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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 – 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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2016 Papers

2016 – Estimating the Probability of Breach for Overtopping of Low to Medium Height Homogenous Earthfill Embankments
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Kelly Maslin, Richard Rodd

As an industry there have been many advances in the assessment of the probability of failure associated with a range of failure modes including embankment piping and stability. However, little work has been done on the development of a meaningful tool to assist in the assessment of probabilities of failure for embankment breach due to overtopping.

In the development of this paper a number of embankment overtopping case studies were reviewed and these were used to anchor the suggested probabilities of failure. The case studies assessed were all low to medium height, homogeneous earthfill embankment dams. Consideration has been given to a range of factors including embankment material and construction, embankment geometry, duration of overtopping and the presence and condition of vegetation on the embankment face.

The results of the analysis of the case studies indicate that the probability of breach due to overtopping, particularly for short duration events, is actually relatively low compared to the typical values being adopted within the industry.

It is the intended purpose of this paper that it provides guidance to the industry on the assignment of the probability of embankment breach due to overtopping to allow more consistent, robust and defensible estimates for dam safety risk assessments.
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2009 Papers

2009 – Burdekin Falls Dam – Testing the Boundary of Hydrology
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Rob Ayre, Simone Gillespie

The Burdekin Falls Dam (BFD) is a SunWater-owned dam completed in 1987. BFD is located in North Queensland, approximately 180 kilometres south of Townsville. BFD is the largest dam in Queensland having a storage capacity of 1,860,000 ML and it has the largest spillway capacity in Australia. The Burdekin River basin drains an area of about 114,770 km2 which is nearly twice the size of Tasmania. Runoff in the catchment is very reliable and flows have overtopped the spillway every year, except one, since it was built. The volume of inflow into the dam during a flood event is considerable, and water spills from the dam for an average of three months each year.

SunWater is investigating the raising of BFD, to increase the storage capacity of the dam by two metres or approximately 30% of its current storage capacity to 2,446,000 ML. In addition SunWater are investigating provisions to further stabilise the concrete gravity main dam to improve dam safety performance by ensuring it complies with current guidelines. Design flood estimation has advanced since BFD was constructed, as the techniques for determining extreme rainfall have been progressively refined. To meet current Acceptable Flood Capacity (AFC) guidelines, the flood discharge capacity at BFD must be increased by 35%. However, whilst this estimate was derived in accordance with current relevant guidelines (ARR, Book VI, 2001) the size of the BFD catchment means that this particular catchment lies on the fringe of the applicability of these guidelines.

Of particular concern is the assignment of the Annual Exceedance Probability (AEP) of the Probable Maximum Precipitation (PMP), which is based upon catchment area. The adoption of the AEP of the PMP for BFD at 1 in 9,000 has implications for the application of risked based approaches for the design.

This paper discusses the existing methodology of design flood hydrology used in Australia and identifies areas of concern for the application of such techniques for large catchments. It also discusses the methodology SunWater utilised in an attempt to meet existing guidelines within these limitations.

Keywords: Burdekin Falls Dam, Flood Hydrology, Probable Maximum Precipitation, Annual Exceedence, Probability
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