A. Swindon, M. Gillon, D. Clark, P Somerville, R. Van Dissen and D. Rhoades
The 45 km long Lake Edgar Fault in south-west Tasmania passes through the right abutment of the Edgar Dam and into Lake Pedder, and within 30 km of three other large dams. In 2004 an independent seismotectonic study concluded that the fault had moved three times in the past 48–61,000 years, with the last movement around 18,000 years ago.
In order to better constrain the risk assessment for the nearby dams, the likelihood of a rupture recurrence along the fault was required. Two independent methods were investigated. The first was a comprehensive review of active faulting and deformation of stable continental region faults within Australia, and a comparison with similar faults worldwide with the well studied behaviour of the Lake Edgar Fault. The study results demonstrated the episodic nature of stable continental region fault activity, separated by much longer periods of quiescence, with a decreasing likelihood of rupture following each event within an active period. The time window of applicability of this paleoseismological study is thousands to tens of thousands of years.
The second study looked for evidence of precursory seismic activity in the vicinity of the fault which could indicate an increasing risk of rupture over the next decade or so. This method does not predict specific earthquakes, but does forecast whether the level of future earthquake activity in the short to intermediate term is relatively low, high or at an average level. Using a catalogue of seismic activity for south-eastern Australia, the study concluded that there is no evidence for precursory seismic activity in the area of the Lake Edgar Fault that would give rise to an elevated forecast rate of occurrence of moderate magnitude earthquakes either in the short to intermediate term. This precursory method has a window of applicability of a decade to perhaps several decades.
The combination of these two studies has advanced the understanding of the Lake Edgar Fault activity by both setting it in the long-term stable continental region fault context and investigating the presence of short-term behavioural activity. This has allowed the seismic hazard to be re-assessed as nearer to ambient levels than earlier postulated. This work has applicability for other fault scarps in Australia, both with regards to better defining the long-term hazard (103-105 years) posed by a fault, and potentially also giving advance (short-term 101 years) notification of increasing risk of fault rupture. Better long- and short-term hazard information allows more complete and thorough engineering decisions to be made.
Keywords: Earthquake, seismic, fault rupture, dam safety, risk assessment, Hydro Tasmania, Lake Edgar Fault.
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David Ho, Chee Wei Tan and Glen Dominish
Upper Cordeaux Number 2 Dam is founded on an igneous intrusion rock mass which overlays sedimentary rock layers above the Wongawilli Coal seam. The coal mining company, BHP Billiton Illawarra Coal, planned to extract coal close to the dam. Although the dam is classified as a low hazard dam, its importance, both as part of the Sydney Catchment Authority’s water supply system and for its significant heritage value, mean that the proposed mining should not have undesirable impact on the structure. This paper describes how the mining impact on the dam was assessed using a nonlinear 3D finite element model. The model considered the pre-existing cracks in the dam wall, uplift water pressure along the dam/foundation interface and the hydrostatic pressure at full supply level. Mining-induced movement such as valley opening, closure and upsidence were applied to the model. Stability and strength assessments were made against a set of acceptance criteria developed for mining impact. The development of different stabilising mechanisms was examined. From the numerical investigation, WorleyParsons was able to provide technical advice to the mining company, the dam owner and the NSW Dam Safety Committee to facilitate the mining application and to satisfy dam safety requirements.
Keywords: Mining subsidence, Arch/gravity dam, Nonlinear numerical analysis, Safety assessment
Stephen McInerney, Donald A. Bruce and John Black
An historical database of North American dam anchoring experience has been recently assembled in the United States. This database clearly shows the historical development of dam anchoring technology, particularly with regard to corrosion protection practices over four decades. The results of this research are significant to dam owners worldwide because of the number of examples in the database.
The paper describes New Zealand experience with dam anchoring against the background of the historical practices in North America and the main conclusions drawn from the United States research.
Keywords: Post-tensioning, anchor, corrosion protection, historic database, dam remediation
Appurtenant structures associated with a dam play and important part to the dam’s operation. For these structures it may be important that their functional and structural integrity is retained in the event of a notable earthquake, particularly when they are required to release water from the reservoir in a controlled manner to lower the storage following an earthquake. Research has been conducted into the current state of practice for the seismic design and analysis of these structures, including review of the main issues for seismic effects, documentation of case histories and review of current research, international guidelines and standards. The general assessment philosophy was found to be relatively consistent internationally, however, the adopted assessment procedures were found to vary. The status of the current ANCOLD earthquake guidelines has been provided in relation to the current international state of practice for various types of appurtenant structures.
Keywords: Appurtenant structures, performance criteria, seismic performance, seismic analysis.
Dörte Jakob, Robert Smalley, Jeanette Meighen, Brian Taylor and Karin Xuereb
Probable Maximum Precipitation (PMP) is one of the required inputs for estimating the PMP design flood. In estimating the PMP, currently no allowance is made for long-term climatic trends. A 2-year project funded jointly by the Australian Greenhouse Office and the Queensland Department for Natural Resources and Water, and with in-kind contributions by the Bureau of Meteorology began in May 2006. This study aims to assess how climate change might affect estimates of PMP. Preliminary results from this work will be presented.
Changes in factors used in PMP estimation, such as storm type and depth-duration-area curves, were assessed using a storm database covering the period 1893 to 2001 (Beesley et al. 2004). Based on the last 50 years, there is little evidence to support the notion that tropical cyclones (connected to major rainfall events) are penetrating further south or have become more frequent. A recent event that led to widespread flooding (Gold Coast, June 2005) was found to have very high storm efficiency. Changes in observed and projected moisture availability were assessed on the basis of a high-quality dataset of surface dewpoint temperatures and climate model output.
It is assumed that PMP received by a catchment is not uniformly distributed over a catchment but rather follows a typical spatial pattern. A pilot study to revise design rainfall estimates is currently under way at the Bureau of Meteorology. The methods developed in the pilot study were used to assess whether the spatial distribution of design rainfall estimates might be changing under a changing climate.
Keywords: Probable Maximum Precipitation, climate change, moisture availability, storm efficiency
Mike Phillips and Karen Riddette
The use of Computational Fluid Dynamics (CFD) models in the dams industry has increased significantly in recent years and conversely the use of physical hydraulic models has decreased. Typical design approaches for an upgrade of similar magnitude to the Hinze Dam Stage 3 project would have allowed for considerable time to develop a preliminary spillway design before hydraulic modelling was introduced, potentially requiring only one type of model. So is there a need for both types of models?
Because of the complex hydraulics associated with the spillway required for the Hinze Dam Stage 3 raise and accelerated schedule, the utilisation of CFD and 1:50 Froude Scale physical hydraulic models was necessary. Both models were constructed independent of each other. Both models complemented each others strengths and weaknesses, and each provided critical information at the following different stages of design:
• Spillway selection and conceptual design stage – the CFD model results were highly valuable in steering the selection of spillway type and configuration, particularly with visual representations of the ranges of flow for each spillway option.
• Preliminary design – in a one week period, 90 to 95% of the final spillway layout was resolved with interactive modifications of the physical hydraulic model.
• Detailed design – both the physical hydraulic model and the CFD model were utilised to determine water pressures, velocities and water surfaces and evaluate cavitation potential as input to detailed design.
In the case of the Hinze Dam Stage 3 project, it was highly advantageous to utilise a CFD and physical hydraulic model to achieve the design outcomes at each phase of the design. The dual-model study approach also provided advantages for project management of the design and stakeholder involvements.
Keywords: Computational fluid dynamics, CFD, physical hydraulic model, spillway, hydraulics