Brendan Sheehan, Chris Topham, Alan White, Rowenna Lagden
Darwin Dam is a 21m high embankment dam constructed on a geologically complex foundation that includes karst limestone features. The dam retains the top 15m of Lake Burbury on Tasmania’s west coast, and borders the Tasmanian Wilderness World Heritage Area. Defensive design of the dam addressed the key failure modes of piping through the complex foundations of limestone, sandstone, gravels and silts, and guarding against sinkholes forming in the limestone foundations. During construction, a comprehensive range of instruments were installed in the dam and foundation, as a long term means of monitoring this structure. A range of surveillance data has been collected since lake filling and this data, along with historic geological investigation information, was used to develop a three dimensional (3D) geological model of the dam and
foundation with phreatic profiles. The software used was a commercially available geographical information system. This tool has assisted Hydro Tasmania to better understand and manage the dam. The paper outlines the need for a 3D model, the methodology for development of the model, resources required, limitations and lessons learned. The benefits of the model, such as aiding understanding of foundation behaviour, assisting with interpretation of surveillance data, supporting decision making, and potential use during incident response are also discussed.
Keywords: Three dimensional, computer model, karst foundation, geology, hydrogeology ,dam surveillance
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Keirnan Fowler, Peter Hill, Phillip Jordan, Rory Nathan, Kristen Sih
Although there are considerable uncertainties in the science of climate change, there is a growing recognition of the importance of the issue. Incorporation of climate change impacts is now required in policy guidance from several government authorities and it is prudent risk management to consider the effects of climate change in planning for water resource infrastructure, including assessment and design of dam upgrades. This paper describes the potential impact of climate change on extreme flood estimates and provides a case study for Dartmouth Dam in south-eastern Australia. Three inputs to flood estimation were considered according to the projected impact of climate change; namely design rainfalls, modelled losses and initial reservoir level. The relative influence of each of these factors is explored. Rainfall and losses had a similar (and opposite) influence on results and for this dam the reservoir level prior to the flood event had the largest influence on results. This case study demonstrates that the insights of climate modellers and hydrologists need to be integrated in order to provide defensible estimates of the impact of climate change in flood hydrology studies. Credible projections of changes in design rainfall intensities are required for the full range of exceedance probabilities across Australia.
Application of Available Climate Science to Assess the Impact of Climate Change on Spillway Adequacy
Shao Kwan Ng
Asset management aims to ensure that assets, such as dams, are sustainable. In order to achieve this, management decisions need to be defensible and the long-term impacts of short-term decisions need to be clearly demonstrated, such that an asset operates and is maintained in an appropriate fashion and in a satisfactory condition. Expert rule systems are becoming widely recognised as powerful and elegant tools suitable for engineering and management decision-making. They are powerful, transparent and flexible tools that mimic how people make decisions, and hence provide a natural way of thinking for decision-making. This paper reviews the current usage of expert systems in asset management, and illustrates the potential of these tools, in conjunction with the available (ANCOLD) guidelines, to assist dam owners in decision-making, such as in condition evaluation and dam hazard assessment applications.
Keywords: Decision-making, expert rule systems.
Tony Harman, Richard Herweynen, Malay Ghosh
Following a number of years of investigation into the condition of the existing 1960’s post tensioned anchors at Catagunya Dam Hydro Tasmania embarked on an options study to determine the best method to restore the dam stability to acceptable limits for the long term. The required solution was intended to not only resolve the issue of anchor deterioration but also to increase the flood capacity of the dam.
Based on preliminary design work a concrete buttress solution was recommended and approved for detailed design. The preliminary design utilised a simplified, 2-dimensional, rigid body model, including crack analysis. As part of the detailed design a finite element model was developed to refine the preliminary design. However, this model did not support the simplified analysis and further non-linear finite element analysis demonstrated that the proposed passive buttress design solution was not technically feasible. The options were reconsidered and the adopted solution was to replace the original anchors with new modern anchors with a high level of corrosion protection.
The new anchors adopted are the largest post tensioned anchor loading currently used for a dam in the world. This along with the existing post-tensioned anchors and the tight geometry of the dam, which has a central spillway with a cantilevered ogee crest, provided significant challenges with the design of this dam upgrade. Some of the key design challenges included:
– Appropriate level of modeling and analysis to be able to make sound design decisions. (Hydraulic modeling and FEA).
