Since the research and development work carried out by the (then) Metropolitan Water Sewerage and Drainage Board for the strengthening of Manly Dam in 1979/80, there has been over twenty years of continuous improvement in the application of advanced post-tensioned anchors for gravity dam rehabilitation.
Up until the Manly Dam remedial works, concerns had been increasing as to the long-term viability of available anchors. Sophisticated monitorable and restressable anchors, with superior corrosion protection afforded by greased and sheathed strands, were developed initially in test-bed conditions. This style of anchor has subsequently been used extensively throughout Australia on dam upgrades.
This paper compares the claims made by the designers with the demonstrated outcomes of installations that have been achieved, with particular emphasis on dams now owned by the Sydney Catchment Authority and Sydney Water Corporation. The original commitments to economy, aesthetics and rapidity of construction have been borne out by experience, with additional environmental advantages also being achieved. With the confidence built up from many successes in the strengthening of older dams, the time appears right to revisit the construction of new dams using the same style of post-tensioned anchors as the primary stabilising force.
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Now showing 1-12 of 26 2965:
M.B.Barker and B.A. Vivian
Tumut Pond and Island Bend Dams are owned and operated by the Snowy Mountains Hydro Electric Authority. These dams, which are gated, have recently had significant electrical supply and control system upgrades. Subsequent reliability analyses performed for the gates provided unexpected results which highlighted issues concerning common mode failures and common cause failures associated with the mechanical systems. A further unexpected outcome of the analyses was the minor affect of human error and response to the emergency operating conditions of the gates in the event of electrical supply failure due to the over-riding mechanical system failures. This outcome was of benefit to the owners who had some concern that centralization of operation and consequent reduction in operating personnel would have an adverse effect on the reliability of the gates. The operation of the automatic control system is an ongoing issue for Island Bend where hunting of the gate operation is yet to be resolved. The preparation of the fault trees, development of failure probabilities and outcomes of the analyses are discussed in the paper which highlights some of the difficulties in design and operation of spillway gates, particularly where human response time is limited and automatic control is essential.
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
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.
The Forth River dams consisting of Cethana (110m), Devils Gate (84m) and Paloona (43m), were constructed between 1964 – 1971. The Population at Risk (PAR) downstream of this cascade system is significant in the event of hypothetical dam failure.
By 1990 a Generalised Method had been fully developed for estimating extreme rainfalls for South East Australia. Using these extreme rainfall estimates, flood estimates were updated for all dams owned by Hydro Tasmania. These estimates indicated that the spillway capacity of the three Forth River dams no longer complied with current practice.
The risk position of these Forth River dams did not comply with the ANCOLD risk based criteria, indicating that some level of upgrade should be considered to reduce the risk associated with flooding. Given the risk position, considerable priority was placed on resolving this issue.
Due to physical constraints within the Cethana Dam site area, it was difficult to upgrade to a “Standards Based” level of upgrade without very high expenditure and imposing additional risk arising from major dam modifications. Instead the ALARP (As Low As Reasonably Practicable) principle was adopted to determine an appropriate level of upgrade, which did not preclude upgrading to a higher standard, should this be necessary at some time in the future.
The spillway upgrade for the three Forth River dams was approved in 1999 and detailed design has commenced with completion of construction planned for 2003. This paper will include discussions on the decision making process, communicating complex dam safety issues to senior management and some interesting details of the design.
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.