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Gregg A Scott
Abstract: The Bureau of Reclamation has been performing quantitative risk analysis as the primary dam safety decision making tool for well over a decade. This paper summarizes some of the key concepts and basic methodology currently used in the dam safety risk analysis process at Reclamation.
Keywords: dam safety, risk analysis, reliability analysis, event trees, subjective probability.
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2008 Papers
2008 – Best practices in dam safety risk analysis at the Bureau of Reclamation
Learn moreGregg A Scott
Abstract: The Bureau of Reclamation has been performing quantitative risk analysis as the primary dam safety decision making tool for well over a decade. This paper summarizes some of the key concepts and basic methodology currently used in the dam safety risk analysis process at Reclamation.
Keywords: dam safety, risk analysis, reliability analysis, event trees, subjective probability.
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2008 Papers
2008 – Risk assessment with uncertainty analysis for an already-initiated failure mode
Learn moreDavid S Bowles, Sanjay S Chauhan, Loren R Anderson, Terry F Glover
Abstract: A nested model is presented for considering variability and knowledge uncertainty in a dam safety risk assessment of an existing dam and interim risk-reduction alternatives (operating restrictions) during the staged implementation of a permanent structural risk reduction measure. The effects of some important aspects of natural variabilities on estimated risks are represented as cumulative distributions of probability of failure, annualised life loss, economic risk cost, and an F-N representation of life loss. Many cumulative distributions are generated to represent the effects of some important aspects of knowledge uncertainties.
An important aspect of the knowledge uncertainty is the current level of development of an already-initiated piping failure mode. Also, an approach to conditioning the system response probabilities (SRPs) for the piping failure mode on the duration of reservoir pool exceedance is included in the failure event tree risk model.
ANCOLD and Reclamation tolerable risk guidelines are evaluated at selected percentiles of variability and percentiles (levels of confidence) of knowledge uncertainty. The incremental cost-per-statistical-life saved and benefit-cost ratio for interim risk-reduction alternatives are estimated and evaluated to examine the case for more-severe levels of operating restriction than the least-severe operating restriction that is estimated to satisfy, at a selected percentile of variability and a desired level of confidence, the limit values in all of the tolerable risk guidelines that were considered.
Keywords: Dam safety, risk analysis, risk assessment, uncertainty analysis, aleatory uncertainty, epistemic uncertainty.
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2008 Papers
2008 – Fairbairn Dam – performance during flood of record
Learn moreRob Ayre, Terry Malone
Abstract: Fairbairn Dam with a storage capacity of 1,301,100 ML is the second largest dam in Queensland in terms of water supply capacity. The dam forms the head works of the Nogoa – Mackenzie Water Supply Scheme operated by SunWater in Central Queensland. Completed in 1972, it consists of a zoned earth-fill embankment 49 m high and 823 m in length. The dam has an un-gated ogee spillway crest that is 4.2 m high and 165 m long, with an original design capacity of 15,600 m3/s.
In January 2008, Central Queensland experienced significant flood producing rains which were generated from low pressure systems associated with monsoonal activity across northern Australia. Rainfall totals over the 16,000 km2 catchment area of Fairbairn Dam varied in depth from around 200 mm to nearly 700 mm during a five day period to 20 January 2008. This resulted in the largest outflow from the dam since its construction and the first spill event from the dam since April 1990. While the dam had a significant mitigating impact, there was still major flooding of the township of Emerald, some 19 kilometres downstream.
This paper describes the performance of the dam during the event. Details of the data collected during and after the event, including assessments of spillway performance, dam safety surveillance and the implementation of the Emergency Action Plan will be presented. In particular, the paper focuses on the flood response concerning downstream communities and the resultant flood effects on Emerald and major infrastructure located in the downstream flood plain. It highlights the need for dam owners to have the capability of forecasting inflows and outflows to their structures and how this information contributes to the overall flood response system.
