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Barton Maher, Michel Raymond, Mike Philips
The Queensland Bulk Water Supply Authority (trading as Seqwater) owns and operates North Pine Dam, situated on the North Pine River in the Northern Suburbs of Brisbane. North Pine Dam is an Extreme Hazard Dam consisting of a concrete gravity dam with earthfill embankments at both abutments and three earthfill saddle dams. The spillway consists of five radial gates which are manually operated. Flood operations at the dam are controlled in real time by the Seqwater Flood Operations Centre.Read More
In January 2011, North Pine Dam experienced the flood of record at the dam site with a peak inflow of approximately 3,500 m3/s and a corresponding outflow of approximately 2,850 m3/s. This inflow was more than double the previously recorded flood of record. The inflow was generated by high intensity rainfall both at the dam and in the upper catchment resulting in a rapid rise of the storage. The system which caused this rainfall was also contributing to the major flooding occurring in the adjacent Wivenhoe – Somerset catchment, also being managed by the Seqwater Flood Operations Centre. The rapid rise and fall of the storage presented difficulties for both the Seqwater Flood Operations Centre and the operators at the dam site.
Following the flood event, an analysis of the rainfall and the resulting inflows indicated a significant difference between the Annual Exceedance Probability (AEP) of the rainfall in the catchment and the estimated AEP of the inflow and peak water levels from previous hydrology studies. A detailed review of the flood event was commissioned by Seqwater and undertaken by URS Australia Pty Ltd.
This paper presents details of the flood event, lessons learned for the operation of the dam, upgrade works undertaken to date, results of the hydrology review and the conclusions of the Acceptable Flood Capacity (AFC) study. A key implication for dam owners was the increase in the estimate of the Probable Maximum Flood (PMF) by over 30% due to changes in calibration of the hydrologic model for the catchment.
Keywords: Probable Maximum Flood, Flood Operations, North Pine Dam, Flood Estimation
Richard R. Davidson, Nate Snorteland , Doug Boyer, John France
The US Army Corps of Engineers (USACE) has embarked upon a monumental journey in applying risk-informed decision making in the management of the safety of the 650 major dams for which it is responsible. This process has shifted safety criteria from fully deterministic to a probabilistic basis. There has also been a shift from de-centralized district-based decision-making to centralized management of resources through the new Risk Management Center (RMC) and the Senior Oversight Group (SOG), a group of senior engineers and managers from across the USACE organization. The risk process began about five years ago with a portfolio prioritisation using screening-level risk assessments of the entire dam inventory, culminating in Dam Safety Action Classifications (DSAC) for each of the dams. Based on this risk prioritisation, Issue Evaluation Studies (IES) were initiated for the highest risk DSAC I and II dams, with each study including detailed failure mode and risk analyses for each dam. Because the Corps was relatively new to dam safety risk analyses, and their dam design history was one of following codified manuals of practice, various risk tools were prepared to provide guidance when assessing the risk of potential static, seismic and flood failure modes, as well as life loss and economic consequences of dam failure. Although these tools provided useful guidance to a relative large population of inexperienced risk estimators, many of these early risk assessments were flawed; they provided unrealistically high estimates of failure probabilities and the tools did not help estimators understand or explain each failure mode. To assist the RMC in bringing more defensible risk estimates to the table and improve consistency of the evaluations, the Quality Control and Consistency (QCC) review process was initiated about two years ago. The QCC process provides high level review of IES activities, including detailed reviews of risk analyses, by a small group of experienced dam safety risk estimators. Not only has this brought risk estimates into a more reasonable range, it has provided valuable training for risk estimators, and important checks and balances on the risk-informed decision making process for moving dam safety upgrade projects forward. The justification for a number of very expensive projects has been challenged and, in some cases, re-prioritised, and other projects have risen to the prominence they deserve.Read More
Dr. Mark Locke, Jiri Herza
Gördes Dam is a nickel and cobalt mine tailings dam situated in a seismically active zone in Manisa Province, Western Turkey. The dam is a conventional cross valley earthfill structure with a fully lined storage basin. The starter embankment with a maximum height of 50 m will be raised in downstream lifts to an ultimate height of 90 m. The total storage capacity is 19 million m3. Construction of the starter embankment is planned to commence in late 2012 and the dam will be commissioned in June 2013.
The tailings will be discharged from the dam crest and return water will be collected by a floating decant pump at the opposite site of the storage. Decant water has high calcium sulphate levels and will require treatment before re-use in the plant or release. The tailings contain about 33 % of solids and are classified as high plasticity silts and clays with more than 90 % of particles passing the 0.075 mm sieve.
