Papers
Papers
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2019 Papers
2019 – Rationale Behind the US Army Corps of Engineers Tolerable Risk Guidelines
Learn moreNathan J. Snorteland, M.S., P.E.
Dams and levees within the U.S. Army Corps of Engineers (USACE) inventory were constructed for a variety of purposes including flood control, navigation, hydropower, recreation, and fish and wildlife conservation. USACE transitioned to using life safety risk as a key input to all dam and levee safety decisions in 2006. This was implemented for many reasons, paramount among them is forming a consistent basis to evaluate the safety of dams and levees and prioritize the implementation of risk reduction measures in a consistent manner across the agency to best utilize available resources. This requires knowledge of what constitutes unacceptable risks that would require risk reduction actions. The Tolerable Risk Guidelines (TRG) were developed for this purpose, and to form a common basis for dam and levee safety evaluations and decisions. Protection of life is paramount, and there are four TRG related to (1) understanding the risks surrounding dams and levees, (2) building risk awareness, (3) fulfilling daily responsibilities, and (4) continually considering actions to reduce risks. The USACE policies have evolved over time, but the fundamental principles that underpin the TRG have been fairly consistent for the past 10 years. The evolution of the TRG have come as a result of the experiences using these principles to support more than 2,500 safety decisions. This paper describes the rationale behind the selection of the TRG.
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2019 Papers
2019 – Performance and Management of the Cromwell Gorge Landslides, Clyde Dam Reservoir
Learn moreDon Macfarlane and Peter Silvester
Prior to filling the Clyde Dam reservoir in 1992-93 large scale stabilisation works were undertaken on several pre-existing landslides along the reservoir margins. Monitoring and visual observations indicate that the landslides are behaving satisfactorily and have confirmed that the stability improvements undertaken have successfully offset the negative effects of the reservoir on the landslides.
This paper presents selected records detailing more than 25 years of landslide behaviour that demonstrate the effectiveness of the stabilisation works. Monitoring has been able to detect increasing water levels, drainage flow changes and, in some cases, deformation following periods of high rainfall.
However, the highly satisfactory performance of the landslides experienced to date does not allow complacency and although the surveillance monitoring has been progressively scaled back to a more focussed strategy, ongoing assessment and reviews will be required. The paper also briefly discusses the current challenges associated with changing personnel and aging instrumentation.
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2019 Papers
2019 – Increasing Airspace Management Flexibility at Hume Dam for Better Flood Operations and Environmental Outcomes
Learn moreAndrew Bishop, Tom Zouch
A new operating arrangement at Hume Dam is being developed to improve the transition from flood operations to the release of water set aside for delivering environmental flow demands. The arrangement also aims to help manage inherent downstream flood risks associated with this transition and with the requirement to fill the storage.
This paper describes particular flood risks and environmental impacts resulting from the current approach required to meet asset and water resource security priorities during airspace management operations at Hume Dam. It then considers how the new environmental demands have interacted with long-standing operating objectives and airspace management during high inflow periods in ways that have altered the dam operations required to meet operating priorities and manage flood risks.Critically, requests by environmental managers to start releases can arise sooner and with greater uncertainty compared with releases for meeting irrigation demand following a period of flood operations or airspace management. This difference has led to a more rapid storage filling curve to maximise water resource during periods when inflow rates remain relatively high and catchments are still responsive to rainfall.
The paper details how the new operating arrangement provides greater volumes and more flexible flood mitigation airspace using a discretionary volume of ‘held’ environmental water without otherwise impacting on the flood operations decision-making process. A number of challenges in defining the potential level of benefit and risk, and in understanding trade-offs were faced in negotiating the arrangement. However, the successful development of the approach and agreement to trial it were ultimately achieved by framing the issue as an opportunity to adjust dam operations in a way that seeks mutual benefits for dam operators and environmental managers.
Full adoption of the arrangement would result in greater airspace flexibility during flood operations to better manage risks without affecting water resource. Simultaneously, it provides environmental benefit due to changes in the pattern of releases during the transition period from flood operations to the commencement of environmental water releases as well as during the pre-spill release period.
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2019 Papers
2019 – A Resilient Dam for a Resilient Community in East Africa: Challenges in Designing Small Hydropower for a Wild River
Learn moreAndrew Noble, Robert Kingsland
The waters that feed the Nyamwamba River in western Uganda start as meltwater from the glaciers high up in the Rwenzori Mountains. A small scale run-of-river hydropower plant, equipped with a low height tyrolean type intake weir, is now operating just upstream of the town of Kilembe, the first large community along this river. History has seen floods cause realignments of the river through the town and major damage to property and loss of life.