Keywords: dam safety, spillway, flooding, Fairbairn Dam, Emerald, SunWater, Queensland.
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2009 Papers
2009 – The internal investigation of conduits through an earth dam
Learn moreTim Logan, Angus Swindon, Chris Topham
Edgar Dam is a 17m high saddle dam forming part of the Gordon River Power Development (GRPD) in south west Tasmania; the smallest of three dams, which created the current Lake Pedder. It is essentially a homogeneous embankment, designed and built between 1970 and 1972. It is assigned a “High A” Hazard Category. An unusual feature of the dam design is a reinforced concrete facing on the upstream face, crest and the upper portion of the downstream face provided primarily as protection against wave overtopping.The upstream facing is bedded on drainage material encapsulating a longitudinal drain 1.6m above the level of the bottom of the concrete. This drain is connected to four transverse drains (100 mm diameter PVC) which run through the body of the dam and discharge through the concrete slab on the downstream face. The screening level risk assessment for Edgar Dam identified piping through the embankment as the predominant failure mode, particularly related to the transverse drains and the uncertainty surrounding the competency of the backfill around the pipes. To address this, the condition of drain backfill has been assessed using geophysical logging, supplemented by an internal video inspection. The information has allowed a more detailed risk assessment to be performed and potential mitigation measures to be assessed.
Keywords: Risk Management, Dam safety, Conduits, Geophysical Logging.
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2009 Papers
2009 – Ross River Dam Upgrade – Flood Commissioning Experience
Learn moreRob Ayre, Simone Gillespie, Peter Richardson, Mark Harvey
In November 2007, NQWater (now Townsville City Council) completed the upgrade of Ross River Dam near Townsville. This upgrade included the installation of a Program Logic Control (PLC) operated radial gate structure in the existing spillway. The purpose of the upgrade was threefold; to meet current design standards regarding dam safety and flood capacity; increase the town water supply to Townsville and Thuringowa; and to maintain the flood mitigation benefit of the dam during flood events in the Ross River catchment.
The designers of the dam upgrade completed a risk assessment at the commencement of the design phase and concluded that a PLC operated gated dam posed less risk than an approach that relied purely on dam operators. This is seen as the “new way forward” in gate operated dams by many designers. The regulator of dam safety in Queensland has indicated a preference for an operator to remain on site full time whilst the PLC system is ‘proved’. As a consequence a flood operations team was engaged to monitor the dam during times of flood through the commissioning period of the upgrade.
This paper discusses the lessons learnt from the experiences of the Ross River Dam during its first few flood events and issues associated with the adopted design approach.
Keywords: Dam Safety, Gated Spillway, Flood Operations, Ross River Dam, NQWater, Townsville City Council, SunWater, Queensland.
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2009 Papers
2009 – Managing dam portfolio risk using a model for World Class Asset Management
Learn moreAngus Swindon, Tony Ang
Managing risk lies at the heart of the asset management processes. All utilities and large asset owners strive to ensure their asset management practices meet the needs of the business. To be able to demonstrate this to internal and external stakeholders including Boards, Regulators and Insurers, some form of bench marking or comparison with industry practice is often undertaken. This inevitably leads to a discussion about what good or indeed best practice might look like and a somewhat philosophical discussion about what aspects of these might be appropriate. If an organisation chooses to define “World Class Asset Management” as “the level of practice that no one else could exceed given the same internal requirements and constraints and external operating environment” then that organisation must develop a framework such that a pathway to achieve this can be defined and an assessment made. This paper will present such a framework and describe a pathway to demonstrate progress towards asset management maturity. Actively managing surveillance practices to monitor dam condition and performance is presented as an example within such a framework.
Keywords: World Class Asset Management, Dam Safety, Surveillance, ANCOLD Guideline.
