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Andrew Richardson, Stephen Farrelly and Phil Farnik

In 2012 an update to the Portfolio Risk Analysis (PRA) was undertaken by State Water Corporation for its 18 major dams in New South Wales. The updated portfolio level risk analysis of all the dams has taken account of the completion of major components of the 2006 dam safety upgrade program, while also incorporating continued engineering research into dam safety performance. This paper will provide an overview of the approach, the challenges faced in the process and it will highlight the innovative advances made representing industry best practice. Some future implications and directions will also be discussed.

The three main components of the PRA update in 2012 have included a significant amount of dam break hydraulic modelling including revised hydrology and flood inundation mapping delivered in-house by State Water with consultant support. The Consequence Assessment was developed with a spatial link to natural flooding and dam failure consequences by Sinclair Knight Mertz (SKM), while the third element in producing the event trees, risk analysis and PRA reports was undertaken by consultants GHD. Peer review of the PRA process and reports and additional technical review of the failure modes and event trees by a panel of industry experts provided the necessary independent input and oversight required by the NSW Dams Safety Committee.

State Water’s PRA update builds on the large body of work undertaken for and since the last PRA in 2002. The update process has applied a systematic and quantitative approach across the Portfolio that provides a robust basis for managing dam safety risk. The results of the PRA have identified further work required to investigate and assess the need for dam safety upgrade options for non-compliant dams. State Water’s investment in the PRA has produced a risk-based position on each dam in the portfolio that can be used to identify a range of measures in a revised dam safety upgrade program for the future.

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2014 Papers

2014 – Advances in Portfolio Risk Analysis for Improved Management of Dam Safety Risk – a State Water Perspective
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Andrew Richardson, Stephen Farrelly and Phil Farnik

In 2012 an update to the Portfolio Risk Analysis (PRA) was undertaken by State Water Corporation for its 18 major dams in New South Wales. The updated portfolio level risk analysis of all the dams has taken account of the completion of major components of the 2006 dam safety upgrade program, while also incorporating continued engineering research into dam safety performance. This paper will provide an overview of the approach, the challenges faced in the process and it will highlight the innovative advances made representing industry best practice. Some future implications and directions will also be discussed.

The three main components of the PRA update in 2012 have included a significant amount of dam break hydraulic modelling including revised hydrology and flood inundation mapping delivered in-house by State Water with consultant support. The Consequence Assessment was developed with a spatial link to natural flooding and dam failure consequences by Sinclair Knight Mertz (SKM), while the third element in producing the event trees, risk analysis and PRA reports was undertaken by consultants GHD. Peer review of the PRA process and reports and additional technical review of the failure modes and event trees by a panel of industry experts provided the necessary independent input and oversight required by the NSW Dams Safety Committee.

State Water’s PRA update builds on the large body of work undertaken for and since the last PRA in 2002. The update process has applied a systematic and quantitative approach across the Portfolio that provides a robust basis for managing dam safety risk. The results of the PRA have identified further work required to investigate and assess the need for dam safety upgrade options for non-compliant dams. State Water’s investment in the PRA has produced a risk-based position on each dam in the portfolio that can be used to identify a range of measures in a revised dam safety upgrade program for the future.

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2014 Papers

2014 – Application of Geomembrane Sealing Systems in Two Tailings Dams
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Behrooz Ghahreman Nejad

In recent years, dam designers have become increasingly interested in application of the geomembrane sealing systems (GSS) in design of tailings storage facilities around the world. The main reasons for this have been the deformation characteristics, environmental aspects (ie seepage minimisation), speed of construction and constructability of geomembrane liners in most climatic conditions. This paper reports the design and application of two types of geomembrane sealing system in Angas Zinc (AZ) and Sarcheshmeh Copper (SC) tailings dams. The former is a 25m high zoned earthfill embankment with an HDPE liner system, located in an environmentally sensitive site in South Australia. The latter is a 94m high zoned rockfill embankment with a PVC liner system, located in a region of high seismicity in central Iran.

The designs of the AZ and SC geomembrane sealing systems including geomembrane liner, drainage layer, anchoring, leak detection system and drains, and instrumentations are discussed in detail. The performance of the liner systems during operation is also presented.

