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.
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Peter Lilley, Kelly Deighton, Don Tate, Craig Scott
In recent years TrustPower has undergone a rapid transition from a part owner of three dams in the Kaimai ranges south of Tauranga and the Hinemaiaia and Wheao schemes near Taupo in 1998, to the present ownership situation. Today TrustPower owns 22 dams comprising a range of structure types, including arch, earthfill, rockfill, concrete gravity and a number of embankment canal systems. The dam classifications for the dam portfolio vary from small to large and the NZSOLD potential impact ratings vary from very low to high. The portfolio includes some of the largest dams in New Zealand, for example Matahina Dam a 70m high central core rock fill, Patea Dam an 80m high earthfill dam and Mahinerangi Dam a 40 m high concrete arch dam with concrete gravity abutments.
The dam structures vary significantly in terms of age, potential impact and risk to TrustPower . The Dam Safety Management Procedures (including monitoring and surveillance systems, inspections and reviews) that existed for each dam also showed considerable variation in comparison.
The approach adopted for dam safety management is described, and the interrelationship with commercial objectives and commonly accepted standard practices.
The report of the World Commission on Dams (WCD), issued in November 2000 was the result of a world-wide study of the development and effectiveness of dams. This paper describes a parallel but much smaller study, specifically relevant to the Asia-Pacific Region, carried out simultaneously for the Asian Development Bank (ADB) by a multi-disciplinary consortium of consulting organisations, whose work was reviewed by an Advisory Panel. This study comprised an analysis of performance compared with design expectations in case studies of four dams spread across the Region and constructed at different times between 1970 and 1999, supplemented by a literature survey. Although the case studies were conducted independently of WCD and to different Terms of Reference, they used similar methodologies, and the results were submitted to WCD to increase their knowledge base. The ADB study’ s recommendations take a somewhat different form to those of the WCD, but there is no conflict between the recommendations of the two studies.
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.
For many years, engineers associated with the design, construction and operation of large dams have been undertaking environmental effects studies in association with their projects in the belief that they were thereby satisfying their obligation to the community whose interests they served. With increasing environmental consciousness of the community in developed countries, methods have been developed one by one for assessing environmental impacts of various kinds, and techniques have been developed for abating them.
However, the issue in November of the report of the World Commission on Dams (WCD) has focussed attention not only on the importance of bigger issues such as regional ecology, national economic disbenefits and social dislocation, but also on the vulnerability of dams to social and political hindsight.
This paper develops the above background, and shows why some excellently conceived techniques developed in the early 1970’s were capable of identifying almost all imaginable environmental impacts of dam projects, but were not applied in such a way as to deal adequately with the larger issues. It is argued that tools for dealing with all known issues now exist, but that responsible and competent application of the tools are not equivalent to successful application of them. d A new approach is suggested both to upgrade the quality of the decision and to make successful adoption of a soundly based decision more likely to withstand long term critical appraisal, by expressly recognising these decisions as ethical ones.
R.J. Nathan, P.I. Hill, and H. Griffith
Recently released ANCOLD and IEAust flood guidelines include provision for the estimation of the Probable Maximum Flood (PMF) as well as the Probable Maximum Precipitation Design Flood (PMP DF). This paper examines the theoretical justification for derivation of these two different types of floods, and discusses how they may be used to characterise the hydrologic risk relevant to dam spillways. Recent experience has indicated that there is some confusion in the industry about the different uses of these estimates, and thus one objective of the paper is to clarify the different concepts involved and to provide an illustration of the differences between flood estimates for the two methods. Examples are provided to illustrate how the different estimates may be derived, and the practical implications for risk analysis are discussed.