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2009 Papers
2009 – Innovative Solution to foundation piping risk at Hinze Dam
Learn moreGavan Hunter, Chris Chamberlain, Mark Foster
Hinze dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is principle potable water storage supplying the Gold Coast. Hinze Dam Stage 3, presently under construction, involves raising the existing embankment almost 15m to a maximum height of 80m.
The foundation geology on the right abutment of the main embankment comprises of a deeply weathered sequence of greywacke and variably silicified greenstone and chert. The deeply (and variably) weathered soil profile below the right abutment of the existing embankment presented an unacceptable piping risk for the embankment in its existing condition. Contributing factors included: 1/ the highly erodible extremely weathered greywacke and presence of continuous defects in the weathered soil mass; 2/ the extremely weathered greenstone in direct contact with highly fractured, highly permeable silicified greenstone and chert bodies aligned normal to the dam axis which provide continuous seepage paths through the foundation.
Works were required as part of the Stage 3 raise to address the foundation piping risk. Significant issues for design included: 1/ the depth of weathering extended up to 25to 40m into the foundation.; 2/ extremely weathered and highly erodible greenstone was present below the right abutment of the embankment and extended down to the lower abutment some 50 to 60 m below the existing dam crest; 3/ the reservoir level could not be drawn down during construction and the probability it would be near full supply level during the works was high; and 4/ the variability of strength in the greenstone form soil to extremely high strength presented challenges for excavation.
The options assessed to address the piping risk included a plastic concrete cut-off wall and an upstream blanketing option. The plastic concrete cut-off wall (220m long and up to 50m deep) and deep filter trench was the selected option. The cut-off wall had been successfully completed ahead of time and below budget. The innovative design required excavation through earthfill core of the embankment under full reservoir level and use of a purpose built trench cutter (by Bauer Foundations Australia) for the variable excavation conditions.
Keywords: dam safety, piping, risk assessment, cut-off wall.
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2009 Papers
2009 – Dam Safety Regulation of Large Farm Dams in Queensland
Learn moreRobert Fowden, Peter Allen, John McKenna
The Large Referable Farm Dam Assessment Program commenced in early 2006 after inspections identified a significant number of Queensland dams that were unknown to the Department of Environment and Resource Management (DERM) and could potentially threaten life if they were to fail. The program is unique given the number of structures under consideration and is understood to be the first widespread, systematic search for dams with a population at risk in the world. The Dam Safety (Farm Dams) team has developed many original solutions to allow the majority of investigations to be undertaken in-house, thus minimising the potentially higher cost and timeframe issues associated with obtaining external engineering and surveying support.
Keywords: Queensland, dam safety, dam failure, regulation, farm dams, surveying, modelling
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2009 Papers
2009 – A Review of the Limitations and Options for Dam Safety Management beyond the ANCOLD Guidelines
Learn moreGiovanni De Cataldo
The ANCOLD Guidelines on Dam Safety Management August 2003 were formulated to ensure that dam owners adopt a responsible approach towards the safe operation and maintenance of their dams.
Is it possible to safely, responsibly and acceptably work outside the regulatory Guidelines/Requirements?
The challenge for dam owners now and into the future in meeting stringent standards, is to cost effectively manage their assets within available financial constraints whilst minimising risks and maintaining acceptable levels of safety.
With the continuing drought and suppressed storage levels in most dams, the risk to downstream communities and to the environment from dam failure is significantly reduced.
Based on various studies, investigations, internal workshops and external “Expert Panel” reviews, this paper puts forward a case for a sound and responsible risk-based approach to routine visual and surveillance monitoring frequencies at varying storage levels for “Sunny Day” conditions and compares it against traditional ANCOLD standards which are based solely on consequences.
Keywords: State Water Corporation, ANCOLD guidelines, risk-based approach, dam safety, regulator.