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2014 Papers

2014 – Beyond Quantitative ALARP – Re-Focussing Paradigms for Dam Safety Risk Management in a Changed Legal Environment
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Dennis C. Green

Current good practice for risk management as represented in ANCOLD guidelines emphasises risk reduction beyond tolerable risk levels to As Low As Reasonably Practicable (ALARP). Risk reduction reflected in key design parameters such as the spillway design flood is monitored on a quantitative basis, while the guidelines also draw attention to a number of non-quantifiable measures.

Recent work health and safety legislation in Australia does not at first appear to relate to dam safety, but it mandates elimination of risk, and, if that is not possible, then it mandates reduction of risk So Far As Is Reasonably Practicable (SFAIRP). It is tempting to believe that this is equivalent to ANCOLD’s approach to ALARP, but the devil is in the detail of the legislation. This paper argues for a change to a more systematic presentation of recording of decisions on dam safety risk management, lest the legislation expose dam owners unwittingly to liability when they thought they were following good practice. In particular, the re-focussing of ANCOLD Guidelines to align more recognisably with the new legal paradigm, including preparation and adoption of a Safety Case, is recommended.

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2014 Papers

2014 – Blowing in the Wind: Reassessment of the Existing Rowallan Dam Spillway Walls – Performance and Design Approach
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Paul Southcott, Tony Harman

This paper addresses structural behaviour of the Rowallan spillway walls and the learning that can be derived from this in the design of critical retaining walls in dams and how this can be applied both to remedial works and new work. The authors propose design criteria suitable for retaining walls in high hazard dams.
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2014 Papers

2014 – Design Rainfalls – Past, Present and Future
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Janice Green, Cathy Beesley, Cynthia The, Catherine Jolly

Design rainfall estimates are essential inputs to the design of infrastructure such as gutters, roofs, culverts, stormwater drains, flood mitigation levees and retarding basins. They are also integral to large dam spillway adequacy assessments undertaken to determine the flood magnitude that existing dams can safely withstand.

The previous design rainfall estimates for probabilities from the 1 year Average Recurrence Interval (ARI) to the 100 year ARI were derived by the Bureau of Meteorology (the Bureau) in the early 1980s using a database comprising primarily of Bureau raingauges and techniques for statistical data analysis that were considered appropriate at the time. More recently, estimates of rare design rainfall estimates for probabilities from 100 year ARI to 2000 year ARI have been derived for each state, with the exception of the Northern Territory, using the CRC-FORGE method.

As part of the revision of the 1987 edition of Australian Rainfall and Runoff: A Guide to Flood Estimation being undertaken by Engineers Australia, the Bureau conducted a five year project to revise the design rainfall estimates for probabilities from 1 year ARI to 100 year ARI. The new design rainfall estimates are based on a greatly expanded database which incorporates data collected by organisations across Australia. These data have been analysed using contemporary statistical methods that are appropriate for Australian rainfall data. These new Intensity-Duration-Frequency (IFD) design rainfalls were released in July 2013.

Over the next 18 months, the Bureau will be deriving design rainfall estimates for probabilities more frequent than 1 year ARI and revising the existing estimates of the CRC-FORGE rare design rainfalls. The estimates for more frequent design rainfalls will replace the current ad hoc estimates that have been derived by organisations in the absence of other estimates. The revised rare design rainfall estimates will replace the current estimates that were derived on a state by state basis and which, for most states, are now in need of revision as a result of the release of the new IFDs.

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2014 Papers

2014 – Developments in Emergency Action Planning for Queensland Referable Dams
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Peter Allen and Kevin Bartlett

One of the recommendations of the Queensland Flood Commission of Inquiry was for the introduction of a legislative requirement for all referable dams in Queensland to have Emergency Action Plans (EAPs) formally approved by the Dam Safety Regulator. Prior to this EAPs were required under the dam safety conditions applied to each referable dam and they were not formally approved. This recommendation has now been implemented as a requirement of the Water Supply (Safety and Reliability) Act 2008. This paper summarizes the emergency action planning system now applicable to Queensland’s referable dams and details the actions involved in implementing this system. It involves significant consultation between dam owners and local disaster managers and gives local disaster managers an opportunity to formally comment on EAPs prior to them being submitted for approval. Development of associated regulatory guidelines to cover all aspects of EAPs was done in order to make EAPs more consistent and more readily understood by users and other stakeholders in emergency situations. Once the guidelines had been developed, the Regulator undertook a state-wide series of seminars to raise the level of awareness of local disaster management groups and dam owners of the new requirements. The legislation also requires the publication of the approved EAPs on the department’s website to raise the public’s awareness of the risks involved and improve their responses in advance of emergency events. This represents a challenge from a public relations perspective because people will become more aware of the risks to which they are exposed. The paper summarises the Regulator’s experience in reviewing and considering the EAPs submitted for approval and it indicates some of the benefits and challenges of the ongoing program.

