1998 – Estimating the Probability of Failure of Concrete and Masonry Gravity Dams
Kurt Douglas, Matt Spannagle and Robin Fell
This paper describes a method for estimating the probability of failure of concrete and masonry gravity dams through the dam or the foundation. The method is based on the research and analysis of historic failures and accidents performed at The University of New South Wales over the last two years. The method accounts for dam type; age; foundation; height/width ratio; dam performance observations; and monitoring and surveillance.
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1998 Papers
1998 – Initial Raising of Borumba Dam
Learn moreRA Geddes
Over the last 30 years, the demand for water storages in Queensland’s Mary’s River has grown significantly. As a result of this growth in demand it was decided to raise Borumba Dam, the major storage on the system, in two stages The first stage was to be approximately 2 metres in I997 and the 25 metre raising be required in about 2010.Borumba Dam was completed in 1964. It is a 43 metre high concrete faced dam with a 32 metre long on the left abutment. The first proposal for initial raising was to install a two metre high air-inflated rubber dam on top of the existing crest. However, it was determined that this method of raising presented a number of prob and a new solution was sought.
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1998 Papers
1998 – Improved Estimates of Hydrologic Risks for Dam – Impacts of the New Flood Guidelines
Learn moreP.I. HILL, R.J. NATHAN, P.E. WEINMANN, J.H. GREEN
The assessment of flood risk is important to the safe design, maintenance and operation of dams. Traditionally, a standards-based approach has been adopted, in which the adequacy of a spillway was assessed by its ability to pass the whole, or a specified fraction, of the Probable Maximum Flood (PMF). More recently, however, the ANCOLD Position Paper on Guidelines for Selection of Acceptable Flood Capacity for Dams has moved towards a risk-based approach, in which attention is focused on establishing the exceedance probability of the maximum flood that can be safely passed by the spillway.
The move to a risk-based approach has led to an increasing focus on the derivation of floods with very low probabilities of exceedance. The chapter in Australian Rainfall and Runoff that gives guidance on the estimation of extreme floods has recently been revised and issued as Book VI. The new guidelines reflect the move from a standards, to a risk based approach and also include recent developments in extreme flood estimation. These recent developments result in an improved estimate of floods in the large to extreme range and hence a more reliable estimate of hydrologic risk.
This paper illustrates the impacts of the new flood guidelines by summarising the results for 7 dams in Southeastern Australia. For the examples presented in this paper the impact of the new guidelines is to reduce the estimated hydrologic risk. The new guidelines have an important effect on the estimation of hydrologic risk and therefore the assessment and management of dams in Australia.
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1998 Papers
1998 – Monitoring Dam Performance Using Tiltmeters
Learn moreM O’Reilly, S A L Read and P F Foster
Electronic (bubble) tiltmeters provide an up-to-date technique for continuously monitoring the deformations of dam and dam-related structures. Tiltmeters, with a sensitivity of (10Imm per length), are currently used in New Zealand at the high concrete gravity Waitaki Dam, and the Ohau A Powerhouse, as well as a short-term installation in the high concrete gravity Aviemore Dam.
This paper outlines the performance of the tiltmeters over a period of up to 7 years. They have been used to monitor the reactions of structures to loading changes such as headwater level variation, and to monitor ongoing performance, including the definition of annual thermal cycles. The results are compared with other monitoring techniques (e.g. plumblines, conventional surveying) to illustrate the usefulness and applicability of tiltmeters to dam safety programmes, either in conjunction with standard monitoring options, or in particular where such options may not be practicable.
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1998 Papers
1998 – Repair of Two Dams Suffering Leakage – Two Case Studies
Learn moreNikifor Petrovic
One of the most important issues during design and construction of an earthfill dam is how to secure a dam against unwanted events which may occur as a result of water flow (uncontrolled seepage, leakage & piping) through the dam.
Although earthfill dams are the largest by volume compared with other types of dams and they are designed to cope with seepage, their integrity is most sensitive to the effects which may be caused by it. The reason being that the earthfill materials are generally extremely heterogeneous and only one “unwanted” pocket is enough to create problems.
Another critical area is the foundation. In many situations it is not possible to avoid the complex geology which includes faults and joints as part of the foundation. An additional complication may be the presence of dispersive clay in the foundation.
In the area of tailings dams, the problems with seepage are slightly reduced as in most cases, tailings provide a degree of sealing. Tailings dams are very often designed as leaky dams. However, there is a hidden danger in approaching the design this way as at any stage of their lives they can retain water.
This paper presents two case histories of repairs carried out to tailings dams suffering leakage. One case describes leakage through the embankment wall while the other describes seepage through the foundation which contains dispersive soil.
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1998 Papers
1998 – Risk Assessment of Dams – Future Directions for Victoria
Learn moreD.Watson, J.Adam
For several years risk management has been promoted by the Victorian Department of Natural Resources and Environment – Water Agencies as the key mechanism for the effective and efficient business management of dams. As part of an extensive water reform program, the Victorian Government announced in October 1997, a financial assistance package for the water industry which included funding for dam improvements covering a majority of large dam owners in the State. One of the conditions for receipt of these improvement funds was the need for each water authority to undertake a Business Risk Assessment of all significant and high hazard dams under its responsibility. This paper discusses the Business Risk Assessment document based on a framework developed by Water Agencies after consultation with the industry and expands on the following reasons why the document was produced:
- To stress the importance and value of risk assessment in managing dams and the major role it plays in business planning and understanding dam performance. Risk assessment is not an alternative to the traditional engineering or deterministic approach but an enhancement,
- To identify the minimum assessment required and examples of different approaches such as portfolio risk assessments,
- To provide preliminary reference risk criteria to assist water authorities to determine appropriate business performance requirements. This preliminary criteria was developed from considering likely (although subject to change) outcomes of the current revision of Australian National Committee on Large Dams (ANCOLD) 1994 Risk Assessment Guidelines and other guidelines involving risk aspects recently or being prepared,
- To indicate future reporting and actions expected over the five year duration of the package including project evaluations and further risk assessments,
- To assist with inputs into ANCOLD’s continuing development and revision of guidelines, and
- To provide a platform for development over the next 18 months of an appropriate regulatory framework for covering all referable dams in the State. Key considerations will included performance based requirements encompassing risk management concepts, appropriate drivers which promote duty of care and associated legal issues and liabilities.