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P.I. HILL, R.J. NATHAN, P.E. WEINMANN, J.H. GREEN
The assessment of flood risk is important to the safe design, maintenance and operation of dams. Traditionally, a standards-based approach has been adopted, in which the adequacy of a spillway was assessed by its ability to pass the whole, or a specified fraction, of the Probable Maximum Flood (PMF). More recently, however, the ANCOLD Position Paper on Guidelines for Selection of Acceptable Flood Capacity for Dams has moved towards a risk-based approach, in which attention is focused on establishing the exceedance probability of the maximum flood that can be safely passed by the spillway.
The move to a risk-based approach has led to an increasing focus on the derivation of floods with very low probabilities of exceedance. The chapter in Australian Rainfall and Runoff that gives guidance on the estimation of extreme floods has recently been revised and issued as Book VI. The new guidelines reflect the move from a standards, to a risk based approach and also include recent developments in extreme flood estimation. These recent developments result in an improved estimate of floods in the large to extreme range and hence a more reliable estimate of hydrologic risk.
This paper illustrates the impacts of the new flood guidelines by summarising the results for 7 dams in Southeastern Australia. For the examples presented in this paper the impact of the new guidelines is to reduce the estimated hydrologic risk. The new guidelines have an important effect on the estimation of hydrologic risk and therefore the assessment and management of dams in Australia.
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1998 Papers
1998 – Improved Estimates of Hydrologic Risks for Dam – Impacts of the New Flood Guidelines
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P.I. HILL, R.J. NATHAN, P.E. WEINMANN, J.H. GREEN
The assessment of flood risk is important to the safe design, maintenance and operation of dams. Traditionally, a standards-based approach has been adopted, in which the adequacy of a spillway was assessed by its ability to pass the whole, or a specified fraction, of the Probable Maximum Flood (PMF). More recently, however, the ANCOLD Position Paper on Guidelines for Selection of Acceptable Flood Capacity for Dams has moved towards a risk-based approach, in which attention is focused on establishing the exceedance probability of the maximum flood that can be safely passed by the spillway.
The move to a risk-based approach has led to an increasing focus on the derivation of floods with very low probabilities of exceedance. The chapter in Australian Rainfall and Runoff that gives guidance on the estimation of extreme floods has recently been revised and issued as Book VI. The new guidelines reflect the move from a standards, to a risk based approach and also include recent developments in extreme flood estimation. These recent developments result in an improved estimate of floods in the large to extreme range and hence a more reliable estimate of hydrologic risk.
This paper illustrates the impacts of the new flood guidelines by summarising the results for 7 dams in Southeastern Australia. For the examples presented in this paper the impact of the new guidelines is to reduce the estimated hydrologic risk. The new guidelines have an important effect on the estimation of hydrologic risk and therefore the assessment and management of dams in Australia.
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2000 Papers
2000 – Development of a Comprehensive Approach to Consequence Assessment
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P.I. Hill, D. Cook, R.J. Nathan, P.A. Crowe, J.H. Green, N. Mayo
This paper describes the development of a comprehensive approach to estimating the consequences of failure of a dam. The approach considers separately the consequences in terms of potential loss of life, economic loss and damage to the environment and the development and application of the method involved professionals from a wide range of disciplines. The method has been applied to 28 dams in NSW.
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2001 Papers
2001 – On the Art of Event Tree Modelling for Portfolio Risk Analyses
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P.I. Hill, D.S. Bowles, R.J. Nathan, R. Herweynen
With the growing emphasis on a risk-based approach to dam safety management, event tree models are increasingly being used as an analysis tool. The simple structure of event trees belies some of the more complex issues associated with their application to dam safety risk analysis. This paper outlines some of the basic principles of event tree analysis and then demonstrates how inappropriate construction of event trees, and particularly oversimplification, can result in a bias in the estimated risk and produce misleading results when used to assess the dam against various risk criteria. Issues considered include the partitioning of the loading event and the impact of conservative assumptions such as assuming the reservoir is initially at full supply level.
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2001 Papers
2001 – Risk Implications of the PMF and PMP Design Flood
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R.J. Nathan, P.I. Hill, and H. Griffith
Recently released ANCOLD and IEAust flood guidelines include provision for the estimation of the Probable Maximum Flood (PMF) as well as the Probable Maximum Precipitation Design Flood (PMP DF). This paper examines the theoretical justification for derivation of these two different types of floods, and discusses how they may be used to characterise the hydrologic risk relevant to dam spillways. Recent experience has indicated that there is some confusion in the industry about the different uses of these estimates, and thus one objective of the paper is to clarify the different concepts involved and to provide an illustration of the differences between flood estimates for the two methods. Examples are provided to illustrate how the different estimates may be derived, and the practical implications for risk analysis are discussed.
