Vicent Espert, Peter Buchanan, Colleen Baker, Malcolm Barker, Mark Locke
Mangrove Creek dam is an 80 m high CFRD constructed between 1976 and 1982 for water supply to the NSW Central Coast area, and is currently operated by Central Coast Council (CCC). The dam is classified as a ‘High A’ Consequence Category dam for both Sunny Day and Flood breach in accordance with ANCOLD guidelines.
Previous assessments of the dam identified that it would not be able to safely pass the ANCOLD Fallback flood capacity of the PMP flood in its current configuration. As such, the dam has been operated at a restricted full supply level for many years.
In 2020, GHD was engaged by CCC to develop a concept and detailed design to increase the spillway capacity using a standards-based approach to achieve the flood capacity fallback position. The first phases of this contract also required GHD to undertake additional investigations and analyses of various aspects of the dam and spillway to confirm the scope of works for the upgrade. During this review, it became evident that although the spillway capacity does not meet the ANCOLD fallback position, the Annual Exceedance Probability (AEP) of the existing capacity was relatively low and could potentially be deemed acceptable from a risk-based position.
A Risk Assessment was subsequently undertaken, with a SFAIRP assessment developed based on the new Dam Safety NSW guidelines. This assessment may be the first one to be completed for a major dam using the Dam Safety NSW guidelines. This paper discusses the different outcomes for a standards-based ‘Fallback’/’Simplified’ criteria and risk criteria based on DS NSW regulations, as well as the investigations developed to maintain confidence in the assessment. In addition, it describes a practical case for the application of SFAIRP criteria to a major dam.
In the case of Mangrove Creek Dam, the application of the new DS NSW Guidelines resulted in the dam being assessed as acceptable in its current state, with the FSL returned to the original design level. The outcome provided significant savings to the client, by avoiding costly upgrade works and avoiding disruption to the operation of the storage – a real success story.
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Anna Hams, Lindsay Millard, Elizabeth Jackson, Zara Bostock, Helena Sutherland
The Queensland dam regulator requires that dam safety risk during construction must not increase from its existing profile. The Stage 2A upgrade of Ewen Maddock Dam required excavation of its homogeneous embankment to retrofit chimney and filter blankets, and also the construction of a concrete parapet wall. Due to the constraints of the embankment profile and a constricted site, it was necessary to excavate the downstream face of the embankment. This excavation increased the risk of embankment failure due to overtopping, piping and instability. This paper discusses the measures taken to manage those dam safety risks, and includes:
● use of a temporary system consisting of six large siphons to regulate the lake level to a Restricted Full Supply Level (Restricted FSL). This encompassed the optimisation of lake level and capacity of siphons required to balance competing risks; dam safety, environmental, community and water security. This optimisation was based on a probabilistic assessment of hydrological inflows and lake levels, the development of a flow management plan;
● implementation of a Dam Safety Management Plan which outlined the roles and responsibilities for
managing dam safety during construction at each pre-determined lake level trigger levels. This includes how the contractor was involved to ensure quick response from the “eyes and ears on the ground”; and,
● development of recommended construction methodologies including a “rolling front” and placing
filters vertically to increase production, maintain quality and limit the extent of embankment excavation underway.
Alberto Scuero, Gabriella Vaschetti, John Cowland
Efficiency in water supply reservoirs, even more so in pumped storage reservoirs, requires good water management and minimisation of water losses. With climate change affecting the quantity of water available for supply and power generation, minimising water losses is becoming more and more crucial, and the most efficient way to achieve this critical objective is to line the reservoir with a watertight geomembrane system. With more than 60 years of use, flexible geomembrane systems have proven to be a dependable technology for new construction as well as for rehabilitation. Efficiency can also be increased by covering the reservoir with a floating geomembrane cover to minimise evaporation losses, and by adding value to the reservoir with the installation of floating photovoltaic panel farms on the surface of the reservoir, to provide or increase electrical power generation. This paper addresses these two aspects of efficiency: water loss minimisation, by presenting concepts and advantages of geomembrane liners, and concepts and application
of floating photovoltaic farms with a case history in a water supply reservoir. The concept of a floating
photovoltaic farm on a pumped storage reservoir, and information on available guidelines for geomembrane systems and floating photovoltaic panels, are also presented.
