Bill Hakin, Phillip Solomon, Geoff Hughes, Peter Siers
Lyell Dam is located on the Coxs River near Lithgow NSW Australia. It was constructed in 1982 to supply cooling water to Delta Electricity’s Mt. Piper and Wallerawang power stations.
In 1994 the storage capacity of the dam was increased by 7,500 MI by raising the embankment height and installing two 3.5m high inflatable rubber dams on an enlarged and slightly raised spillway sill.
Two significant failures of the rubber dams in 1997 and 1999, led the dam owner, Delta Electricity, to seek a more reliable way of maintaining the increased FSL whilst still providing spillway capacity for the design flood.
Following a detailed review of options, Delta Electricity chose to reinstate the storage capacity with the Hydroplus Fusegate System. The Hydroplus System consists of a series of fusible units that progressively tip off the spillway as flood magnitude increases, thereby forming a controlled breach in the spillway and providing for passage of the design flood. At Lyell Dam it has been designed such that no units tip until the 20 000 AEP flood. The System is designed to act as a normal free overflow spillway up until extreme events when it is required to commence operation. Key factors in the selection process were safety, reliability and operation/maintenance.
This is the first installation of the Hydroplus Fusegate System in Australia or New Zealand. There are currently 35 installations throughout the world. The System has wide application with dam owners either seeking to store additional water and/or to increase the capacity of their existing spillways for safety reasons in an economical and efficient manner.
This paper examines the decision and selection process adopted by Delta Electricity. It also presents a case study for the design and construction stages of this unique solution for Lyell Dam.
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Dr Bradford Sherman, Dr Phillip Ford, Allison Mitchell, Gary Hancock
Recent reports from the World Commission on Dams have highlighted the relative lack of knowledge regarding the release of greenhouse gases (GHGs) from reservoirs. In order to be considered eligible to receive carbon credits in the future, hydropower facilities probably will be assessed using some sort of life cycle analysis of net GHG emissions.
Unfortunately, empirical data regarding GHG emissions is available only for a few reservoirs none of which are located in temperate or semi-arid climates.
We report preliminary observations on the vertical distributions of methane and carbon dioxide in Chaffey Reservoir (Tamworth, NSW) and Dartmouth Reservoir, two temperate zone reservoirs located in southeastern Australia. In Chaffey, the diffusive methane flux from the hypolimnion to the epilimnion (where it is oxidised by bacteria) was estimated to be 220-1760 mg-CH, m’ d’. Operation of a destratification system released 43 t of CH, to the atmosphere in 3 days. The carbon dioxide flux to the atmosphere via the surface of Dartmouth was 21-168 mg-CO, m’ d’, and 530 mg-CO, m° d’ through the turbine. The impact on GHG emissions of common reservoir management techniques such as destratification and hypolimnetic oxygenation is discussed.
P.I. Hill, D.S. Bowles, R.J. Nathan, R. Herweynen
With the growing emphasis on a risk-based approach to dam safety management, event tree models are increasingly being used as an analysis tool. The simple structure of event trees belies some of the more complex issues associated with their application to dam safety risk analysis. This paper outlines some of the basic principles of event tree analysis and then demonstrates how inappropriate construction of event trees, and particularly oversimplification, can result in a bias in the estimated risk and produce misleading results when used to assess the dam against various risk criteria. Issues considered include the partitioning of the loading event and the impact of conservative assumptions such as assuming the reservoir is initially at full supply level.
R.A. Vreugdenhil, G. Gibson, M.R. Jorgensen, A. Brown and P.G. Somerville
For the first time for any region of Australia, a modern site specific seismic hazard assessment has been completed for six major dams, incorporating fault mapping and trenching to assess fault source characterisation and likely slip rates. A combination of modern ground motion attenuation relations appropriate for stable continental regions was used. The work was performed in a probabilistic context, and includes significant advances in Australia for all aspects of seismic hazard evaluation. The study found that for a short recurrence interval, a large earthquake distant from the site may have a greater probability of contributing to a low PGA, than a low magnitude event closer to the site. At longer recurrence intervals, the magnitudes that contribute most to the hazard have dropped significantly below the previous levels of magnitude for several storages. The outcome of this work is an understanding of the likely strength and duration of ground shaking at each of the six dam sites for any design earthquake, and an understanding of the contribution of each source zone to the seismic hazard. Ground motion parameters produced by the study have been used as a reasonable basis for subsequent seismic analysis of embankments, towers and spillway structures.
Graeme Bell, Robin Fell and Mark Foster
Standards based, dam safety management has always been about managing risks. Risk based approaches attempt to quantify the risks in a formal manner, but are based on the same requirement for good investigation and engineering, and understanding of the physical processes, as standards based methods.
This is demonstrated by the assessment of the potential for internal erosion and piping of Eucumbene Dam. The assessment is a combination of semi-quantitative risk based, and standards based approaches, and considers the likelihood of initiation, continuation, progression to form a pipe, and breach. The filter transition zones are coarser than required to meet modern filter design criteria, but it has been demonstrated by laboratory testing, and relation to the performance of other dams, that if, in the low likelihood erosion of the core initiates, it will after some erosion, seal on the filter transition zone. The downstream zone of rockfill has sufficient permeability to discharge any potential leakage which might occur, so the likelihood of breach of the dam by piping is
P L Campbell, J W Walker and J T Mills
The results of a questionnaire on deformation surveys sent to dam owners around the world are presented. An analysis of the large variation in current international practice is made. The link between geodetic surveys and displacement instrumentation is established. The comparison with practice within a recent major New Zealand dam owner is drawn and a deformation survey policy is developed. Recourse is made to dam safety guidelines. Application of the policy is then described for a selection of differing types of hydraulic structures. It is shown that with the application of the policy a more rational approach resulted, surveys better reflected actual performance and there was better integration with the overall dam safety monitoring programme.