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Papers 2017
2017 – A Turkey’s Nest Dam on Top of a Waste Rock Dump – An Innovative Solution for the Kidston Pumped Storage Project
Learn moreRichard Herweynen, Jamie Campbell, Mohsen Moeini
Hydropower storage plays an expanding role in integrated power systems internationally and can enable increased use of intermittent renewable energy sources such as wind and solar.With an increased amount of renewable energy within the Australian grid, pumped storage has gained increased focus in the past 2years. Entura have been working with Genex Power Ltd. to investigate, evaluate, optimise and design the Kidston Pumped Storage Project, located at the old Kidston gold mine in Northern Queensland. Through this design process, the final arrangement developed included an upper reservoir turkey’s nest dam to be built on the existing waste rock dump on the northern side of the Eldridge Pit, using the existing waste rock dump material and lining it with an HDPE liner. The original waste rock dump was formed during the mining operation by progressively dumping the waste rock predominantly from the Eldridge Pit excavation, with the haul truck traffic being the only compaction that occurred. Since the closure of the mine about 20 years ago, some consolidation of the waste rock dump has occurred.As a result, the key risks identified for the construction of the turkey’s nest dam on top of the waste rock dump were: (1) the stability of the slopes of the waste rock dump, which were generally at the angle of repose for the rockfill material; (2) the absolute settlement of the waste rock dump as the final dam crest level requires a settlement allowance in excess of the flood freeboard requirements; and (3) the differential settlement as excess differential settlement could cause fatigue stress cracking within the liner.This paper presents the investigation and modelling undertaken to confirm the feasibility of constructing this turkey’s nest dam on top of the existing rock waste dump, utilising the historical data on dumped rockfill dams. The paper also presents the feasibility design developed for the upper storage.
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Papers 2017
2017 – A Unique Experience with Liquefaction Assessment of Impounded Brown Coal Ash
Learn moreRadin Espandar, Mark Locke and James Faithful
Brown coal ash has the potential to be a hazard to the environment and local communities if its storage is not well managed. The risk of releasing contained ash from an ash tailings dam due to earthquake induced liquefaction is a concern for mining lease holders, mining regulators and the community.Ash tailings dams are typically raised by excavating and compacting reclaimed ash to form new embankments over slurry deposited ash, relying on drying consolidation and minor cementation for stability. Understanding the post-earthquake behaviour of the brown coal ash is necessary to assess the overall stability of an ash tailings dam during and after seismic loading events.A particular concern is the seismic motion may break cementation bonds within the ash resulting in a large reduction in shear strength (i.e. sensitive soil behaviour) and potential instability. There is limited information available for black coal ash however, brown coal ash has different properties to black coal ash and no known work has been carried out to date in this area.The dynamic and post-earthquake behaviour, including liquefaction susceptibility, of the brown coal ash was studied, specifically for Hazelwood Ash Pond No. 4 Raise (HAP4A) in Latrobe Valley, Victoria. In this study, different well-known methods for liquefaction susceptibility, including the methods based on the index parameters, the cone penetration test (CPT) and the cyclic triaxial testing, were used and the results were compared.It was found that the impounded brown coal ash is susceptible to liquefaction and /or cyclic softening. Triggering of the liquefaction or softening was assessed based on the results of cyclic triaxial test.In this methodology, the relationship among axial strain(εa), Cyclic Stress Ratio (CSR) and number of uniform cycles (Nequ) was determined based on the triaxial test results. Then, asite-specific CSR was determined using the ground response analysis. The CSR and number of uniform cycles (Nequ) for each ash layer was calculated and added to the εa-CSR-Nequgraph to determine the expected axial strain during an MCE event. It was found that the calculated axial strain for the ash embankment and ash deposits during site specific Maximum Credible Earthquake (MCE) are less than the axial strain of the ash material required for triggering of liquefaction and the brown coal ash in HAP4A does not liquefy and/or soften the material during an MCE event. Also it was found that the insitu tests which break the cementation between particles(such as CPT)does not provide accurate results on triggering or sensitivity.