A devastating flood occurred during the design phase for the scheme prior to any construction commencing, which caused loss of life and significant damage to roads, bridges and buildings within the town, including the hospital. Design changes to improve resilience of all riverine connections were made, including relocation of the diversion weir to a stronghold point within the basic protection zone of a natural island. A flood diversion dyke was constructed across one of the river branches that flows around the island, with its alignment, type and height optimised to capture low flows for energy generation while deflecting large flows away from the weir to mitigate flood damage.
Another major flood arrived three months after completion. No damage was sustained which provided confidence in the resilience of the headworks. A major river dredging program contributed to the overall resilience of this reach of river through the town.
This paper describes the challenges for the development of the project site in terms of physical considerations to work with the river, adopting some lessons learned from the pre-construction floods.
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2019 Papers
2019 – Waimea Community Dam: Design of a resilient CFRD in a Highly Seismic Environment
Learn moreD. A. Fletcher, J.O. Grimston, M.C.N. Taylor, D. J. Andrews, H. J. Bowen, P. Cazalis de Fondouce, E. Guilleminot, I. G. Walsh
The Waimea Community Dam will be the largest multipurpose concrete face rockfill dam (CFRD) to be constructed in New Zealand. This 53 m high CFRD will impound a reservoir of 13 Mm3 and is essential to securing the future water needs of the community and environment of the Waimea Plains and wider Tasman/Nelson region.
The design of this unique large dam in the New Zealand context was a long-term collaboration of local dam design expertise and international experience that took the ‘historic precedent based design approach’ for CFRD’s and supplemented this with modern embankment design techniques for the highly seismic environment at the dam site. Design of this High Potential Impact Category dam presented a range of technical challenges for the designers and wider project team, which required new and innovative design solutions and approaches.
The dam features a number of unique arrangements in the New Zealand context including:- A 4 m high parapet wall with horizontal movement joint connection to the concrete face and designed to accommodate displacements and embankment deformation due to seismic induced crest accelerations of up to 1.9g.
- A twin barrel diversion culvert through the dam designed for the high seismic loads and with geometry to accommodate construction diversion floods and following closure, installation of two pressure pipe conduits within the culvert that consider future maintenance and accessibility requirements.
- A mass concrete starter dam at the upstream toe to provide regular geometry for the plinth and concrete face above the diversion culvert.
The project had its origins in the early 2000’s. Detailed design commenced in 2010, and was externally peer reviewed. The detailed design stage was undertaken in an Early Contractor Involvement (ECI) process which was completed in February 2019.
This paper covers the important seismic design aspects for this large dam, including understanding and designing for the potential range of displacements and embankment deformations to inform the crest parapet wall and diversion culvert designs, and understand how differing rockfill properties might affect the dam performance. Quantifying the range of potential dam performance enabled a more resilient dam design.
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2019 Papers
2019 – The Lessons Learned and Construction Challenges for the Leslie Harrison Dam Upgrade
Learn moreRyan Cantrill, Peyman Andaroodi, Colin Thompson
Leslie Harrison Dam is located on Tingalpa Creek in the Redlands region, approximately 18 km southeast of Brisbane. It is classified as an extreme hazard category dam with a large population at risk only a short distance downstream.
The dam comprises a 25 m high zoned earthfill embankment, with a dry well concrete intake tower and an outlet conduit located at the base of the dam near the old river channel. The spillway has a 43 m wide concrete gravity ogee crest, with a concrete lined chute terminating in an energy dissipator structure.
Seqwater is undertaking a staged upgrade of Leslie Harrison Dam to address deficiencies identified during the Portfolio Risk Assessment (URS 2013) and Geotechnical Investigations (GHD 2016).
While the dam has met the water supply needs of the community for the past 50 years, the upgrade ensures local residents will be well served into the future. Additionally, the structure will meet the most up to date requirements of dam safety management and national industry standards.Construction of the Stage 1 upgrade commenced in June 2018 and involved the removal and replacement of liquefiable material in the foundation, modernisation and extension of the outlet works, addition of a new downstream filter buttress to the embankment, and lastly, the installation of both active and passive anchors within the spillway ogee and lower chute floor.
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As with any major project, the works involved a number of challenges that had to be addressed. This paper provides an insight into the key challenges encountered and how these were overcome by the design and construction teams using practical engineered solutions. The intent is to provide the reader with an account of the “lessons learned” during the construction phase, along with recommendations for future dam upgrades. -
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2019 Papers
2019 – Understanding Reservoir Sedimentation in South East Queensland
Learn moreDeb Gale, Michel Raymond, Nathaniel Deering, Simon Albert, Alistair Grinham
Sedimentation of reservoirs is acknowledged as a global issue and likely impacts water storage capacity in Australia. This major challenge to our future water supply is a highly complex process with deposition leading to infilling of the reservoir of course sediments in headwaters following major inflows, progressively to finer fractions towards dam walls. Wave action and catchment inflows during drawdown conditions will further transport and redistribute sediments into the main body of the reservoir.