The dam is founded on a complex formation of altered sedimentary and metamorphic rocks including mudstones, siltstones, limestones and serpentines. The mudstone blocks, the predominant foundation materials, are juxtaposed with siltstones and serpentines via a complex arrangement of faults. Where exposed, the mudstones are highly to completely weathered with a well-developed structure of smooth bedding surfaces leading to anisotropic strength characteristics. Several landslides, likely associated with the anisotropic character of the mudstones, were identified within the area including a significant landslide under the upstream shoulder of the dam.
Mining development in Turkey has a complex legislative environment. There is also standard practice which is not legislated but expected, this can be considerably different to normal design practice in Australia. The Turkish legislation is based on waste management guidelines and may be more appropriate to landfills than large tailings storages. The legislation is very prescriptive in some aspects and silent in others, with little consideration of risk or consequence based design.
This paper discusses the design difficulties associated with the challenging foundation conditions, which have been magnified by the requirements and limitations embedded in the approval documentation and the legislative environment in Turkey. It will also address some of the key differences between the design philosophy in Australia and in Turkey with a focus on the major risk elements of the design.
Keywords: Tailings, Turkey, Liner, HDPE, Nickel laterite
A concrete-rockfill composite dam consists of two zones: a slender concrete gravity section and a rockfill embankment section. Each zone behaves according to its stiffness and geometry during earthquake shaking. At the abrupt interface a structure behavioural discrepancy results. To mitigate such this discrepancy, a transition interface is introduced by gradually tapering the concrete section down and burying into the central part of rockfill embankment. However the behaviour of the interface is complex due to the two intermeshing of the different materials. Previously, the interface was not designed with any serious theoretical approach, but with the intuitive belief that the transition structure can play the role of mitigating behavioural difference between concrete and rockfill sections. This study seeks to characterize the dynamic behaviour of each section and to understand the performance of the interface using centrifuge model test and numerical analyses. The centrifuge model, which was reproduced by scaling down D dam in Korea, were loaded with adjusted seismic forces based upon seismic coefficient of 0.098g and 0.154g required in the dam design criteria. The legitimacy of the model test was verified by the comparison of the test results with those of numerical analyses, and the most appropriate input values for the interface elements were proposed through a systematic parametric analysis. The key findings of the paper are as follows: Numerical parameters study of the interface-element was carried out, the friction angle depends on rockfill zone material and normal and shear stiffness coefficient of the two materials (concrete and rockfill), the average values were found to be the most appropriate. The findings of this study can be used to design new composite dams, rehabilitate current dams, or design additional spillways to current rockfill dams.
Keywords: Composite dam, Centrifuge, Interface-elementRead More
Sofia Vargas, Robert Wark
Logue Brook Dam, 130 km south east of Perth, was completed in 1963 and comprises a 49 m high main embankment with a crest length of approximately 335 m and the reservoir impounds 24.59 GL of storage. The outlet works comprise an inlet tower, an outlet pipe (DN 1100 mm) and a valve house. Water from the dam is released through a clam shell valve and there is a sluice valve upstream of the clam shell which acts as a scour isolation valve.Read More
Previously Logue Brook Dam supplied water into the Harvey irrigation system by releasing water down the river which was then drawn off downstream and pumped into the piped network. The scheme planning had identified that constructing a pipeline from the dam outlet to connect directly into the piped irrigation system would eliminate the need for pumping as the system could then be gravity fed directly from the dam.
The outlet works upgrade comprised the refurbishment of the Inlet Tower, refurbishment of the Valve House, installation of new valves, environmental release and magnetic flow meters, electrical, communications, SCADA, instrumentation and security upgrades.
This paper describes the diving inspection and above water inspections of the inlet tower, refurbishment of the existing installation, challenges of the design, adopted solutions, connection to the Harvey Water pipeline and construction issues. The project represents an interesting case history of improving dam safety standards to current ANCOLD guidelines to provide a modern and safe facility.
Keywords: Outlet works, diving, OH &S Issues, safety, deterioration
Krey Price, David Moore, John Palensky
Cold water pollution (CWP) occurs when dam releases draw from lower-temperature regions of a reservoir, potentially impacting fish survivability in downstream waterways. Declining fish populations along the Missouri River have prompted recent investigations into solutions for CWP prevention.Read More
Fort Peck Dam is an 80-metre high, 7-kilometre long dam located along the Missouri River; completed in 1940, it is one of the world’s oldest and largest hydraulically filled earthen dams and is listed on the U.S. National Historic Register. Inflow temperatures to Fort Peck Reservoir are significantly warmer than the outflow temperatures through the dam during the months of March through August. A water temperature of 18°C has been identified as critical for spawning and recruitment of locally threatened fish species; however, downstream temperatures typically remain below 14°C during critical time periods. This difference is due to the current deep-water withdrawal from Fort Peck Reservoir.
