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Thomas Zink, Michael Howat, Clive Anderson, Richard Davidson
This paper describes the refurbishment of Diversion Gate No 2 at Roxburgh Dam on the Clutha River, New Zealand. This 53m high concrete, gravity dam constructed in the 1950’s had three diversion gates fitted with stoney rollers which when opened into flow allows the rollers to disengage from the gate precluding subsequent closure. Diversion gate No. I was sealed off with a concrete plug. The remaining two diversion gates are required to provide additional flow control flexibility at the dam. Key aspects of the refurbishment design and construction include the system used to remove silt from upstream of the gate, and the condition of the gate itself and the concrete diversion channel after nearly fifty years of service. Brief details of the commissioning testing are also presented.
M.A. Foster, R. Fell, R. Davidson, C.F. Wan
The probability of failure of embankment dams by internal erosion and piping can be estimated using historic performance, and event tree methods. Event tree methods are preferred for all except preliminary assessments, because they can better model the characteristics of the dam. This paper provides guidance on how to estimate the conditional probabilities within the event tree based on an understanding of the process involved, the historic performance of dams, and experience in recent risk analyses. This includes methods for representing the reservoir water level; assessing the likelihood that piping initiates; assessing the likelihood that erosion will be controlled by the filters or transition zones; and assessing the likelihood of development of a pipe and breach. The paper will be useful for those carrying out risk analyses and will also have relevance to those who are assessing piping of dams using traditional methods.
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.
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.
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.