Mike Taylor and Doug Halloran
Candowie Dam is a 15m high embankment dam with a storage capacity of 2182 ML. It is the primary source of water for the Westernport Region Water Authority which includes Phillip Island and the town of Cowes south- east of Melbourne.
The existing spillway, comprising a 21m long concrete ogee profile crest discharging into a concrete chute which converges to a width of 7m, has a capacity to only accommodate the I in 6 000 annual exceedance probability (AEP) flood, well short of the required capacity of the 1 in 40 000 AEP flood.
In addition, Westernport Water would like to increase the yield of Candowie Dam as far as economically possible, within the scope of the spillway works.
A solution has been developed whereby the spillway capacity could be increased to accommodate the 1 in 40 000 AEP flood and at the same time the full supply level could be raised by 900mm resulting in an increase in storage of 573 ML and an increase in yield of 580 ML per year.
The solution comprises the following:
The fusegates are designed to tip off incrementally with the initial tip off occurring when the flood exceeds the 1 in 200 AEP flood. The tip offs are actuated purely by hydrostatic pressure developed by the rising flood level and programmed so that at no stage does the outflow flood peak exceed the inflow flood peak.
Westernport Water can accommodate the risk (0.5% per year) of the occasional loss of the existing top 830mm of storage resulting from a tip-off.
The total cost of the augmentation is estimated to be in the order of $ 700 000.
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The regulatory environment for dams in Queensland will change when the new provisions of the Water Act 2000 are proclaimed in late 2001 or early 2002. The definition of a ‘referable dam’ has shifted from a simple height and storage criteria to one that requires a population at risk (PAR) before dams are considered referable. Additionally hazardous waste dams such as tailings dams will no longer be considered as referable dams and under the Act regulatory control will be transferred to the Environmental Protection Agency.
Referable water dams will be assigned a Failure Impact Category of 2 if they have a PAR greater than 100 and a Category of I if they have a PAR greater than or equal to 2 and less than 100. This has required the development of guidelines for the assessment of ‘population at risk’. These guidelines have been written to suit a wide variety of dam impact situations and a range of dam owner resources. The guidelines require certification of the failure impact assessment by a Registered Professional Engineer in the state of Queensland.
The Queensland Dam Safety Management Guidelines have also been re-written to make them more amenable for reference in dam safety conditions.
New dams will require development permits to be issued under the Integrated Planning Act and will have development permit conditions applied in accordance with their Failure Impact Category. There is a range of transitional provisions for existing dams.
This paper covers all of the above issues as well as providing an indication as to how these statutory guidelines relate to the various ANCOLD guidelines.
G. A. Pickens, J. O. Grimston
The Opuha Dam Project is a multipurpose water resources development, for irrigation and other uses. The 50 m high irrigation dam incorporates a 7.3 MW hydro installation, enhances summer low flows downstream, increases potable water supply security, is a significant recreational facility and provides flood attenuation. Opuha Dam was the first large dam permitted under NZ’ s Resource Management Act, for which sustainability is the cornerstone. It was also built under a design-build contract arrangement. Although breached by a flood during construction, the dam was successfully completed and performance has met or exceeded expectations. Experiences of potential value to future developments are outlined including the positive features of design- build. Technical features which contributed to the cost-effectiveness and performance of the project, are described, including downstream reregulation to enable “on-off’ peak hydro operation, Obermeyer type spillway gates to maximise flow capture for hydro and a stepped service spillway.
Lelio Mejia, Murray Gillon, Jim Walker, Tom Newson
This paper describes the criteria for developing seismic loads for the safety evaluation of dams of two New Zealand owners. The criteria were constrained to satisfy the requirements of the NZSOLD Dam Safety Guidelines and to be consistent with international practice in countries with levels of seismicity and socio-economic development similar to New Zealand. In selecting the criteria, dam seismic load standards from several countries were surveyed and summarized. The selected criteria follow a standards-based approach to the seismic safety evaluation of dams. Guidelines for the use of deterministic and probabilistic procedures to develop seismic loads were formulated as a function of the Potential Impact Classification of a dam. In addition to the traditional deterministic definition of evaluation earthquakes, the selected criteria allow the use of a probabilistic definition in cases where the deterministic definition yields very low probability evaluation events.
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.
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.