“The move to a risk-based approach to the management of dam safety requires robust estimates of the consequences of failure, and particularly the potential loss of life.” (Hill et al. 2007) In Australia to date, the empirical method developed by Graham (1999) is the most widely applied approach for estimating loss of life from dambreak flooding. However, as the move to risk-based approaches of dam safety management has gathered momentum internationally, increasingly sophisticated techniques for estimating loss of life have emerged. For example, Utah State University has developed the LIFESim model (Aboelata et al. 2002, 2003, 2004) and BC Hydro the Life Safety Model (Johnstone et al. 2003, 2005), while the United States Army Corps of Engineers have incorporated a simplified version of LIFESim into a software package they use to simulate the impacts of dambreak flooding (HEC-FIA). One advantage of the LIFESim, LSM and HEC-FIA models is that they can be used to estimate loss of life attributable to both natural and dambreak flooding. These models, along with empirical methods developed by Graham (2004, 2006), HR Wallingford (Pennning-Roswell et al. 2005, Priest et al. 2007) and Jonkman (2007) for estimating loss of life from flooding are reviewed in this paper, with an eye to their applicability in Australian contexts. This research was conducted with support from the 2009 ANCOLD travel bursary for young professionals.
Keywords: loss of life, dam safety risk analysis.
Gavan Hunter, Chris Chamberlain, Mark Foster
Hinze dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is principle potable water storage supplying the Gold Coast. Hinze Dam Stage 3, presently under construction, involves raising the existing embankment almost 15m to a maximum height of 80m.
The foundation geology on the right abutment of the main embankment comprises of a deeply weathered sequence of greywacke and variably silicified greenstone and chert. The deeply (and variably) weathered soil profile below the right abutment of the existing embankment presented an unacceptable piping risk for the embankment in its existing condition. Contributing factors included: 1/ the highly erodible extremely weathered greywacke and presence of continuous defects in the weathered soil mass; 2/ the extremely weathered greenstone in direct contact with highly fractured, highly permeable silicified greenstone and chert bodies aligned normal to the dam axis which provide continuous seepage paths through the foundation.
Works were required as part of the Stage 3 raise to address the foundation piping risk. Significant issues for design included: 1/ the depth of weathering extended up to 25to 40m into the foundation.; 2/ extremely weathered and highly erodible greenstone was present below the right abutment of the embankment and extended down to the lower abutment some 50 to 60 m below the existing dam crest; 3/ the reservoir level could not be drawn down during construction and the probability it would be near full supply level during the works was high; and 4/ the variability of strength in the greenstone form soil to extremely high strength presented challenges for excavation.
The options assessed to address the piping risk included a plastic concrete cut-off wall and an upstream blanketing option. The plastic concrete cut-off wall (220m long and up to 50m deep) and deep filter trench was the selected option. The cut-off wall had been successfully completed ahead of time and below budget. The innovative design required excavation through earthfill core of the embankment under full reservoir level and use of a purpose built trench cutter (by Bauer Foundations Australia) for the variable excavation conditions.
Keywords: dam safety, piping, risk assessment, cut-off wall.
David Ryan, Simone Gillespie
The Burdekin Falls Dam is the largest of the 19 dams owned by SunWater. The dam is located on the Burdekin River at AMTD 159.3km, approximately 210 km south of Townsville and supplies water for irrigation, urban and industrial development in the lower Burdekin Region. The dam has such unique features as the largest spillway of any dam in Australia and a catchment area of 114,770 km2, which is equivalent to about 1.7 times the land area of Tasmania. It is proposed to raise the dam to provide a more certain water supply for the North Queensland region. This paper outlines the features of the existing structure, the influence of the revised hydrology since the time of its construction and the options considered in the planning and design of the raised structure.
Keywords: Burdekin Falls Dam, unique features, spillway, fuse plug.
Russell Paton, Peter MacTaggart, Lee Benson
The Nathan Dam project has been identified by the State Government of Queensland as a potential water supply option to facilitate future growth in central Queensland. The proposed storage is located approximately 69 km downstream of the township of Taroom and would have a storage capacity of 1,080,000 ML which would make it Queensland’s fourth largest storage.
