ANCOLD Bulletin No.116 (December 2000)
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Papers 2000
2000 – Spillway Gate Reliability
Learn moreGlen 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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Papers 2000
2000 – Lyell Dam Rubber Dam Incidents
Learn moreI. R. Forster
Lyell Dam is a concrete-faced rockfill dam, located on the Coxs River, near Lithgow, NSW. The dam forms part of the Coxs River Water Supply Scheme, which supplies water to Delta Electricity’s Wallerawang and Mount Piper Power Stations. In 1994, the spillway capacity of the dam was upgraded, and the storage augmented with the addition of two 40 m long by 3.5 m high inflatable rubber dams to the spillway crest. An automatic deflation system, controlled by a programmable logic controller, was installed to provide a staged bag deflation sequence during flooding, and hence minimise the downstream impact of rubber dam operation.
Although the rubber dams and control system initially operated as designed, more recently, two uncontrolled bag deflations have occurred, which have caused flooding downstream and loss of significant storage volumes. In the first incident, a spontaneous uncontrolled deflation of the rubber dams released about 1600 ML, before the bags re-inflated automatically. An investigation revealed that the incident was most likely the result of design deficiencies in the control system. Recommendations were made for improvements to the system.
During the most recent deflation, one of the rubber dams failed by spontaneous rupture, and approximately 6000 ML of water was released from the dam. The Dam Safety Emergency Plan was activated to ensure persons at risk downstream were notified of the impending flood wave. A post- failure inspection of the ruptured bag suggested that the likely cause of failure was a manufacturing defect, which allowed air to penetrate the layers of rubber forming the bag. The rupture most likely occurred when the resulting air pocket expanded on exposure to the sun.
The paper examines the two deflation incidents in detail, and analyses the emergency response to the second incident.
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Papers 2000
2000 – Advances in the Practice and Use of Portfolio Risk Assessment
Learn moreD. S. Bowles
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Portfolio risk assessment (PRA) can now be considered to be a standard of practice in Australia. In this paper various advances in the state-of-the-practice for performing PRA’s are reviewed, including some pitfalls and limitations. The uses of PRA outcomes by owners are discussed, along with some ways to improve the value derived from PRAs. The challenges that are common in seeking to achieve an integration of the PRA process into the owner’s dam safety management program and with broader business processes, and the importance of targeting PRA outcomes to an owner’s specific business needs, are emphasised.
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Papers 2000
2000 – Risk and Standards Based Approach to Rip Rap Design Alternative for Grahamstown Dam Stage 2 Augmentation
Learn moreM. 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.
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Papers 2000
2000 – Protecting Environment Flows While Catering for Urban Water Supply Demand – The Cowarra Off-Creek Dam Experience
Learn moreMurray Thompson and Geoff Chenhall
The Hastings District Water Supply Augmentation Scheme [HDWS] includes a 10GL off-creek storage dam, which is currently under construction and due for completion in October 2001. The Cowarra off-creek storage dam is required to meet predicted long-term urban growth demands for water supply and to ensure protection of environmental flows in the Hastings River.
Since 1985 the Hastings Council has progressively developed a strategy for the augmentation of the water supply scheme. A very successful ongoing consultation process with both the local community and key government agencies during the planning and implementation phases of this project has highlighted a number of key issues including:
“That the impact upon aquatic flora and fauna in the Hastings River should be minimised and appropriate safeguards developed by maintaining minimum river flows to ensure that the river habitat is not adversely affected”
The subsequent HDWS Environmental Impact Statement, 1995 was one of the first in NSW to recognise the importance of environmental river flows in the assessment of the aquatic ecological effects of water supply schemes. This paper to be presented to the ANCOLD Conference on Dams will detail the investigation, planning, implementation and current construction activities associated with the Cowarra Off-Creek Storage Dam.
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