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David S. Bowles, Loren R. Anderson, Michael E. Ruthford, David C. Serafini, Sanjay S. Chauhan, Utah State University, Logan, Utah, U.S. Army Corps of Engineers, Sacramento, CA

In 2005 the Sacramento District of the US Army Corps of Engineers implemented an operating restriction to reduce the risk of an earthquake-induced failure of Success Dam, which could cause significant life loss and property damage. This paper describes an update of the 2004 risk-based evaluation of operating restrictions for Lake Success, which incorporated new information obtained by the District and enabled a re-evaluation of the level of the operating restriction and provided a basis for a possible modification of the restriction.

A RISK-BASED RE-EVALUATION OF RESERVOIR OPERATING RESTRICTIONS TO REDUCE THE RISK OF FAILURE FROM EARTHQUAKE AND PIPING

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2010 Papers

2010 – A risk-based re-evaluation of reservoir operating restrictions to reduce the risk of failure from earthquake and piping
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David S. Bowles, Loren R. Anderson, Michael E. Ruthford, David C. Serafini, Sanjay S. Chauhan, Utah State University, Logan, Utah, U.S. Army Corps of Engineers, Sacramento, CA

In 2005 the Sacramento District of the US Army Corps of Engineers implemented an operating restriction to reduce the risk of an earthquake-induced failure of Success Dam, which could cause significant life loss and property damage. This paper describes an update of the 2004 risk-based evaluation of operating restrictions for Lake Success, which incorporated new information obtained by the District and enabled a re-evaluation of the level of the operating restriction and provided a basis for a possible modification of the restriction.

A RISK-BASED RE-EVALUATION OF RESERVOIR OPERATING RESTRICTIONS TO REDUCE THE RISK OF FAILURE FROM EARTHQUAKE AND PIPING
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2011 Papers

2011 – Western Australian Dam Safety Challenges for ANCOLD Part 2
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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
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2011 Papers

2011 – Towards increased clarity in the application of ALARP
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Roger Vreugdenhil, Peter Hill, Siraj Perera, Susan Ryan

All Australian water authorities have in place dam safety programs that seek to ensure the ongoing safety and serviceability of their dams along with the benefits they secure for the wider community. Many have progressed multiple dam safety upgrades over the past decade and embraced risk assessment as a helpful tool in prioritising upgrade investment.

The ANCOLD Guidelines on Risk Assessment (2003) have been applied across the country and, coupled with State regulation, have supported dam owner efforts in reducing risks below the ANCOLD “Limit of Tolerability”. However, it is generally acknowledged that in their current form, the ANCOLD guidelines provide limited guidance to dam owners for determining appropriate levels of risk reduction and timing of dam safety improvements. This has contributed to a range of guideline interpretations and inconsistency in subsequent dam safety investment decisions across Australia. Having achieved priority risk reduction, a number of owners are beginning to assess their dams against the ALARP principle, bringing dam safety investment within an owner’s portfolio into more direct competition with other important and urgent organisational investment decisions.

This paper outlines the outcomes of a recent study commissioned by the Victoria Department of Sustainability and Environment into risk reduction principles and the application of ALARP by a number of Australian and international dam owners and regulators, hazardous industry owners and regulators, and the interaction of ALARP with whole-of-organisation investment. The paper highlights the study process and significant points of interest regarding risk reduction principles and current application of ALARP and some options for refinement and clarity.

Towards increased clarity in the application of ALARP
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2011 Papers

2011 – Hume Dam Spillway Southern Junction Filter and Drainage Works
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Rod Westmore, Andrew George& Robert Wilson

A 2007 risk assessment of Hume Dam concluded that the dam did not satisfy the ANCOLD societal risk criteria for existing dams. The Spillway Southern Junction (SSJ) and its associated failure modes was one of the main contributors to the risk profile.

Upgrade works at the SSJ involved the retro-installation of additional filter and drainage materials in the 40m high embankment immediately downstream of the tower block and central core wall by installation of more than 10,000m of secant caisson drilled columns backfilled with filter and/or drainage materials.

