Probable Maximum Precipitation

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David J. Walland, Jeanette Meighen, Catherine Beesley, Karin Xuereb

The method for estimating Probable Maximum Precipitation in areas of Australia affected by tropical storms has been revised. The method that it replaces, designed in the 1970s is considered outdated and based on limited data.

The entire Bureau rainfall record has been examined objectively for the largest rainfall events. These events have been analysed and modified to enable storm transposition across a large region. The modifications are based on local topography, moisture and location. Once the storm data is transposed to a single location it can be meaningfully compared and used to construct an upper estimate on the possible rainfall. This estimate can then be used in conjunction with information about a specific catchment in order to estimate Probable Maximum Precipitation at that location.

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2002 Papers

2002 – A New Method for Estimating Probable Maximum Precipitation in Tropical Australia
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David J. Walland, Jeanette Meighen, Catherine Beesley, Karin Xuereb

The method for estimating Probable Maximum Precipitation in areas of Australia affected by tropical storms has been revised. The method that it replaces, designed in the 1970s is considered outdated and based on limited data.

The entire Bureau rainfall record has been examined objectively for the largest rainfall events. These events have been analysed and modified to enable storm transposition across a large region. The modifications are based on local topography, moisture and location. Once the storm data is transposed to a single location it can be meaningfully compared and used to construct an upper estimate on the possible rainfall. This estimate can then be used in conjunction with information about a specific catchment in order to estimate Probable Maximum Precipitation at that location.
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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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2005 Papers

2005 – Revised Extreme Floods in Tropical Regions
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R.A. Ayre and T. L. McGrath

SunWater as an owner of 25 major dams in Queensland has completed a programme to update the design flood hydrology of all of its referable structures in accordance with the latest methodology for estimating extreme design floods. This programme ensures the adequacy of existing spillways is included in an overall dam safety portfolio risk assessment in a consistent fashion.

This paper describes the methodology adopted in the re-assessment of the design flood hydrology of the storages. Principally this has meant the use of a design hydrograph approach utilising runoff-routing methods as described in Australian Rainfall and Runoff (1999). Design rainfall inputs have been based on generalised techniques derived by the Bureau of Meteorology such as the Revised Generalised Tropical Storm Method and the Generalised Short Duration Method for the estimation of Probable Maximum Precipitation. These estimates, coupled with the use of a regional design rainfall estimation technique known as CRC-Forge that is used for determining large to rare design rainfall estimates, have been used to derive a complete estimate of the inflow/outflow flood frequency curve for each dam.

The paper also provides an insight into the significant factors and relationships that are involved in the changes resulting from this process. Overall, there has been an increase in design rainfall depth estimates for the extreme events, and a general reduction to neutral change in the large to rare rainfall range. These changes plus the influence of temporal effects and the assignment of Annual Exceedance Probability (AEP) has led to substantial changes from previous estimates of design floods. The implication of these changes is profound for
an organisation such as SunWater.
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2015 Papers

2015 – Recent Developments in Estimating the Annual Exceedance Probability of Probable Maximum Precipitation
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R. Nathan, P. Jordan, M. Scorah, S. Lang, G. Kuczera, M. Schaefer, E. Weinmann

This paper describes the development and application of two largely independent methods to estimate the annual exceedance probability (AEP) of Probable Maximum Precipitation (PMP). One method is based on the Stochastic Storm Transposition (SST) approach, which combines the “arrival” and “transposition” probabilities of an extreme storm using the total probability theorem. The second method – termed “Stochastic Storm Regression”(SSR) – combines frequency curves of point rainfalls with regression estimates of areal rainfalls; the regression relationship is derived using local and transposed storms, and the final exceedance probabilities are derived using the total probability theorem. The methods are used to derive at-site estimates for two large catchments (with areas of 3550 km2 and 15280 km2) located in inland southern Australia. In addition, the SST approach is used to derive regional estimates for standardised catchments within the Inland GSAM region. Careful attention is given to the uncertainty and sensitivity of the estimates to underlying assumptions, and the results are compared to existing AR&R recommendations.
Keywords: Annual exceedance probability, Probable Maximum Precipitation.
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2018 Papers

2018 – A Quick Probable Maximum Flood Estimation Method for Queensland
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Samantha Watt, Michael Hughes & Daniela Sciacca

This paper presents an updated simplified technique for estimation of extreme floods in Queensland. This technique will be of use to practicing hydrologists and engineers working on early phase investigations of dams, weirs, and other infrastructure that requires flood resilience for extreme floods. The equations presented in this paper will provide practitioners with a robust yet simple to apply technique to rapidly estimate peak PMPF and PMF flows (inflows for dams) for Queensland catchments. This will assist feasibility and optioneering of infrastructure without significant cost overheads associated with often complex and time-consuming extreme flood estimates.
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2007 Papers

2007 – Climate change and Probable Maximum Precipitation
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Dörte Jakob, Robert Smalley, Jeanette Meighen, Brian Taylor and Karin Xuereb

Probable Maximum Precipitation (PMP) is one of the required inputs for estimating the PMP design flood. In estimating the PMP, currently no allowance is made for long-term climatic trends. A 2-year project funded jointly by the Australian Greenhouse Office and the Queensland Department for Natural Resources and Water, and with in-kind contributions by the Bureau of Meteorology began in May 2006. This study aims to assess how climate change might affect estimates of PMP. Preliminary results from this work will be presented.

Changes in factors used in PMP estimation, such as storm type and depth-duration-area curves, were assessed using a storm database covering the period 1893 to 2001 (Beesley et al. 2004). Based on the last 50 years, there is little evidence to support the notion that tropical cyclones (connected to major rainfall events) are penetrating further south or have become more frequent. A recent event that led to widespread flooding (Gold Coast, June 2005) was found to have very high storm efficiency. Changes in observed and projected moisture availability were assessed on the basis of a high-quality dataset of surface dewpoint temperatures and climate model output.
It is assumed that PMP received by a catchment is not uniformly distributed over a catchment but rather follows a typical spatial pattern. A pilot study to revise design rainfall estimates is currently under way at the Bureau of Meteorology. The methods developed in the pilot study were used to assess whether the spatial distribution of design rainfall estimates might be changing under a changing climate.

Keywords: Probable Maximum Precipitation, climate change, moisture availability, storm efficiency
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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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Samantha Watt, Michael Hughes & Daniela Sciacca