Managing reservoir sedimentation requires an understanding of the sediment types and deposition patterns across the reservoir. Once the location and type of sediment is known, strategies to mitigate the effects on the reservoir can be determined. Methods typically used for determining sedimentation of a reservoir are empirical or modeling techniques that rely on detailed data from inflow events, suspended solids loads and flow rates. In the absence of this data, more direct measurements to quantify the amount of sediment present can be used. Direct measurements are more robust than modelling approaches that utilise rating curves that can result in over estimations of the sediment present. This study combined several measurement techniques to produce high spatial coverage of the reservoir floor. Detailed validation of this approach was undertaken in one representative reservoir prior to adopting this approach across multiple reservoirs.
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2019 Papers
2019 – Estimating Design Floods with a Specified Annual Exceedance Probability Using Bayesian Analysis
Learn moreC. Haden Smith and Brian E. Skahill
Design floods for most dams and levees typically have an annual exceedance probability (AEP) of 1:100 (1E-2) or less frequent. In the U.S., high hazard dams are designed to pass the Probable Maximum Flood (PMF), which typically has an AEP of 1:10,000 (1E-4) or less frequent. In order to reduce epistemic uncertainties in the estimated AEP for extreme floods, such as the PMF, it is important to incorporate as much hydrologic information into the frequency analysis as reasonably possible. This paper presents a Bayesian analysis framework, originally profiled by Viglione et al. (2013), for combining at-site flood data with temporal information on historic and paleofloods, spatial information on precipitation-frequency, and causal information on the flood processes. This framework is used to evaluate the flood hazard for Lookout Point Dam, which is a high priority dam located in the Willamette River Basin, upstream of Portland, Oregon. Flood frequency results are compared with those from the Expected Moments Algorithm (EMA). Both analysis methods produce similar results for typical censored data, such as historical floods; however, unlike the Bayesian analysis framework, EMA is not capable of incorporating the causal rainfall-runoff information in a formal, probabilistic manner. Consequently, the Bayesian method considered herein provides higher confidence in the fitted flood frequency curves and resulting reservoir stage-frequency curves to be used in dam and levee safety risk assessments.
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2019 Papers
2019 – Estimating Extreme Rainfall Probabilities for Large Catchments in Northern Australia
Learn moreS. Lang, R. Nathan, D. O’Shea1, M. Scorah, J. Zhang, G. Kuczera, M. Schaefer
If a risk-based approach is used to assess the spillway adequacy for large dams, then an estimate of the annual exceedance probability (AEP) of extreme rainfalls is required up to and beyond the Probable Maximum Precipitation (PMP). This paper describes how two site-specific approaches described by Nathan et al. (2015; 2016) were used to estimate the AEP of extreme rainfalls for seven catchments, ranging from 1300 km2 to 114,000 km2, in the northern-coastal region of Australia. The results are then compared with the regionally-based Laurenson and Kuczera (1999) relationship for estimating the AEP of the PMP, which is recommended by the Australian Rainfall and Runoff 2019 guide to flood estimation (Nathan and Weinmann, 2019). This shows that the site-specific assessments have produced a rarer estimate of the AEP of the PMP compared with Laurenson and Kuczera (1999), particularly for the catchments >10,000 km2. For some of these locations, this has allowed the dam owners to plan risk-based upgrades with more confidence.
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2019 Papers
2019 – Using Risk Communication to Address the Dam Safety Knowledge Gap in Communities
Learn moreSophie Walker, Amisha Mehta, Clinton Weeks, Aimee Tutticci, Ellen Tyquin
International emergency agencies such as the Federal Emergency Management Authority (FEMA) in the U.S. highlight a lack of public awareness of hazards relating to dams (FEMA, 2012). This is an issue faced by emergency management agencies around the world, including in Australia and New Zealand. Without hazard awareness, communities who live downstream of large dams are potentially more vulnerable to possible risks, and are likely to be less resilient when hazards arise. One way to address this knowledge gap is risk communication or the meaningful and purposeful exchange of information about risk among relevant parties (Covello, von Winterfeldt, & Slovic, 1984).