This paper describes the design and construction issues associated with the upgrade works, the equipment and methodologies developed to achieve the principal design objectives of coverage and connectivity of filter and drainage columns, and optimisation of compaction of the backfill materials. It also describes how these requirements were met whilst minimising adverse affects such as vertical deviation, excessive vibration, subsidence of secant filter columns during construction, and clay smearing of the perimeter of individual columns.

Hume Dam Spillway Southern Junction Filter and Drainage Works

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2011 Papers

2011 – How SunWater, as a commercial dam owner makes investment decisions for dam safety upgrades
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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
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2011 Papers

2011 – How do you solve a problem like retarding basins? An asset owner’s perspective
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Kirsty Carroll, Kelly Maslin, Richard Rodd

Melbourne Water manages over 210 retarding basins across Greater Melbourne ranging in size from 4ML to 4700 ML with embankment heights from 0.3m to 10m. Over the years the basins have been designed and constructed by a range of different owners and authorities. Varying design and construction standards with the majority of retarding basins generally being located in highly urbanised areas, has resulted in Melbourne Water having a large portfolio of assets that have potential to pose a significant risk to the downstream communities they are designed to protect.

High level hazard category assessments completed over the last10 years identified that approximately 90 structures were either High or Extreme hazard categories based on the ANCOLD Guidelines on Assessment of the Consequences of Dam Failure.

In an attempt to identify retarding basins requiring priority consideration for remedial works Melbourne Water embarked on a process of completing a dam safety risk assessment for five of the retarding basins in accordance with the ANCOLD Guidelines on Risk Assessment. The objective of the risk assessment was to develop an understanding of the key risk issues that might affect retarding basins as distinct from water supply storages, identify potential remedial works and develop a prioritised risk management strategy for the five basins considered. In completing the risk assessment there was also significant discussion about ways to streamline the process to allow assessment of the remaining basins.

This paper details the results obtained from the risk assessment, investigates the application of the base safety condition and implementation of a risk management strategy. It also looks at similarities between sites to enable common upgrades to be implemented across the range of retarding basins. This paper also discusses the need for guidelines specific to retarding basins to be developed.

How do you solve a problem like retarding basins? An asset owner’s perspective

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2011 Papers

2011 – Investigating the Piping Risk Associated with Seepage at Monbulk Saddle Dam of Silvan Reservoir, Victoria
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Monique de Moel, Mark Arnold, Gamini Adikari

Monbulk Saddle Dam, built in 1929, is one of two saddle dams located at the southern end of Silvan Reservoir, near the township of Monbulk, Victoria. The saddle dam is a 5.3m high earthfill embankment with a 230mm wide, centrally located, concrete core wall. The reservoir retained is located in the valley of Stonyford Creek, and impounds approximately 40,500 ML of water at FSL.

Excessive seepage at the right abutment of Monbulk Saddle Dam has been an issue since the early 1970’s. The reservoir has been operating with a level restriction since then to reduce the seepage flows. However; this restriction limits the operational flexibility of the storage. Early investigations concluded that the most likely mechanism for these excessive seepage flows was a defect in the concrete core wall.

Melbourne Water Corporation, (the owner and the operator of the reservoir), undertook a risk assessment for Silvan Reservoir as part of a review of its dams asset portfolio. Based on the information then available, the risk assessment was undertaken using the criteria and guidelines developed by ANCOLD. The result was that the piping risks associated with the seepage from the west abutment at Monbulk Saddle Dam was unacceptable. The risk assessment Panel also cast doubt on the likelihood of the seepage being caused by a defect in the concrete core wall. Melbourne Water therefore engaged SMEC Australia to investigate the likely causes and mechanisms for this seepage and to develop suitable remedial measures for the dam.

The investigations have included a desktop review of historical information, test pit investigations, Sonic borehole drilling, dynamic cone penetration tests, an infrared thermal imaging investigation and an electromagnetic groundwater seepage flow mapping investigation.

These investigations have shown that the most likely cause of the seepage is the presence of permeable foundation layers located beneath and around the existing core wall as the core wall does not extend over the full length of the embankment and becomes shallower towards the abutments.

To satisfy the ALARP principle; risk reduction remedial works Concept Designs are being developed and reviewed.

2011 – Investigating the Piping Risk Associated with Seepage at Monbulk Saddle Dam of Silvan Reservoir, Victoria

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