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2002 Papers
2002 – Probabilities of Extreme Rainfall – Past, Present, and Future
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J.H. Green, P.E. Weinmann, G.A. Kuczera, R. J. Nathan and E.M. Laurenson
Assigning an Annual Exceedance Probability (AEP) to the Probable Maximum Precipitation (PMP), and subsequently to the PMP Design Flood, is an integral part of the risk assessment process for large dams. Laurenson and Kuczera (1998) conducted a review of existing PMP risk estimation practices in Australia and concluded that, in the absence of any better information, the work by Kennedy and Hart (1984) provided the most appropriate estimates to adopt but with the proviso that the method should be viewed as interim pending the outcomes of ongoing research.
This paper gives an overview of a joint research project that is working towards obtaining credible estimates of exceedance probabilities of extreme rainfalls using the concept of storm arrival probability and storm transposition probability. It also outlines the work to be carried out over the next 12 months that will culminate in the combining of the outcomes of the two components and the application to test catchments. Finally, the paper discusses desirable follow-up action to promote the adoption of the research results by practitioners.
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2002 Papers
2002 – Use of a Monte Carlo Framework to Characterise Hydrologic Risk
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R.J. Nathan, P.E. Weinmann and P.I. Hill
Current practice for estimation of design floods is typically based on the “design event” approach, in which all parameters other than rainfall are input as fixed, single values. Considerable effort is made to ensure that the single values of the adopted parameters are “AEP-neutral”, that is, they are selected with the objective of ensuring that the resulting flood has the same annual exceedance probability as its causative rainfall. While this approach represents current best practice in Australia (and overseas), it does suffer from a number of limitations.
This paper describes the development and application of a Monte Carlo (or joint probability) framework which offer an alternative to the design event method. This technique recognises that any design flood characteristics (e.g. peakflow) could result from a variety of combinations of flood producing factors, rather than from a single combination. The approach mimics “mother nature” in that the influence of all probability distributed inputs are explicitly considered, thereby providing a more realistic representation of the flood generation processes.
The advantages of the technique are illustrated by application to a hypothetical dam located on a real catchment. The manner in which standard design inputs are incorporated are discussed, as is the relationship of the approach to current guidelines.
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2004 Papers
2004 – TOWARDS INCREASING OBJECTIVITY IN THE PROBABLE MAXIMUM FLOOD
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Selected aspects of the guidelines dealing with the estimation of dam safety floods are being
considered for revision as part of a larger overhaul of “Australian Rainfall and Runoff”. One section
that warrants attention is the guidance provided on estimation of the Probable Maximum Flood
(PMF). The section of the current guidelines that deals with this issue received little attention in the
last revision as the focus was on estimating floods for risk-based design, that is on floods of known
annual exceedance probability. However, it is now common to jointly consider both risk-based and
standards-based components in the design process, and this has highlighted problems associated with the manner in which the PMF is defined and estimated. This paper examines the current problems involved with estimation of the PMF, and presents concepts and practical estimation approaches that could be considered for incorporation in the guidelines. The purpose of this paper is to promote discussion and elicit feedback from industry to aid the revision process.
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2004 Papers
2004 – Towards Increasing Objectivity in the Probable Maximum Flood
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R.J. Nathan and P.E. Weinmann
Selected aspects of the guidelines dealing with the estimation of dam safety floods are being
considered for revision as part of a larger overhaul of “Australian Rainfall and Runoff”. One section
that warrants attention is the guidance provided on estimation of the Probable Maximum Flood
(PMF). The section of the current guidelines that deals with this issue received little attention in the
last revision as the focus was on estimating floods for risk-based design, that is on floods of known
annual exceedance probability. However, it is now common to jointly consider both risk-based and
standards-based components in the design process, and this has highlighted problems associated with the manner in which the PMF is defined and estimated. This paper examines the current problems involved with estimation of the PMF, and presents concepts and practical estimation approaches that could be considered for incorporation in the guidelines. The purpose of this paper is to promote discussion and elicit feedback from industry to aid the revision process.
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2011 Papers
2011 – ACHIEVING CONSISTENCY IN DERIVATION OF THE PROBABLE MAXIMUM FLOOD
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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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