David S Bowles, Sanjay S Chauhan, Loren R Anderson, Terry F Glover
Abstract: A nested model is presented for considering variability and knowledge uncertainty in a dam safety risk assessment of an existing dam and interim risk-reduction alternatives (operating restrictions) during the staged implementation of a permanent structural risk reduction measure. The effects of some important aspects of natural variabilities on estimated risks are represented as cumulative distributions of probability of failure, annualised life loss, economic risk cost, and an F-N representation of life loss. Many cumulative distributions are generated to represent the effects of some important aspects of knowledge uncertainties.
An important aspect of the knowledge uncertainty is the current level of development of an already-initiated piping failure mode. Also, an approach to conditioning the system response probabilities (SRPs) for the piping failure mode on the duration of reservoir pool exceedance is included in the failure event tree risk model.
ANCOLD and Reclamation tolerable risk guidelines are evaluated at selected percentiles of variability and percentiles (levels of confidence) of knowledge uncertainty. The incremental cost-per-statistical-life saved and benefit-cost ratio for interim risk-reduction alternatives are estimated and evaluated to examine the case for more-severe levels of operating restriction than the least-severe operating restriction that is estimated to satisfy, at a selected percentile of variability and a desired level of confidence, the limit values in all of the tolerable risk guidelines that were considered.
Keywords: Dam safety, risk analysis, risk assessment, uncertainty analysis, aleatory uncertainty, epistemic uncertainty.
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Abstract: Queensland’s regions need additional water supply infrastructure to underpin the future economic growth of the state, and secure the water supplies of existing communities in light of climate variability.
Queensland’s economy has consistently been the fastest growing of all the States for the last decade. The major component of Queensland’s growth is underpinned by resources and industry, in regional areas of the State. If the enabling infrastructure is absent, or if timeframes for development do not align with the needs of the industry, this growth will be constrained, or will go elsewhere. Future investments that support growth cannot and will not occur without certainty around the availability of future water supply.
Queensland’s regions are emerging from the worst drought in recorded history, this has revealed serious deficiencies in the resilience of our regional water supplies. For the first time in living memory, major regional centres have been forced to face the prospect of running out of water.
The Queensland Government has been proactive in identifying these issues and has responded resolutely through the release of Statewide Water Policy which supports a $451 million investment in regional water infrastructure, which aims to meet the needs of urban and rural users.
As part of this policy SunWater has been named proponent for a range of projects considered essential for Queensland’s future economic sustainability which include the Nathan Dam on the Dawson River, Water for Bowen & Proserpine Pipeline Projects, Rookwood Weir & Eden Bann Weir Raising (with RRC & GAWB), Connors River Dam & Pipelines, Nullinga Dam and Kinchant Dam Raising. SunWater is also investigating the raising of Queensland’s largest dam, Burdekin Dam. This paper will outline the need for water supply infrastructure within regional Queensland, and the challenges faced by the projects being progressed by SunWater including the technical issues, environmental sustainability and approvals, and commercial development challenges.
Keywords: Queensland, Water Supply Infrastructure, Dams
Chris Topham, Paul Southcott, Tim Cubit
Abstract: Dee Dam is a 15 m high and 270 m long central core earth and rockfill dam on the upper reaches of Hydro Tasmania’s Tungatinah Power Scheme. The dam is assessed to have a High A hazard category. Hydro Tasmania’s portfolio risk assessment found that a risk based upgrade was warranted to protect against both piping and flood overtopping failure modes.
A $4.2M modernisation project was implemented in 2008 comprising the installation of a full height downstream filter with rockfill buttress, repairs to cracking in the diversion conduit and raising of the core for improved flood capacity. Lowering of the Dee Lagoon to facilitate full height excavation of the downstream shoulder of the dam was impractical to Hydro Tasmania for production, environment, cost and stakeholder reasons. Hence, careful analysis and risk management was required to ensure the safety of the dam during the construction of the above works against a full storage.
This paper presents the risk objective for the upgrade work, modelling and analysis undertaken to assess dam safety during the works. A wide range of construction risk mitigation measures were employed prior to and during the upgrade works. The response to latent conditions and potential dam safety incidents that occurred during construction are described. The continuous adaptation of the construction methodology to suit site conditions encountered during the project is also presented. The project was successfully completed in June 2008. The approach of detailed investigations and design coupled with a strong risk based approach during the construction proved to be effective in managing the dam safety risks of construction work with a full reservoir.
Keywords: earth and rockfill dam, filters, construction risk mitigation, dam safety, dam safety incidents.
