Since publication in 2003, the ANCOLD Guidelines for Risk Assessment have reached broad acceptance and use in Australia. In practice, dam owners use the principles of risk assessment to drive business investment decisions. As the guidelines undergo revision, it is timely to assess whether our practices need to evolve to more holistically consider all types of consequences, rather than our current focus on loss of life, in decision-making. This paper aims to prompt dam owners and consultants alike to re-assess our focus on loss of life in risk assessment decision-making, and whether we should more meaningfully consider alternative or broader indicators.
An industry survey was undertaken which found that large dam owners are generally happy with the current system of dam safety decision making. However, the survey responses did identify difficulties in relation to justifying investment below the limit of tolerability that are subject to ALARP principles. In a small number of cases, dam owners found it difficult to justify investment when life safety was not important.
Building on the industry survey and subsequent discussions with practitioners, this paper discusses how the current approach to risk based decision making may result in sub optimal decision making. Further it is discussed how there is an important role that economics should play in providing a universally accepted framework for assessing trade-offs and providing consistent evidence to support decision making.
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Oroville Dam is located on the Feather River in northern California (USA). At 234.7 m (770-ft) tall, this earth embankment is the tallest dam in the United States. With its 4.3 billion m3 (3.5 million acre-feet) of storage, Lake Oroville is the second largest reservoir in California, supplying water to cities as far south as Los Angeles. The Oroville Dam, reservoir (Lake Oroville), and hydropower plant facility is the flagship of the State Water Project (SWP), which is owned and operated by the State of California, Department of Water Resources (DWR).
In 2015, a study was undertaken where recommendations were made to provide protection to the exposed rock in the unlined channel of the spillway at Burdekin Falls Dam. The protection included a matrix of anchor bars which extended the full 504 m width of the spillway and 25 m in the downstream direction. Over 1,200 anchors were proposed comprising 36 mm diameter bar extending up to 15 m into the foundation.
A value engineering study was undertaken in 2017 where a review of the rock scour potential was undertaken. The study was based on a methodology developed by Pells (2016) as part of a research grant funded under an Australian Research Council (ARC) Linkage Project which was jointly financed by the Federal Government of Australia, various state government bodies and engineering consultancies involved in dam design, operations and management.
This paper describes the approach taken as part of the value engineering study, the methods used in the assessment and the benefits of both innovative thinking and challenging the more traditional approach of rock scour assessment, the outcome of which resulted in a $11 m plus saving to the owner of the asset.
Deformation Survey is a simple and widely implemented technique to identify the early signs of dam failure and is regularly undertaken on many dams. Thanks to advances in equipment and more accurate survey records, there is now a better understanding of measurement and movement of embankments and previous records.
However, the “expected” range of transverse deformation and implications for failure modes of dams is not particularly well researched or understood.
This paper collates a case history of transverse deformation for a number of Tasmanian dams and examines the relative behaviour of the embankment dams. From this the “expected behaviour” of an embankment dam can be estimated and related to key influencing factors, such as observed settlements, height and age of the dams, and thereby providing guidance on when transverse deformation is considered unusual for similar dams.
An assessment of dam failure consequence for Jandowae Water Supply Dam in South-West Queensland was performed using HEC-LifeSim. The purpose of the assessment was to investigate the applicability of the software to inform decisions on an appropriate regulatory pathway for the dam that reflects the consequences of failure. This paper details the development of the hydrologic and hydraulic models behind the HEC-LifeSim simulations, the assignment of key parameters and their sensitivities, and a comparison of predictions to existing procedures for assessing potential loss of life and populations at risk. The paper reflects upon the level of effort required to develop HEC-LifeSim assessments and the relative benefits gained using this information in the regulatory space.
Two tailings storage cells were raised by constructing new embankments upstream of the existing
embankment walls. The performance of the new embankments was mainly dictated by the underlying tailings that consisted of a thick layer of very soft to soft fine tailings. The fine tailings in one cell was capped by a layer of sand for more than 30 years hence the tailings had mostly consolidated under the load of the capping. The fine tailings in the other cell was under consolidated because the cell had only been capped for about 18 months before the construction of the new embankment. The capping material was sand extracted from the tailings.
Stratification of the tailings was determined by CPT. Undisturbed samples of fine tailings were obtained by a piston sampler for CIU and oedometer testing to obtain parameters required for advanced soil models SHANSEP and Soft Soil (SS) models. These models were incorporated in full 2-D FE models to analyse the stability and settlement of the new embankments at various locations.
The application of advanced soil models such as SHANSEP and Soft Soil by hand calculation and
conventional slope stability analysis is considered cumbersome and labour intensive. This paper
demonstrates that with the help of FE software (PLAXIS in this case), it is practical to implement such advanced soil models to simulate the behaviours of soft fine tailings with reasonable accuracy. A similar approach could be used to model other fine tailings and soft clays. One should be reminded that the reliability of any analysis method relies on validation of the analysis model and parameters adopted.