R.J. Nathan, P.I. Hill, and P.E. Weinmann
The current definition of the Probable Maximum Flood (PMF) is open to subjective interpretation, and this lack of objectivity can lead to inconsistencies in the application of risk-based and standards-based criteria. This paper summarises the different approaches used to estimate the PMF, and highlights how these reflect differences in the availability of design information and local tradition and experience. A number of approaches are available that can aid the objective definition of the PMF. These approaches attempt to define the “reasonableness” of the manner in which the various flood producing factors are combined by reference to the relative shift in the annual exceedance probability of the event. The implications of the different approaches to deriving the PMF are summarised for a number of dams from across Australia. Guidance on deriving the PMF is provided in the paper with a view to seeking feedback from industry and consideration for inclusion in relevant guidelines.
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Mojtaba E. Kan and Hossein A. Taiebat
Abstract: The simplified procedures for evaluation of earthquake induced displacement in earth and rockfill dams are widely used in practice. These methods are simple, inexpensive, and substantially less time consuming as compared to complicated numerical approaches. They are especially recommended to be used as a screening tool, to identify embankments with marginal factor of safety, assuming that these methods always give conservative estimates of deformation. However recent studies show that application of these methods may not be conservative in some cases, especially when the tuning ratio of a dam is within a certain range. In this paper the fundamental theory behind the simplified methods is critically reviewed and practical guidelines are presented that can be used to identify cases where the simplified procedures may not be conservative.
2011 – Reliability of simplified methods for evaluation of earthquake-induced displacement in earth and rockfill dams
Robert Keogh RPEQ, CE Civil (Hon), Mal Halwala, Peter Boettcher, Renee Butterfield
SunWater is a Government Owned Corporation (GOC), operating in a competitive market on an equal commercial footing with the private sector. SunWater owns 23 referable dams. Over the last fifty years there has been significant development of the methodologies used to estimate extreme rainfall events. These have resulted in substantial increases in probable maximum flood (PMF) estimates for most of SunWater’s dams.
SunWater has undertaken a Comprehensive Risk Assessment program across its portfolio. SunWater now has a good understanding of the deficiencies and available risk reduction options for each dam under all load conditions. The total cost to rectify all deficiencies is several hundred million dollars and well beyond the financial capacity of the organisation in the short term.
ANCOLD and Regulators have different published opinions on decision making criteria for dam safety upgrades. Once the conditions for the tolerability of Societal and Individual Risk are satisfied the onus remains with the dam owner to meet the ALARP principle. The decision making process is complicated by uncertainties in inputs to risk assessments. The authors have considered these uncertainties as well as the legal implications, differing ANCOLD and Regulator requirements, and business and economic loss, in formulating the decision making process. The methodology is simplified but effective. If the process is followed the dam owner’s investments will meet ANCOLD, Regulatory, legal and business requirements.
This Paper details a logical decision making process designed to allow a non technical Board to balance social, legal and financial objectives. The process considers overall risk, tolerability, the ALARP principle, and project prioritisation. The process is being used by SunWater to determine the Acceptable Flood Capacity of each dam, which dams will be upgraded, priorities and scheduling of each upgrade.
How SunWater, as a commercial dam owner makes investment decisions for dam safety upgrades
Dan Forster, Murray Gillon
A robust and defensible dam surveillance process is considered to be the ‘front-line of defence’ in ensuring dams do not present an unacceptable risk to people, property and the environment. The concept of a ‘Quality Chain of Dam Surveillance’ describes the surveillance process as a multi-linked chain where each step in the process forms a critical link. Without rigorous attention given to quality assurance links in the chain can become tenuous or broken and thus compromise the integrity of the whole chain. Hydro Tasmania is currently re-engineering its existing surveillance process using the Quality Chain of Dam Surveillance as a basis.
This paper presents the concept of the quality chain and uses the Hydro Tasmania improvement initiative as an example application of the concept. The paper is intended to provide a fresh perspective on what is sometimes considered a stale topic and reinforces the need for a considered approach to dam surveillance.
2011 – The Quality Chain of Dam Surveillance
Michael 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
George Bolliger and Clare Bales
Traditionally, the dams engineering profession has been a career path for engineers of civil/structural or geotechnical persuasion. As dams are constructed there is understandably a predominate focus on the civil requirements. Beyond the first few years of the dam’s life, effective operation and maintenance becomes increasingly important. A number of mechanical/electrical components and plant items form part of the critical infrastructure of the dam. A good maintenance routine is an essential requirement of the dam safety management program.
State Water Corporation, as the owner of 20 large dams and over 280 weir and regulator structures, runs a dam safety management program that is in line with the Australian National Committee on Large Dams Guidelines and NSW Dams Safety Committee requirements. The maintenance procedures and outcomes are audited through an internal maintenance audit program.
The maintenance audits form an integral part of the total asset management plan as well as the dam safety program. They are used to identify areas of strength as well as common errors or defects. Using State Water’s internal maintenance audits as case studies, the paper elaborates the role of maintenance audit program in enabling a cultural change to further include mechanical/electrical aspects and thereby enhance the longevity and safety of the assets.
Cultural Change – A Mechanical Perspective on Dam Safety Management