Ten alternatives were proposed to increase the temperature of the discharge, and an options analysis narrowed the results to a single, preferred alternative that consisted of a flexible, submerged weir around the intake. This paper documents the design efforts undertaken for temperature control measures at Fort Peck Dam, including a description of the modelling methods, design criteria, and effectiveness of the submerged weir alternative.
The use of a submerged weir to increase discharge temperatures relies on the process of passing warmer water from the upper portion of the water column over the weir crest into the intake area, rather than drawing from the bottom of the reservoir. For reservoirs with fluctuating levels, such as those at Fort Peck Dam, a flexible curtain can be suspended a set distance from the water surface using a float system, with the bottom of the curtain anchored to the lake bottom with ballast and cables. The crest elevation is set relative to the thermocline; as the lake level fluctuates, the flexible curtain folds and unfolds in response.
The impacts of CWP are increasingly recognised as an environmental risk worldwide. This paper draws upon the results of similar, implemented projects around the world, including a comparison to cold water pollution prevention measures and costs that have been assessed by CSIRO for application to Australian reservoirs.
Keywords: Cold water pollution, reservoir stratification, thermocline, curtain, fish health
Mike Phillips, Kelly Maslin
A spillway upgrade conceptual design and selection process was undertaken to identify options for upgrading the Dartmouth Dam to pass the Probable Maximum Flood (PMF). A number of upgrade options were investigated, including variations of dam raise heights and spillway modifications. One of the options, the piano key weir, was initially developed from the limited available publications on the weir design, and further developed with the use of a 1:60 scale model. The piano key weir, a variation of the labyrinth weir, is a passive spillway that utilises a total weir length several times that of the effective spillway width. For the Dartmouth Dam study, the piano key weir design that was developed consisted of a 7-cycle, 9 m high structure, with a total weir length of nearly 600 m, or more than 6 times the existing effective spillway width of 91 m. The spillway was designed to pass the routed PMF outflow of approximately11,500 m3/s with a head of approximately 11 m.
The piano key weir design was developed using the following analyses:
Initial 1:60 scale physical model of the piano key weir based on published papers on piano key weirs and design manuals for labyrinth weirs;
Structural analysis and weir member sizing using initial physical model results;
Computational Fluid Dynamics (CFD) modelling to improve the hydraulic efficiency of the weir for the range of flows;
Revised 1:60 scale physical model of the piano key weir; and
Confirmation of conceptual structure design.
This paper describes the process of developing the piano key weir option for the Dartmouth Dam spillway and lessons learned.
Keywords: Piano key weir, CFD, spillway, physical model
Eric Lesleighter, Peyman Andaroodi, Colleen Stratford
In January 2011 major flooding was experienced across a large part of Southern Queensland. The flood discharges through the Wivenhoe Dam spillway caused extensive erosion of the rock in the plunge pool. While not an issue in relation to the spillway structure’s security, the rock erosion experience was dramatic for a number of reasons. The paper presents details of the extent of erosion under head conditions that can be classed as moderate only when compared with many taller dams. The discharges over several days resulted in a pile of huge rock blocks downstream of the plunge pool.Read More
The paper describes the plunge pool design dimensions, the geology, the hydrology of the releases, the hydraulics of the plunge pool, the surveys of the pool and rock mound, and moves on to discuss the mechanism of the fracturing and transport of the rock. Similar relevant experiences will be cross referenced, especially from details of recent experiences at the Kariba Dam and the study of remedies in the context of the dam’s actual safety.
From an actual major experience of erosion, and the sheer volume of rock that was lifted up and out of the plunge pool, the occurrence stands as a timely demonstration of what can happen in similar spillway situations, and suggests the type of awareness that spillway design needs to accommodate for energy dissipation facilities in unlined spillways plunge pool.
Keywords: Spillways, plunge pools, rock erosion, scour, plunging jets, pressure transients.