Rob Ayre, Terry Malone
Abstract: Fairbairn Dam with a storage capacity of 1,301,100 ML is the second largest dam in Queensland in terms of water supply capacity. The dam forms the head works of the Nogoa – Mackenzie Water Supply Scheme operated by SunWater in Central Queensland. Completed in 1972, it consists of a zoned earth-fill embankment 49 m high and 823 m in length. The dam has an un-gated ogee spillway crest that is 4.2 m high and 165 m long, with an original design capacity of 15,600 m3/s.
In January 2008, Central Queensland experienced significant flood producing rains which were generated from low pressure systems associated with monsoonal activity across northern Australia. Rainfall totals over the 16,000 km2 catchment area of Fairbairn Dam varied in depth from around 200 mm to nearly 700 mm during a five day period to 20 January 2008. This resulted in the largest outflow from the dam since its construction and the first spill event from the dam since April 1990. While the dam had a significant mitigating impact, there was still major flooding of the township of Emerald, some 19 kilometres downstream.
This paper describes the performance of the dam during the event. Details of the data collected during and after the event, including assessments of spillway performance, dam safety surveillance and the implementation of the Emergency Action Plan will be presented. In particular, the paper focuses on the flood response concerning downstream communities and the resultant flood effects on Emerald and major infrastructure located in the downstream flood plain. It highlights the need for dam owners to have the capability of forecasting inflows and outflows to their structures and how this information contributes to the overall flood response system.
Keywords: dam safety, spillway, flooding, Fairbairn Dam, Emerald, SunWater, Queensland.
Mark Hore, Joseph Matthews
Abstract: Substantial flooding occurred in the Gippsland region in late June 2007, following the severe bushfires experienced during the previous summer. Major damage was sustained to the regions infrastructure, which included the gated storages at Glenmaggie Dam and Cowwarr Weir, located in the neighbouring Macalister and Thomson river catchments respectively. The flood event at Glenmaggie Dam produced a record peak inflow of well over 250,000 ML/day, which was more than twice the previous record.
The magnitude and intensity of the flood event created a number of issues for the staff at Southern Rural Water (SRW), who are engaged in the safe operation of the facilities. The event tested the organisations emergency management systems and the ability of the organisation to effectively manage events at multiple sites. Some of challenges faced included: a rapid rise in storage volumes; the loss of upstream warning gauges; the accumulation of large volumes of debris; the development of suitable release strategies and the communication and engagement with the local community.
The event caused a significant amount of damage to key infrastructure at both sites, with the Cowwarr Weir storage being the worst affected. A number of high priority projects have been completed since the event, with spending to date totalling $3 million. The unusual nature of the flood event provided the opportunity to review previous flood assessments and to identify deficiencies with elements of the existing infrastructure. Of particular interest was the subsequent hydrology review which provided a comparison with previous modelling assessments. The review included a flood frequency assessment which showed that the magnitude of the Glenmaggie flood was in the order of a 1:200 AEP(Annual Exceedance Probability) event, which was disproportionate to the rainfall event frequency assessed as having a 1:50 AEP, when averaged across the catchment. The aim of the paper is provide a case study for other dam engineers who may be preparing for future flood events in similarly affected catchments.
Keywords: flood, gated storage, dam performance, remedial works.
David M Schaaf, Jeffrey A Schaefer, Rick W Schultz, Jason T Needham
Abstract: As one of the main federal agencies with responsibility to build, operate, and maintain large dams in the United States, the US Army Corps of Engineers (USACE) is developing a risk based framework to better manage their portfolio of 600+ dams in terms of risk management and prioritization of funding. A key element to this effort is the development of risk-based analytical tools to evaluate primary features for applicable failure modes. These are used in conjunction with loading and consequence modules to assess the overall risk associated with the dam in terms of lives and economic damages. The focus of this paper is on the engineering analysis modules used to generate fragility curves for dam features.
The analysis modules are broken into three main categories by engineering discipline: geotechnical, structural, and mechanical/electrical. The risk based assessment tools associated with geotechnical failure modes include Seepage & Piping, Embankment Stability, Seismic Performance, and Erosion of Unlined Spillways. The structural assessment tools include Concrete Monolith Stability, Spillway Gates, Scour of Concrete Lined Spillways, Spillway Training Wall Stability, Performance of Pipes through Dams, Hydropower Superstructures and Intake Towers. The mechanical and electrical are primarily focused on the performance of machinery used to operate dam gates.
This paper gives a broad overview of the main characteristics and methods used for each of these analysis tools. Some of the modules use historical performance to establish failure rates, while others are more analytically based. The context of each within the framework of the overall risk assessment effort of USACE dams is covered.
Keywords: risk based analytical tools, fragility curves, risk assessment, US. Army Corps of Engineers, portfolio, dams.