Chris Nielsen, Ron Guppy, Donna Dunn, David Murray
Following several years of investigations and analysis a serious safety issue with the stability of the primary spillway during major flood events was identified at Paradise Dam that required urgent risk reduction works. The response to this safety issue was significant.
The Inspector General Emergency Management conducted a review into the effectiveness of emergency response if a dam safety event were to occur, taking into consideration process and communications to manage around 40,000 population at risk, comprised mostly of residents within the city of Bundaberg.
An essential works program to reduce the risk was urgently prepared then executed effectively within a calendar year. This short timeframe required significant and novel amendments to Queensland’s laws to bypass normal legislated process for such a major project.
The Paradise Dam Commission of Inquiry was established to identify the root cause of the issues, the facts and circumstances that contributed to them and recommendations to consider for future dam projects. All recommendations from the commission were accepted by the Queensland government and, following an extensive stakeholder engagement exercise, have been implemented through changes in policy and methodology and described in published guideline revisions.
For future dam projects the lessons learnt highlighted the need for early and ongoing engagement of
independent technical review, project governance that is cognisant of risk and the ownership and capacity to bear of that risk, the need to consider testing to confirm critical design parameters and the need for an effective regulator. The essential works program has established a precedent for the timely and appropriate application of risk reduction measures.
Dan Clark, Joanne Stephenson, Trevor Allen
We present earthquake ground motions based upon a paleoseismically-validated characteristic earthquake scenario for the ~ 48 km-long Avonmore scarp, which overlies the Meadow Valley Fault, east of Bendigo, Victoria. The results from the moment magnitude MW 7.1 scenario earthquake indicate that ground motions are sufficient to be of concern to nearby mining and water infrastructure. Specifically, the estimated median peak ground acceleration (PGA) exceeds 0.5 g to more than ~ 10 km from the source fault, and a 0.09 g PGA liquefaction threshold is exceeded out to approximately 50-70 kilometres. Liquefaction of susceptible materials, such as mine tailings, may occur to much greater distances. Our study underscores the importance of identifying and characterising potentially active faults in proximity to high failure-consequence dams, including mine tailings dams, particularly in light of the requirement to manage tailing dams for a prolonged period after mine closure.
Ryan Singh, Jiri Herza, James Thorp, Michael Ashley
Performance-based risk-informed decision making is an underlying principle of the Global Industry
Standard on Tailings Management (GISTM). While owners make significant efforts to align with this
principle, commonly used risk assessment and management practices in the mining industry have largely been based on the HSE principles, which consider more frequent, lower consequence incidents.
As a result, the existing risk assessment frameworks do not provide the owners with a comprehensive understanding of the risk profiles of their tailings storage facilities (TSFs). Without the understanding of a facility’s risk profile, the owners cannot appreciate how changes to their facility, processes and operational activities may impact the risk profile. A large step-change in thinking is therefore required in risk assessment practices for the owner to align their TSF management with GISTM requirements.
Beyond risk assessments, the mining industry has other valuable concepts to manage the safety of their tailings management practices, such as Critical Controls, however, commonly used risk assessment and management practices do not incorporate these concepts.
This paper explores commonly used risk assessment practices and the concepts of Critical Controls. It proposes how these concepts can be linked, with Critical Controls being embedded in the risk assessment process. The outcomes of linking these concepts result in an estimation of the effectiveness of the Critical Controls and how they can be improved to demonstrably reduce the risk presented by a TSF. A case study has been included to demonstrate the benefits of linking risk assessment with Critical Controls and how owners can readily identify deficiencies and efficiently manage the risk profiles of their facilities.