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.
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.
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.
PJ. Cummins, P.B. Darling, P. Heinrichs, J.Sukkar
The Department of Land and Water Conservation, Town Water Treatment and Recycling Branch, had identified a number of local government council-owned dams throughout NSW with deficiencies. SMEC was engaged to undertake a portfolio risk assessment to assist in the development of an appropriate program of remedial works.
The portfolio risk assessment methodology relies on the development of consistent assessments of failure probability, consequence level and cost estimate for mitigation measures. This tool enables a large amount of data on a portfolio of dams to be drawn together so as to provide decision makers with a coherent and robust basis for the development of a program of remedial works.
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.
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.