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2014 Papers

2014 – Does Your Rating Curve Hold Water: The Consequence of Rating Curve Errors
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Wark, Bob; Thomas, Louise

This paper discusses the rating curves developed for several case studies from the Pilbara and Kimberley, including the Harding Dam, Moochalabra Dam and Ophthalmia Dam. The paper will discuss the impact of underestimated rating curves on the design of infrastructure. An example has occurred at Harding Dam where the pump station was designed to be inundated at a 1:100 AEP and this is now estimated to occur at a lower AEP. The paper will also discuss methods to improve the accuracy of rating curves and the challenges associated with determining accurate rating curves.
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2014 Papers

2014 – Evaluation of Existing Arch Dam Design Criteria in Lieu of ANCOLD Guidelines
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Marius Jonker and Dr Radin Espandar

This paper provides a summary of the current state of practice for arch dam design criteria that have been adopted by some international dam organizations, and where relevant, compares that with the criteria provided in the updated ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams, with the view to provide a basis for consistent and unified design criteria for arch dams in Australia.

The paper draws on the authors’ experience with arch dams, including recent experience with a number of arch dam safety reviews in Australia, their past experience with arch dams over 200 m height, as well as their involvement with the development of the mentioned updated ANCOLD Guidelines.

Since the last arch dam was constructed in Australia, a number of international publications have been released on arch dam design practices, providing general information and guidance for the design of new dams and evaluation of the safety and structural integrity of existing arch dams. This paper compares these publications and proposes criteria that are aligned with the ANCOLD gravity dam guidelines.

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2014 Papers

2014 – Hardap Dam – Responses to the 2006 Floods
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A C Mostert, D J Hagen, P C Blersch

The changes in flood operations since the 2006 flood, covering weather monitoring, hydrological flood station monitoring, and downstream monitoring, are discussed in detail in the paper.
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2014 Papers

2014 – Innovative Curtain Solution for an Environmental Issue
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Guresh Ahuja, Brian Cooper, Olaf Rutgrink and Andrew Thomson

This paper presents details of the State Water Corporation of NSW Australian first temperature control curtain project aimed at dealing with the environmental issue of cold water pollution in dam offtakes. This is a key project focused on dealing with a known environmental issue and reflects changing environmental and social expectations for dam operators. In the concept phases this project was part of the NSW Rivers Environmental Restoration Program (RERP) sub-program ‘better delivery of environmental water’.

The paper presents the development of the design concept including early concept screening and selection, scale model testing, concept design development, hydrodynamic, structural and computational fluid dynamic analyses leading to the final design that has now being constructed. The concept design work was completed in 2011 and following a tender process a design and construct contract was let for the detailed design and construction of the temperature control curtain system. This phase of the work was recently completed and the temperature control curtain is now fully operational. The actual performance and effectiveness of the temperature control curtain will only become evident after a few years of operation and will also hinge on the degree of stratification in the dam.

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2014 Papers

2014 – Nonlinear Time-History Seismic Assessment of 112-Year-Old Barossa Dam
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Francisco Lopez and Michael McKay

At 36 m high and completed in 1902, Barossa Dam is one of the first true concrete arch dams in the world. During the 1954 Darlington Earthquake the dam sustained some damage, in the form of several vertical cracks on both dam’s abutments. In 2013, GHD conducted a nonlinear time-history seismic assessment of Barossa Dam. The analyses, carried out using finite element techniques, included ground motion loading corresponding to Maximum Design Earthquakes (MDEs) with 1 in 10,000 Annual Exceedance Probability (AEP).

