Glen Hobbs and Danny Azavedo
Recent years have seen a growing awareness and understanding of the factors that contribute to the reliability of spillway gates and the incorporation of reliability data into overall dam risk studies.
The study of a number of spillway gate failures shows that no one component or incident leads to gate failure, but rather a combination of factors have resulted in gate failure. A rigorous reliability assessment should consider all factors, not only the equipment condition and performance but the complete system, from the receipt of data through to the actuation of the gates. It should take into account issues such as human factors, poor design, maintenance history and policy. Unfortunately one of the main hindrances to quantifying gate reliability is the lack of information on spillway gate equipment and system performance and failures.
This paper considers a number of gate failures, then looks at some of the tools of reliability assessment and the role of human factors in gate reliability.
The paper then discusses a recent study of four gated dams. For this study a systems approach was adopted and human factors were considered. The results compare favourably with other similar critical structures, and show that for these well designed and maintained structures human factors are the limiting criteria in multiple gate operations. The study also shows that the probability of opening all the spillway gates at a dam improves with time (2-4 hours) during the flood operation, and it is considered that time based reliability provides a more meaningful and useful assessment of overall spillway gate reliability.
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Now showing 1-12 of 27 2964:
M. B. Barker and D. Holroyde
A detailed study was completed for the Stage 2 works of the Grahamstown Dam augmentation to investigate various alternatives for the slope protection of the Saddle Dam and Subsidiary Dam embankments, including a standards based and a risk management approach. The standards based approach required an evaluation of the slope protection level and least cost option based on the hazard rating of the dam. Due to the sand construction of the embankments, it was possible to apply a wave erosion model SBEACH to develop an economic risk model for optimising the slope protection alternatives. The erosion model included the effects of the wind direction, reservoir level and wind speed variation during flood events, embankment profile and material parameters. The risk management approach clearly showed that significant cost savings could be achieved by using the risk management approach. Furthermore, the cost curves indicated the sections of the embankments for which present capital works would not be economically justified and for which ongoing maintenance works would be economically advantageous.
David Dole and Brian Haisman
The $75 million remedial works at Hume Dam on the Murray River near Albury have been of national significance. The rehabilitation program associated with the structure itself and with its appurtenant works is now in the final steps of construction. The authors summarise this program with an emphasis not on technical details, but on decision processes. Equity in this dam is, in effect, held in equal parts by three State governments and by the Commonwealth government. At the same time, in response to the national water reform agenda, the governments have agreed upon new cost sharing arrangements that more nearly reflect the value of services to each government. The particular problems of decision-making within this evolving inter-State environment are discussed.
Lessons from experiments with application of risk analysis are discussed. Finally, the matter of adequacy of the structure for extreme floods is still under consideration. Hume Dam will presently pass the Design Flood developed in accord with Book VI (1999) of Australian Rainfall and Runoff, and the Dam Crest Flood has been estimated to have an annual exceedance probability of 1:110,000. _ Retrofitting a spillway to pass the estimated Probable Maximum Flood will double the cost of remedial works and is estimated at 10 times the cost of similar capacity built into original construction. The authors discuss the public policy elements of this pending decision.
Mark Locke, Buddhima Indraratna, Phillip Cummins and Gamini Adikari
ABSTRACT: Australia has a large number of older embankment dams, which have been in service and performed adequately for over 50 years. However, current industry practice in embankment dam design predicts that the granular filters within these dams may not be adequate. This may require refurbishment of the dam by retro-fitting a new filter to ensure the continued safety of the structure. This paper outlines the potential problems with older embankment dam designs, and the reasons for constructing a new filter. Potential problems may include inadequate or non-existent filters, risk of failure due to earthquake, piping, or excessive foundation seepage. Design methods for granular filters are described briefly, concentrating on whether an existing filter is adequate, and the potential improvement by constructing a new filter. Construction issues for placing filters on existing dams are also discussed.
A new analytical method, developed to describe the time dependent erosion and filtration within embankment dams, is described briefly. The model predicts particle erosion, transport and retention based on fundamental fluid mechanics and geotechnical concepts. The application of this model to the design of filters for new and existing dams will be described. The predictions of such analytical modelling can give a designer a significantly clearer picture of the purpose of a granular filter, the extent of core erosion that can be expected, and the effect of retrofitting a new filter to an existing dam.
R. E. Saunders, J. Roberts, B. W. Omundson
Ross River Dam is located immediately upstream of the twin cities of Townsville and Thuringowa. The population at risk from failure of the dam is approximately 110,000. A recently completed risk assessment has confirmed earlier studies that the dam does not satisfy current safety criteria and presents high risk levels in a number of areas. Importantly, the risk assessment has enabled the extent of these risks to be clearly identified. This paper summarises the risk assessment highlighting notable methodologies employed and the key findings of the study.
Richard R. Davidson, Roger Vreugdenhil and Mark Foster
Significant cracking was observed on the crest of the main embankment at Lake Eppalock for many years, but in recent years increasing movement upstream during low reservoir levels indicated a progressively deteriorating stability situation. Investigations also revealed cohesive filter material that would allow a crack to propagate. A fast-tracked remedial works program was completed in 1999 to rebuild the highly vulnerable upper rockfill shells and filters, both upstream and downstream. To manage construction risk, the works were carried out directly by G- MW with innovations in removal, protection and replacement of the downstream shoulders, and placement of a new multi-zone filter.