The proposed dam arrangement includes a central concrete gated spillway section across the river in order to maximise the storage volume and limit the flood rise upstream such that flood levels at Taroom are not increased during major flood events. A high level fixed crest spillway, to assist in the passage of rare flood events, forms the right abutment portion of the dam wall. It is proposed that the bulk of the concrete sections of the dam be constructed using roller compacted concrete (RCC).
The investigations to progress Nathan Dam are complicated by the existence of the Boggomoss Snail (Adclarkia dawsonensis) within the proposed inundation area. The snail is listed as a critically endangered species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), and a proportion of the snail’s known population is located on a Boggomoss (the colloquial name for an artesian spring) that will be inundated should the project proceed.
SunWater has engaged Australia’s foremost expert on land snails to design a translocation process aimed at relocating the species to alternative habitat outside the inundation area. The process will seek to not only protect the snail from the dam development, but to increase both its numbers and distribution thereby reducing risks to the currently isolated population from threats such as fire and predation. It is the first time in Australia that such a trial has been attempted, and SunWater is working closely with the Federal Department of Environment, Water, Heritage and the Arts (DEWHA) to ensure that the process is consistent with their policies and guidelines.
The paper will discuss the engineering and environmental challenges of the dam and how the Environmental Impact Study process can influence the delivery of a project.
Keywords: Nathan Dam, Environment, Engineering
Ben Ross, Jason Brown, Richard Rodd
Goulburn Weir was constructed in 1891 forming Lake Nagambie on the Goulburn River, approximately 8km north of Nagambie in Victoria. It is a key asset in the irrigation network diverting water to 352,000ha in Northern Victoria. The weir was remodelled between 1983 and 1987, replacing 21 overshot gates with nine radial gates. A series of 28 post tensioned bar ground anchors were installed to secure the radial gate concrete support piers to the weir’s mudstone foundations. On 8 March 2006 during routine testing of the pier bar ground anchors, failure of one anchor occurred. It posed a possible risk to pier stability. Subsequently investigations into the cause of failure and its implications was undertaken consisting of a program of data review, site investigations, metallurgical testing, geotechnical investigation, design reviews and stability assessments. It was recommended to replace the failed anchor and 10 other under performing anchors with 8 cable strand anchors at the cost of approximately $1million.
Key words: Risk, bar anchor failure, stability assessment, anchor construction.
Gavan Hunter and James Toose
Hinze Dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is the principal potable water storage supplying the Gold Coast. The Stage 3 raise, presently under construction, involves raising the embankment almost 15m to a maximum height of 80m.
The central core earth and rockfill embankment is founded on competent greywacke rock within the valley floor and left abutment. On the right abutment it is founded on extremely weathered greywacke and rockfill stability berms were constructed upstream and downstream on this weak foundation.
Key issues for the design of Stage 3 embankment raise on the right abutment were: 1/ removal of the existing downstream stability berm and deep excavation at the toe of the Stage 2 embankment to connect into the blanket filters under the downstream shoulder; 2/ the soil strength properties of the weathered greywacke foundation and the presence of pre-sheared defects with strength properties significantly below the strength of the soil mass; and 3/ undertaking the works while the reservoir remained in operation and close to full supply level.
It was not possible to undertake large scale excavation at the downstream toe of the right abutment as the factor of safety for the excavation condition was below the design criterion for slip surfaces extending back to the upstream shoulder. The innovative design solution was for a staged excavation and back fill operation up the right abutment. In this ay the stability requirements were achieved by a 3-dimensional buttressing support and reduced the time that critical excavation sections were exposed.
The construction risk is being managed under a dam safety management plan. Key elements of this plan include instrumentation monitoring and increased surveillance for early detection of a potential incident, a series of trigger levels and responses to these levels, a clear hierarchy of contacts and adequate preparation for a dam safety emergency (including materials, personnel and equipment).
The embankment construction is presently in progress. The most critical sections have been successfully completed without incident and displacements are within the predicted range. Communication and planning within the Alliance between the designers and constructors has been a key element in the successful construction works to date.
Keywords: construction risk, embankment design, embankment construction, dam safety management