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.
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L Joyce and G Macdonald
Claims for negligence against professional advisers are increasingly common. Large dams can, in the event of failure, give rise to large claims by a large number of affected parties. This paper highlights from the perspective of lawyers practising in the field of professional indemnity litigation some of the ways of managing the risk of such claims being made.
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.
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.
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.
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.