Maree Dalakis, Dr Saman de Silva, Siraj Perera and Dr Gamini Adikari
This paper describes the results of a statistical and qualitative analysis on historical dam safety incidents in Victoria, the first study of its kind conducted in the State. The study investigates trends arising from qualitative dam safety incident data collected by the Department of Environment, Land, Water and Planning since the year 1996. The reported incidents are categorised based on their severity and statistical trends are identified in relation to the types of incidents common to regulated and unregulated dams, as well as common responses to incidents, including their post-incident operation. The geographical distribution of incidents across the State is also analysed to determine the effects of seismicity on dam safety incident rates. Furthermore, the unique Victorian conditions of sustained drought and subsequent flooding and their impact on incident rates are investigated through the combined analysis of geographical incident distribution and streamflow data. The incident data is further assessed according to the frequency of visual inspection and reporting of the structures in order to gauge the relative influence of these practices, and dam regulation in general, on mitigating incident risk in dams. An understanding of dam safety incident trends and the impact of inspection and reporting practices is increasingly important given the increasing expectation for dam owners to properly operate and maintain their assets with minimal resources and finances.
Keywords: dam, safety, incident, historical, failure.
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Chahnimeh reservoirs with 1.4 billion cubic metres storage capacity have a critical role in water supply for both drinking water and agricultural purposes for the whole Sistan region in eastern Iran. Sistan river used to be the only source for agricultural purposes, so that several gated diversion weirs were constructed on the river in the past 50 years. Because of climate change and upstream development causing flow fluctuations, the river alone is no longer a reliable source for irrigation purposes. So the idea of storing water in Chahnimeh reservoirs and optimised operation of reservoirs have become a necessity. In order to achieve this, development of structures to have efficient operational plan of the river and reservoirs system is underway.
Several projects have been built for more efficient use of the reservoirs, some projects still being designed. One of the latest is the project of “Development of Operational Infrastructures for Chahnimeh Reservoirs” designing a structure to regulate flow between Chahnimeh I and III reservoirs. This kind of structure operating between two connecting reservoirs is so rare, so that innovation is needed to design a cost effective structure covering different operational conditions. Different structures were investigated and the summary of selection of structure types are presented. The paper illustrates challenging design of the project, useful for engineers who might be or will be dealing with such a project. By designing gates with pre-compressed rubber sealing, huge amount of costs associated with having two different gates for different directions of flow are avoided. Because of saturated foundation, by designing a diversion system between two reservoirs, it is possible to undertake pre-consolidation of foundation soil and to drain saturated foundation water. This would reduce settlement of the foundation of the structure after construction to the extent that by construction of a pile group, the gated structure will perform with high reliability for gates function. This type of structure is so rare and the methods and experiences of the presented design can be used by other engineers and consultants in similar projects. The estimated cost of the project is 15 million dollars and with construction under way, completion is expected in 2017.
Keywords: regulating structure, gates, reservoirs, reservoir operation
R. Nathan, P. Jordan, M. Scorah, S. Lang, G. Kuczera, M. Schaefer, E. Weinmann
This paper describes the development and application of two largely independent methods to estimate the annual exceedance probability (AEP) of Probable Maximum Precipitation (PMP). One method is based on the Stochastic Storm Transposition (SST) approach, which combines the “arrival” and “transposition” probabilities of an extreme storm using the total probability theorem. The second method – termed “Stochastic Storm Regression”(SSR) – combines frequency curves of point rainfalls with regression estimates of areal rainfalls; the regression relationship is derived using local and transposed storms, and the final exceedance probabilities are derived using the total probability theorem. The methods are used to derive at-site estimates for two large catchments (with areas of 3550 km2 and 15280 km2) located in inland southern Australia. In addition, the SST approach is used to derive regional estimates for standardised catchments within the Inland GSAM region. Careful attention is given to the uncertainty and sensitivity of the estimates to underlying assumptions, and the results are compared to existing AR&R recommendations.
Keywords: Annual exceedance probability, Probable Maximum Precipitation.
Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Makeena Kiugu, Siraj Perera
Dam owners are influenced by drivers such as ensuring economic efficiency, achieving industry good practice, and meeting regulatory or due diligence obligations when making decisions on how to manage their dams. While these drivers can be inter-related, the decisions finally made by dam owners are reflected in planned and completed dam safety activities.
In Victoria, dam owners update the regulator on the status of their dam safety management programs every year. Victorian dam safety regulation is underpinned by risk management principles. Benchmarking of dam safety management practices is also promoted within the industry. The information provided to the regulator includes risk levels of dams, scheduled upgrades and associated cost estimates, interim risk reduction measures, and details of surveillance, emergency management and operation and maintenance programs. A considerable amount of information has been collected over the past few years allowing trends in dam safety management activities to be examined at a State-wide level.
This paper will consider how dam safety management decisions, and the drivers behind those decisions, are reflected in the dam safety practices of Victorian dam owners. Trends in dam safety activities will be observed and linkages made to prevailing industry-wide challenges.
Dam owners are increasingly being required to address a wider range of issues in an environment of limited resources. Ensuring due diligence and improving emergency preparedness are some current challenges facing dam owners. This paper also examines how these emerging drivers may influence dam safety activities into the future.
Keywords: Dam safety management
Simon Lang, Chriselyn Meneses, Kelly Maslin, Mark Arnold
It is now common practice for dam owners in Australia to take a risk based approach to managing the safety of their large dams. Some dam owners are also using risk based approaches to manage other significant assets. For example, Melbourne Water manage the safety of their retarding basins in a manner similar to their water supply dams.
Assessing the risks posed by retarding basins using methods developed for larger dams can raise challenges. For example, the Graham (1999) approach to estimating potential loss of life (PLL) is generally applied when estimating the consequences of dam failure. However, Graham (1999) may not be the most suitable model for estimating PLL downstream of structures with relatively low heights and storage volumes (e.g. retarding basins), given the characteristics of the case histories used to develop the method.
In this paper six potential methods for estimating PLL are tested on four retarding basins in Melbourne. The methods are Graham (1999), the new Reclamation Consequence Estimating Methodology (RCEM), the UK risk assessment for reservoir safety (RARS) method, a spreadsheet application of HEC-FIA 3.0, and empirical methods developed by Jonkman (2007) and Jonkman et al. (2009). Results from the methods are compared, and comment is made about which is most suitable.
Keywords: potential loss of life, dam safety, risk analysis, retarding basins.