D.N.D. Hartford and R.A. Stewart
It seems perfectly logical, obviously desirable and apparently sensible to prioritise dam safety reviews, deficiency investigations and capitalised remediation projects in terms of risk. However, our experience in attempting to apply the various deterministic and risk- based approaches in prioritising dam safety activities has revealed that, while it may appear to be quite logical and desirable to prioritise in terms of risk, it is rather less feasible than it appears.
This paper explores why different prioritisation processes can lead to different priority rankings across the same portfolio of dams. B.C. Hydro’s Preliminary Risk Exposure Profile process, which utilises the best and most robust attributes of risk analysis process at the preliminary level but avoids the pitfalls associated with estimating risks which will often have little or even no basis is presented. The paper explains how this process provides a “fail-safe” backup which will identify non-conservative and erroneous facility risk estimates; thereby allowing for correction in a timely fashion. The paper also raises some awkward philosophical issues which the profession will have to address in order to permit confident dam safety decision-making on the basis of risk analyses. Not the least of these is the following issue – “If preliminary estimates of risk are reasonably good, then there should be little need for more detailed risk analysis for confident and defensible decisions concerning making or not making dam safety improvements”.
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Increasingly, owners of ageing dams are having to reconcile with the notion of involving others in decisions affecting the management of their dams. Previously recognised as ‘expert’ exclusive arenas, doctors, lawyers, scientists and engineers are now expected to respond to enquiring consumers and communities. Individuals and communities are expressing their need to share responsibilities.
Events at Hume Dam provide an illustration of the potential challenges and opportunities that all Dam Owners may face. This paper is a narrative of the processes of involving the wider ‘community’ in the Hume Dam remedial work project. It remains for the stakeholders to rate the effectiveness of the process.
R J Westmore and P J Cummins
Wartook Reservoir is owned and operated by the Wimmera Mallee Rural Water Authority in western Victoria. The reservoir was constructed in the period 1887 to 1890 on the Mackenzie River within the Grampians National Park. It has a capacity of 29400 ML, is the sole supply of water to the City of Horsham, and also supplies stock, domestic and irrigation water to the Wimmera and Mallee Regions of Victoria.
The embankment is 1100 m long, 12 m high and is constructed of loose to medium density silty fine sands which are susceptible to liquefaction during a seismic event due to the combination of high pore water pressures and low density. Active seepage from the embankment and foundations render the embankment susceptible to failure by piping.
The outlet works were constructed of sandstone masonry and comprise a tower and cut-and- cover conduit buried within the embankment. Inflow of fine sands from the embankment into the masonry tunnel render the embankment susceptible to failure by piping through the joints in the masonry tunnel.
Design concepts for the rehabilitation of the embankment, outlet and spillways have been developed jointly between Wimmera Mallee Water and SMEC Victoria adopting a risk based approach. The design involves partial rehabilitation of the works, providing acceptable levels of risk to the Authority and community, at an economically justifiable cost.
Mark Foster, Robin Fell and Matt Spannagle
This paper describes a method for estimating the probability of failure of embankment dams by piping. The so called “UNSW method” is based on the results of an analysis of historic failures and accidents of embankment dams. An estimate of the probability of failure of a dam by piping is made by adjusting the historical rates of failure by piping by applying weighting factors which take into account the dam zoning; filters; age of the dam; core soil types; compaction; foundation geology; dam performance; and monitoring and surveillance. The method is intended for preliminary assessments only and is ideally suited as a risk ranking method for portfolio type risk assessments to identify which dams to prioritise for more detailed studies and as a check on event tree methods.
J. H. Green and P. I. Hill
Early Probable Maximum Flood (PMF) studies and spillway adequacy assessments for Hume Dam adopted the standards based approach of the time. Since then considerable work and thought has gone into the estimation of extreme floods – both the philosophy and the practice. These changes include the general change in emphasis away from a standards based approach and towards risk assessment; the move towards an AEP-neutral approach for the transformation of extreme rainfalls to extreme floods; and the redefinition of both the PMP and the PMF.
This paper details the effect these and other changes to extreme flood estimation techniques have had on the perceived adequacy of the Hume Dam spillway to pass extreme floods.
David Dole and Brian Haisman
The Murray-Darling Basin Commission recently created River Murray Water, an internal business unit, as a step towards the micro-economic goals of the COAG Water Reforms.
The assets which regulate the River Murray, have a replacement value around $1.4 billion. They range from the 4000 gigalitre Dartmouth Dam in the headwaters, to the 7.5 kilometres of barrages near the Murray mouth and are presently held in trust for the Contracting Governments of the Basin Initiative by one or other of the three riparian states. River Murray Water is bringing the assets together into a single, integrated business with the aim of securing long-run sustainability, funded through pricing for services provided. Broad institutional and pricing principles are described along with the special challenges of an inter-government environment.
These challenges are being met by adopting clarity and simplicity as driving principles, supported by best practice asset information. The paper describes the upfront development of explicit guiding principles and policies, including risk management and dam safety; coordination of activities; generation of life cycle information; and introduction of contestable service provision for the business.