Gavan Hunter, Robin Fell, Chris Topham
Backward erosion piping is a failure mode that can affect water retaining structures with earthen cores of very low or no plasticity. Backward erosion involves the progressive detachment of soil particles as seepage through a core material exits to a free surface or unfiltered zone. In contrast to other piping failure mechanisms, backward erosion does not require a defect to be present for initiation, and is heavily influenced by the inherent characteristics of the core materials and the available hydraulic head. For dams with non-plastic or very low plasticity core materials, backward erosion can be a material contributor to the overall piping risk and warrants careful consideration during quantitative risk assessments of such dams. However, there is very little published literature for evaluating the potential for backward erosion piping, particularly in broadly graded soils. This paper concerns one such dam where backward erosion of the glacial till core needed to be assessed in the context of a detailed risk assessment for the facility. The backward erosion mechanism was tested in laboratory tests set up to model the situation in the core of the dam at a range of hydraulic heads. The paper describes the core material and objectives for the testing, presents the apparatus used, summarises the findings, and explains how they contributed to the risk assessment for the dam. Recommendations are also made for future similar testing and research needs.
Keywords: Backward erosion, piping, embankment dam, laboratory testing, quantitative risk assessment, glacial till.
Kelly Maslin, Mark Foster, Len McDonald
A key requirement of assessing the tolerability of dam safety risks is the assessment of individual risk. The ANCOLD Guidelines on Risk Assessment provides guidance on acceptable levels of individual risk and some general guidance on the calculation of individual risk.
Individual risk is a key measure in the consideration of the tolerability of risk, ALARP and development of risk mitigation works. It is essential that there is consistency in the approach to estimating individual risk used across the dams industry.
This paper reviews the approaches taken to estimating individual risk across the dams industry both locally and internationally as well as the experience of other industries.
The paper includes a review of the various methods for estimating the vulnerability of individuals subjected to flood inundation based on historical fatality rates as well as identification of the individual most at risk
The paper then describes a method that has been developed based on the principles used for assessing individual risk due to other hazards, such as landslides. The method includes consideration of a range of factors such as warning time, temporal variation and vulnerability of the individuals most at risk. The method developed provides a transparent, defensible and pragmatic approach to estimating individual risk. Practical guidance and examples are also provided on the application of the method.
Keywords: individual, risk, exposure, fatality
M. Tooley, D. D’Angelo, B. Priggen, K. Sih, N. Vitharana, R. Mouveri
As the urban sprawl of residential and commercial businesses expand to meet rising population, consideration must be given to the frequency and intensity of storm events and changes in tidal levels, to mitigate the risk of flooding and damage associated with the failure of hydraulic structures.Read More
This paper outlines the design method undertaken to ensure the ageing structure (founded on timber piles) meets modern dam safety criteria, extends the life of the 8 gates operating mechanisms and provides overall inherent reliability for the whole structure. The design method included updated hydrological assessment of the upstream catchment, geotechnical investigation, liquefaction review, consequence category and AFC assessment, hydraulic assessment and stability analysis.
These assessments are being undertaken to introduce inherent reliability in their operation in particular during king tide or storm water events, or a combination of the both, minimising leakage and breakdowns and ensuring the risks of flooding to low lying residential areas upstream of the structure and major airport are minimised. The Glenelg Gates structure is an integral part of a larger regulating system for the catchment.
The findings of the design upgrade would be useful to dam designers and owners faced with the upgrading of gated structures with flooding risks in residential areas.
Keywords: Gated Glenelg Gates structures, upgrade, dam design guidelines.
The design of tailings dams under earthquake loading is quite challenging due to the nature of the tailings materials which are generally liquefiable under earthquake shaking. The design will be more complicated when the dam foundation is also liquefiable material. While assessment of liquefaction potentials is well developed in practice, assessment of liquefaction induced deformation varies from the simplest Newmark’s displacement method to the more complex effective stress dynamic analysis approach. It is generally accepted that the simplified method can be used for cases involving non-liquefiable materials. However, the use of this method for cases involving liquefaction may generally result in overly conservative designs to cater for the many simplified
assumptions in the method. With the advance of computer technology, time and cost are no longer obstacles for using the more appropriate method for estimating liquefaction-induced deformations of a tailings dams and achieving an optimum dam design.
This paper attempts to critically discuss issues in seismic design of tailings dams and provide an example of the use of the effective stress dynamic analysis method to estimate the liquefaction-induced deformations of a tailings dam and its importance in optimizing the design. The approach used is capable of estimating pore pressure response of liquefiable materials at any given time during the shaking. The effective stress analysis method used herein is embedded in FLAC software using a specially written FISH routine. Using this method, it can be demonstrated that although liquefaction is an issue, it does not necessarily mean that we must prevent its occurrence. As long as the deformation is acceptable, liquefaction is not necessarily a ‘show stopper’ for the project.
Keywords: liquefaction, seismic deformation, tailings dam design.Read More