– Congestion due to the tight geometry of the original design.
– Anchor head block detail to ensure the loads would be adequately secured and dispersed into the dam body
– Crest cantilever support to ensure that structural integrity was retained during construction and later in service. Innovative installation of carbon fibre reinforcement was used.
– Strain compatibility. It was important to ensure the structural contribution of new and old working together and that the consequences of application of new large stresses was manageable.
– Existing anchor degradation. The design needed to ensure that stability compliance was achieved for complete to zero effectiveness over time.
– Maintaining operability of dam and power station during construction.
– Achieving an effective long term maintainable solution.
This paper will present the risk associated with committing to a solution too early and the design challenges and the solutions finally developed, providing the dam industry with a valuable reference for future similar projects.
Analysis and Design Challenges Associated with the Catagunya Dam Restoration Project
Jim Walker, Jamie Macgregor
The Pukaki Canal Inlet structure is a large gated culvert and stilling basin structure, it is a High PIC appurtenant structure to the Pukaki Dam, located in the Mackenzie Basin area of New Zealand’s South Island.
The 560m3/s capacity inlet structure is founded on glacial moraines. It controls flow from the178 km2 Lake Pukaki storage into the 80m wide, 22km long Pukaki/Ohau canal. It is the owner’s (Meridian Energy) most important valve, as it feeds 1550MW of hydro generation on the Waitaki River.
A risk assessment in late 2009 identified a previously unrecognised trigger for a potential failure mode for the stilling basin. Principally, ongoing erosion of the reinforced concrete base slab could lead to failure of water stops in the slab joints potentially leading to slab uplift, foundation erosion, and ultimately, catastrophic failure of the Pukaki Dam. To better define the risk to the structure, further inspection of the stilling basin was recommended.
A dewatered inspection of the stilling basin was required, as further dive inspections would not improve our understanding of structure condition. Because the stilling basin cannot be isolated from the canal, this requires dewatering the entire Pukaki/Ohau canal, presenting significant risks of damage to the canals from slumping and lining failure. A dewatered outage also has major business revenue impacts.
This paper describes how Meridian were able to take advantage of a transmission network outage, scheduled for just six days after the risk was identified, to plan, safely dewater, inspect, and rewater 22km of hydro canal, and not just to inspect the Pukaki Canal Inlet structure, but also to implement repairs to the stilling basin slab which have successfully mitigated the structure safety and operational risks. This huge undertaking involved mobilising an army of people, plant and materials, and cost over NZ$1.8m. From identifying the risk to the structure, to completing repairs took just 13 (very busy) days.
Lessons learned in the areas of dam safety and asset management are presented. As well as those contributing to the success of the project in seizing an opportunity to mitigate the identified dam safety and operational risks.
M. Tooley, N. Anderson, N. Vitharana, G. McNally, C. Johnson and D. Moore
There is a significant stock of aging concrete dams in Australia which would not meet the requirements of the current recognised dam safety practices applicable to concrete gravity dams.
In this paper, field and laboratory investigations undertaken for two concrete gravity dams are presented, these being Middle River Dam and Warren Dam both owned and operated by the South Australian Water Corporation. The field investigations included a comprehensive drilling program recovering core samples ranging in diameter from 61mm (HQ) to 95mm (4C), continuous imaging (RAAX) of the drilled holes and installation of piezometers. Geological logging of the holes and mapping of the unlined spillway were also undertaken. The laboratory program included the testing of concrete lift joints and concrete samples in direct tension, shear and compression.
Concrete in Middle River Dam is suffering from extensive Alkali Aggregate Reaction (AAR), and consequently a suite of laboratory testing is being undertaken to determine the current level of deterioration and residual reactivity so that potential future AAR-induced expansion can be incorporated into any upgrade design solution.
The main purpose of the study is to determine whether site-specific parameters can be used to re-assess the stability of these two dams as calculations, based on the current standards, have shown that the dams have exceeded the allowable factors of safety values at the storage water levels experienced to date.
The findings may be useful to dam designers and owners faced with the upgrading of concrete dams, where traditional assumptions can result in no upgrade or an upgrade costing several million dollars.