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.
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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.A. Maisano, P.A. Miskell
Design and construction issues relating to Cadiangullong Dam were reported to ANCOLD at
the 1998 Conference on Dams.’ First filling of the dam occurred in July 1998. Monito
ring of installed thermocouples, piezometers, joint and crack meters, movement targets and
seepage measurement devices, has been regularly undertaken by the dam owner and
reviewed by the designer. Annual inspections over these first 3 years of service, with
condition/performance reporting by the designer, has been undertaken in terms of the
Piezometers revealed excessive foundation uplift pressures near the right abutment.
Pressure relief holes drilled into the foundation were subsequently shown to have been successful in lowering foundation uplift pressures. Stability analyses confirmed the dam to
be stable under the new uplift regime.
The paper presents the performance of the structure and the reservoir in meeting the
designer’s and owner’s expectations as an assured process water supply for the Cadia Hill
Gold Mine, one of Australia’ s largest gold mines.
As one of the first roller compacted concrete (RCC) dams to use grout enriched RCC as an
impermeable facing to the upstream face and a durable surfacing to the stepped spillway,
the paper will also provide a performance review of this material which has since been
adopted on a number of other RCC dams worldwide.
Jenny Stewart, Murray Gillon
This paper describes decommissioning studies carried out as part of a dam safety improvement project by Coliban Water. The project results from a Portfolio Risk Assessment of 20 referable dams and the selection of 10 dams for safety improvements. Due to future water supply commitments and possible alternative supplies, eight of the reservoirs were subject to a decommissioning analysis as part of the dam safety options studied. The decommissioning studies included alternative uses, flora and fauna and other environmental issues, and European and aboriginal heritage studies.
As a result of the studies, five of the reservoirs will no longer be required for water supply. Two will be upgraded and handed over to others to manage as recreation sites and one will be decommissioned. Two are still being considered for either decommissioning or hand-over to others at a reduced capacity for habitat and heritage benefits.
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.
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.