This paper will explain the purpose of the study, the material investigation phase, the methodology, model results, the anticipated seismic behaviour of the dam wall, as well as the predicted level of damage under the MDEs. The paper examines the dam construction practices of the beginning of the 20th century, and how such practices affected the material properties and the structural performance of Barossa Dam.

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2014 Papers

2014 – Oaky River Dam Failure
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George Samios and Steve Gough

Following an extreme flood event late on Friday 22nd February 2013, water overtopped the dam crest leading to total collapse of the dam’s right embankment and a span of the nearby intake tower access bridge as well as the cutting of power to the dam gates. The dam’s left embankment was also severely damaged. Options considered for remediation of the dam include decommissioning or reinstatement and upgrading to NSW Dams Safety Committee requirements.
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2014 Papers

2014 – Siting and Engineering Constraints of a New Dam in an Alpine Resort
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Damon Miller and Grant Jones

Mt Buller Alpine Resort has significant constraints on its capacity to store and supply potable water during times of peak demand, which impacts the ability to sustain and grow visitation to the resort, limiting optimal functioning and future development of the resort.

A proposed new 100 megalitre dam would primarily supply the resident and visitor populations with a reliable potable water source while also maintaining through snowmaking, the Resort’s amenity and functionality during winter for skiing and snow-play.

Standard dam engineering criteria of technical feasibility and environmental impact influenced the site selection and design of the new off-stream storage whilst additional key drivers unique to an alpine resort, framed around impact to existing skiable terrain and resort functionality, were critical to satisfy the resort stakeholders. The need to minimise the visual impact of the dam and integrate with the resort environment was also of high importance.

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2014 Papers

2014 – Some Lessons Learned from Gate Operations and Testing
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Peter To

This paper outlines lessons learned from 8 years of regular operations and testing of 111 gates at 22 sites. It points out that the implementation challenges involved are not only technological in nature, but also encompass human factor and organizational issues. This is perhaps understandable since the initiative is part of the cultural shift to sustain gate reliability long-term.

An increase in gate testing frequency has led to the identification of more performance anomalies, ranging from deficiencies to operational failures. This finding may not be unique to a single dam owner. It leads to the following question to the general dam owner community: Are we testing our gates enough?

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2014 Papers

2014 – Stochastic Simulation of Inflow Hydrographs for Wivenhoe and Somerset Dams
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Phillip Jordan, Alan Seed, Rory Nathan, Peter Hill, Eva Kordomenidi, Clive Pierce, Michael Leonard

This paper discusses the stochastic framework that was used to generate the 5449 sets of inflow hydrographs, to develop and stress test a dam operations model. The stochastic simulations were driven by 600 different space-time patterns of rainfall generated using a stochastic space-time multiplicative cascade model. Eight significant storms were identified in the radar archive to identify parameter sets for the stochastic generation algorithm and 600 replicates of space-time rainfall were generated. The statistical properties of spatial patterns of 48-hour rainfall bursts on eight major subcatchments of the Brisbane River catchment from the 600 stochastic replicates were verified against the same statistics derived from 38 major flood causing rainfall events observed in the catchment. The hydrographs were generated using an URBS rainfall runoff routing model of the Brisbane River catchment, which was calibrated to 38 historical flood events (between 1955 and 2013) and tested on a further 10 historical flood events (between 1887 and 1947).

The stochastically simulated sets of inflow hydrographs were then used to assess the impact of variations in flood operation rules for Wivenhoe and Somerset dams. The stochastically generated events exhibit substantial variability in runoff hydrographs but with variability that is statistically consistent with observed events. The stochastically generated hydrographs provide a considerably more realistic basis for testing the outcomes for different flood operations strategies than the single design event approaches that have previously been adopted.

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2014 Papers

2014 – The Impact of Mining Iinduced Subsidence on Water Storages
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William Ziegler and Heather Middleton

This paper presents the collation of over 20 years of data on vertical and horizontal movements around Cataract Dam in the Southern Coalfield of New South Wales, reporting subsidence that continues 25 years after extraction in the area ceased. The occurrence of increased vertical movement over old goaf areas as the result of extraction in the same seam at greater than 1km distance has been observed. Together with a change in the behaviour of measured head of water 6 years after extraction ceased in the area. These points raise the question, how long should subsidence monitoring continue after extraction has ceased in areas of important infrastructure?
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