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2010 Papers
2010 – Overview of Recent Dam Safety Activity on the Waikato Hydro System
Learn moreJustin Howes, Peter Amos
For many years Mighty River Power has operated an intensive Dam Safety Assurance Programme with respect to our nine large hydro assets, a unique run of river cascade system built between 1927 and 1972. From 2001 to 2007 the Arapuni Foundation Enhancement Project was a high profile activity, but there has also been much dam safety analysis and minor mitigation work that could be classified as “Business As Usual Dam Safety Activity” – this paper seeks to give a high level overview of the work carried out from 2000 to 2010. Items covered include; an overview of the hydraulic structures, their hydrological and geological setting, and the current dam safety regime. Examples of typical issues identified by the Programme are given on a structure by structure basis along the river. Seismic, Flooding, Emergency Planning, Documentation, Monitoring, Control, Electrical and Mechanical type issues are covered.
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2010 Papers
2010 – Innovations in resolving construction challenges on the Catagunya Dam Restoration Project
Learn moreCubit T, Swindon A, Tanner D
Catagunya Dam is located on the Derwent River in Tasmania’s south east. During construction of the dam in early 1960’s 412 post-tensioned anchors were installed, however the integrity of the original anchors can no longer be assured. The stability of the dam was restored between 2008 and 2010 using 92 modern, large diameter, load monitorable and corrosion protected post-tensioned anchors. These are the most highly stressed anchors applied to a dam at this time.
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Some of the key construction challenges included installing 53 anchors within an operating spillway, utilising a very limited construction window and replacing severed surface reinforcement using carbon fibre rods.
This paper details how these challenges were resolved and presents a number of innovative solutions developed along the way. -
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2010 Papers
2010 – Hinze Dam: numerical modelling prediction versus performance
Learn moreGavan Hunter and James Toose
Hinze Dam, located on the Gold Coast in Queensland, is an Extreme hazard storage under the authority of Seqwater (Southeast Queensland). The Stage 3 works, which are coming to completion, require raising the existing 65 m high central core earth and rockfill embankment almost 15 m to a maximum height of 80 m. The reservoir has been near full supply level for the construction period.
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Numerical modelling and empirical predictive methods were used to estimate the deformation at three key embankment sections during construction; the right abutment of the main embankment, the maximum section and the main to saddle embankment connection. The results of the analysis were incorporated into the dam safety management plan to provide a framework for evaluation of the monitored deformation during construction.
This paper summarises the numerical modelling and outlines the framework of the dam safety management plan. It then compares the actual deformation measured during construction against the predictions. Overall, the modelled deformation has compared very well in terms of trend and reasonably well in terms of magnitude with the actual deformation to date. On one occasion the deformation has exceeded the estimates and triggered a response to elevate the review to higher levels within the Alliance. Concluding comments are provided on the useful aspects and limitations of the numerical modelling at Hinze Dam. -
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2010 Papers
2010 – Emergency Release Capacity for Dams – International to Local Perspectives
Learn moreC.Johnson, D.Stephens, M.Arnold and N.Vitharana
As part of Melbourne Water’s dam safety upgrade program, emergency release capacity is being investigated at a number of dams. Recent work undertaken by the Water Resources Alliance (WRA) for Melbourne Water has highlighted the lack of current Australian guidelines for appropriate emergency release capacity. With no relevant ANCOLD Guidelines, current practice still references the 1990 USBR guidelines which relate the length of time to empty a reservoir to the hazard and risk associated with dam failure. As hazard category assessment criteria has been improved since and dam design and safety standards are more stringent, the applicability of the USBR criteria in today’s environment is under consideration.
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With the prevailing climatic conditions requiring the augmentation of Melbourne’s water supplies, the Tarago Reservoir was recently brought back into service. However, the dam lacked adequate emergency and environmental release capacity, with this being critical to manage construction flood risk for a pending filter raising project. Through an analysis of recorded inflow data, it was evident the existing scour facility had insufficient capacity to handle the recorded inflows, and would not be able to maintain the reservoir at an appropriate level during the proposed works. The length of time to empty the reservoir for the existing scour facility and the preferred scour upgrade option were calculated and it was found that by providing a new 1200mm scour facility, USBR emptying times were met or exceeded. The enlarged outlet capacity was also required to meet the new environmental flow requirements for the dam.