This study adopted a mental models approach (see Lazrus et al., 2016) to identify community members’ knowledge of dam failure by comparing their views with those of experts. Data were collected via depth interviews with dam safety experts (n=5) from across Australia, and community members (n=26) living downstream of dams in South East Queensland in Australia. Participants were asked to discuss knowledge about dam failure and to evaluate a dam safety message taken from a U.S. dam authority that was verbally read to them. Interviews were transcribed and analysed to identify the gaps between expert and community member knowledge.
Analysis showed some convergence on general dam operations but, less comprehensive community understanding of the causes of dam failure and dam safety management. Response to the U.S. dam safety message was mixed, with some participants believing it delivered the message appropriately, and others feeling it overstated risk or that its intended use was primarily to protect dam operators. Notably, these varied responses were often related to participants’ level of knowledge of dams. Combined, the findings highlight an opportunity to close the gap in knowledge. These findings will inform the strategies and materials for the South East Queensland bulk water authority Seqwater in engaging with communities downstream of their 26 dams. The research will guide the approach in conveying knowledge with an appropriate tone to support ongoing community engagement activities and increase resilience.
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2019 Papers
2019 – Leveraging Open-Access Remote Sensing Imagery to Monitor Dam Infrastructure: Case Study of the Cadia Tailings Dam Collapse, Australia
Learn moreSean Minhui Tashi Chua, Thomas Fuhrmann, Matthew Garthwaite
Satellite remote sensing data can be used to monitor environmental processes and inform disaster risk reduction and hazard early warning. This paper describes the analysis of satellite remote sensing images to investigate the partial wall collapse of a tailings dam at the Cadia gold-copper mine in Australia that occurred on 9th March 2018. Our case study uses freely available remote sensing imagery acquired by the Copernicus Sentinel-1 (radar) and Sentinel-2 (multispectral) satellite constellations to monitor land surface changes in the Cadia mine area before and after the collapse. In this paper we discuss the benefits of utilising both radar and multispectral remote sensing imagery in a holistic approach to remote sensing, which could be used for continuous, near-real time monitoring of risk-related infrastructure such as dams without the need for in-situ measurement equipment.
We applied the Interferometric Synthetic Aperture Radar (InSAR) technique to measure surface displacements and interferometric coherence maps from a stack of Sentinel-1 radar images acquired between 2nd December 2015 and 25th June 2018 at regular 12 day intervals. The time series of surface displacements show a significant increase in the rate of movement of the dam wall in the area that eventually breached in the two months prior to the collapse. This change in movement behaviour was not observed at parts of the dam wall that remained intact. This analysis demonstrates the potential for InSAR monitoring to identify issues in advance of infrastructure failure, which could allow risk mitigation strategies to be implemented by the mine operator. We used interferometric coherence data to observe changes in the dam wall and surrounding areas before and after the collapse. A drop in coherence occurred in the breached section of dam wall, indicating the surface change caused by the collapse. Coherence for unaffected parts of the dam wall remained stable. Sentinel-2 multispectral imagery acquired between 2nd July 2017 and 24th June 2018 show the timing, extent and effects of the collapse as well as the rate of tailings movement.
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2019 Papers
2019 – The Role of InSAR for Tailings Dam Safety Assessments
Learn moreJessica Morgan, Davide Colombo, Giacomo Falorni
The importance of building and maintaining safe, resilient tailings dams has become increasingly apparent with the rise in catastrophic failures in recent years. According to the World Mine Tailings Failures (WMTF) data base, 11 major failures have occurred over the past decade, often with devastating impacts to nearby communities in terms of loss of life and impact to the environment. With the occurrence of these types of events only expected to increase in coming years, there has been a corresponding increase in global calls to action to develop monitoring systems to better predict and wherever possible, prevent these failures from occurring.
With up to an estimated 20,000 tailings dams around the world, the development and implementation of a worldwide monitoring protocol is a daunting task, particularly as many of these structures are remote and difficult to access. This is where a technology like InSAR can make an immediate impact. InSAR is a remote sensing technique that uses radar satellite imagery to measure ground movement with up to millimetric precision. Radar systems are active, meaning they collect information from reflections of the radar signal off the ground and therefore do not require the installation of any equipment. As satellite images cover areas that extend thousands of square kilometres, they can provide information not only on the stability of dams, but also entire regions. Global archives already exist due to the Sentinel constellation of satellites, which provide coverage since 2014 over most parts of the world.
In an ideal world, tailings dams are safe and constructed to provide permanent containment of mining by- products. However, experience has shown that they can fail, often with dire consequences, especially if these failures occur without warning. The development of an internationally accepted standard for tailings dam monitoring is imperative to ensure the safety and resiliency of these structures is continuously tracked. This paper explores the role InSAR can play in the development of a global protocol for tailings dam monitoring.
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