacted.

Hydro Tasmania's existing commitments is to release a minimum of 18 cumex from Meadowbank Dam at all times and makes its best endeavours to achieve an average outflow of 32 cumex over a rolling five-day period.