The paper will review international guidelines, share the experience of several Australian water authorities in assigning emergency release capacity for their dams, and discuss the specific work undertaken to provide suitable emergency release capacity at Tarago Reservoir for Melbourne Water. -
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2010 Papers
2010 – Deformation and dam safety of earthfill dams
Learn moreGavan Hunter and Robin Fell
Earthfill embankments in Australia have been widely used in dam construction since the start of the 20th century replacing the older puddle type and concrete corewall embankment designs. Most Australian dam portfolios will have one or more of these embankment types. A key component to the dam safety assessment of these dams is understanding their deformation behaviour, in particular the assessment of the future performance of these structures as they now reach ages of 40 to 80 years or more.
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This paper presents the findings of a study on the deformation behaviour of earthfill embankments. It draws on a database of 54 case studies from mainly Australia, the United States and Europe. It is a component of a broader study on the deformation behaviour of embankment dams undertaken as a research project at University of New South Wales earlier this decade.
The data presented in this paper allows dam owners and their consultants to compare the deformation behaviour of their dam to the performance of other similar earthfill dams in evaluating dam safety. Typical patterns of deformation behaviour are presented and guidance is provided on identifying trends in deformation behaviour that are assessed as “abnormal” and that may be potentially indicative of marginal stability. -
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2011 Papers
2011 – Western Australian Dam Safety Challenges for ANCOLD Part 2
Learn moreMichael Somerford and Steven Fox
The Water Corporation of Western Australia has been implementing the risk assessment process promulgated by ANCOLD for 12 years. This approach has been central to a $310 million dam safety remedial works program that has reduced life safety risk across the Water Corporation’s portfolio of dams by an order of magnitude. However, whilst this process has provided a rational basis to prioritise dam safety upgrades, there are still questions that have not been fully answered and further development of the application of risk assessment to dams is desirable.
This paper revisits some of the key concerns that remain evident with the process and argues that unless further guidance is provided it may be that dam safety upgrades have effectively “hit the wall”; and upgrade programs commenced may never be completed as envisaged by ANCOLD.
2011 – Western Australian Dam Safety Challenges for ANCOLD Part 2
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2011 Papers
2011 – Towards increased clarity in the application of ALARP
Learn moreRoger Vreugdenhil, Peter Hill, Siraj Perera, Susan Ryan
All Australian water authorities have in place dam safety programs that seek to ensure the ongoing safety and serviceability of their dams along with the benefits they secure for the wider community. Many have progressed multiple dam safety upgrades over the past decade and embraced risk assessment as a helpful tool in prioritising upgrade investment.
The ANCOLD Guidelines on Risk Assessment (2003) have been applied across the country and, coupled with State regulation, have supported dam owner efforts in reducing risks below the ANCOLD “Limit of Tolerability”. However, it is generally acknowledged that in their current form, the ANCOLD guidelines provide limited guidance to dam owners for determining appropriate levels of risk reduction and timing of dam safety improvements. This has contributed to a range of guideline interpretations and inconsistency in subsequent dam safety investment decisions across Australia. Having achieved priority risk reduction, a number of owners are beginning to assess their dams against the ALARP principle, bringing dam safety investment within an owner’s portfolio into more direct competition with other important and urgent organisational investment decisions.
This paper outlines the outcomes of a recent study commissioned by the Victoria Department of Sustainability and Environment into risk reduction principles and the application of ALARP by a number of Australian and international dam owners and regulators, hazardous industry owners and regulators, and the interaction of ALARP with whole-of-organisation investment. The paper highlights the study process and significant points of interest regarding risk reduction principles and current application of ALARP and some options for refinement and clarity.
Towards increased clarity in the application of ALARP
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