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Papers 2017
2017 – An Innovative Approach to Determine Particle Size Distribution for Rockfill Material
Learn moreZerui Lu, Behrooz Ghahreman-Nejad, Mahdi M. Disfani
Particle characterisation like size distribution and shape can greatly affect the mechanical behaviour of granular materials, and is closely related to the economics for engineering projects. For rockfill material in embankment dam construction, the particle size distribution (PSD) is fundamental to the design, quality control and numerical modelling. Traditionally, particle size distribution for engineering materials is obtained through physical sieving. However, with rockfill material, the size varies significantly and can range from gravels (+2mm) to cobbles (+60mm) and boulders (+200mm) with the maximum size usually limited to 1m, which makes the conventional sieving process considerably difficult to conduct as well as being time-consuming. Meanwhile, the advanced technology in computer image processing has created many possibilities in characterising particles within digital photographs, and therefore can be utilised as an effective alternative to the conventional sieve analysis. This method has been in use mainly in the mining industry over the past two decades to assist with rock fragmentation and process monitoring and control. Notwithstanding, the use of this technique in the dam industry for quality control of rockfill material has been rare. Thus, an innovative approach is proposed in this paper to estimate the PSD curves for rockfill material using image analysis along with the latest developments in aerial photography. The results of PSD analysis using the image processing software Split Desktop are presented and compared with the results from sieve analyses for verification. Recommendations are made to improve the process and increase the accuracy of the outcome. It is demonstrated that the proposed method has a reasonable accuracy and is a viable option for quality control in construction of rockfill structures such as rockfill embankment dams.
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Papers 2017
2017 – An Updated National Seismic Hazard Assessment for Australia: Are We Designing for the Right Earthquakes?
Learn moreT. Allen, J. Griffin, M. Leonard, D. Clark and H. Ghasemi
Geoscience Australia (GA) has embarked on a project to update the seismic hazard model for Australia through the National Seismic Hazard Assessment (NSHA18) project.The draft NSHA18 update yields many important advances on its predecessors, including: 1) calculation in a full probabilistic framework using the Global Earthquake Model’s OpenQuake-engine; 2) consistent expression of earthquake magnitudes in terms of moment magnitude, MW; 3) inclusion of epistemic uncertainty through the use of alternative source models; 4) inclusion of a national fault-source model based on the Australian Neotectonic Features database; 5)the use of modern ground-motion models; and 6)inclusion of epistemic uncertainty on seismic source models, ground-motion models and fault occurrence and earthquake clusteringmodels.The draft NSHA18 seismic design ground motions are significantly lower than those in the current (1991-era) AS1170.4–2007 hazard map at the 1/500-year annual ground-motion exceedance probability (AEP) level. However, draft values at lower probabilities (i.e., 1/2475-year AEP) are entirely consistent,in terms of the percentage area of land mass exceeding different ground-motion thresholds,with other Stable Continental Regions(e.g.,central & eastern United States). The large reduction in seismic hazard at the 1/500-year AEP level has led to engineering design professionals questioning whether the new draft design values will provide enough structural resilience to potential seismic loads from rare large earthquakes. This process underscores the challenges in developing national-scale probabilistic seismic hazard analyses (PSHAs)in slowly-deforming regions, where a 1/500-year AEP design level is likely to be much lower than theANCOLD Maximum Credible Earthquake (MCE) ground motions. Consequently, a robust discussion among the Standards Australia code committee, hazard practitioners and end users is required to consider alternative hazard and/or risk objectives for future standards.Site-specific PSHAs undertaken for owners and operators of extreme and high consequence dams general-ly require hazard evaluations at lower probabilities than for typical structural designas recommended in AS1170.4.However, modern national assessments, such as the NSHA18, can provide a benchmark in terms of recommended seismicity models, fault-source models, ground-motion models, as well as hazard values, for low-probability site-specific analyses.With a new understanding of earthquake processes in Australia leading to lower ground-motion hazard values for higher probability events (e.g.,1/500-year AEP), we should also ask whether the currently recommended design probabilities provide an acceptable level of seismic resilience to critical facilities (such as dams)and regular structures.
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Papers 2017
2017 – Application of Risk Assessment to an Emergency Levee Design in an Urban Environment
Learn morePetros Armenis, Malcolm Barker, Peter Christensen, Graham Harrington
The Canterbury Earthquake Sequence in September 2010 and February 2011 caused large areas of land to change by differing amounts throughout Christchurch, New Zealand. Land levels fell by more than 300 mm in some areas. This increased flood risk in the tidal reaches of the Avon River. Urgent repairs were completed with the objective to restore the tidal river defences to a crest level equivalent to a 1% AEP tide level. This work needed to be completed prior to impeding spring tides.
The levees will be required for up to 20 years and then probably be rebuilt on a new alignment. To better understand the risks associated with the ongoing reliance of the levees for flood protection in the interim, a risk assessment was undertaken using conventional Australian National Committee on Large Dams (ANCOLD) practices and levee design procedures. Careful consideration was made to the performance of the existing levees under seismic, flood and tidal loading from which the societal and individual risk profiles were derived. The work included the following:
- The identification of critical sections along both sides of the 11 km levee river alignment through consideration of the foundation and embankment construction
- Combining seismic events with tides and flood events with tides using levee lifetimes of 1, 5, 10 and 20 years
- Consideration of overtopping failure and piping through the levee embankment, foundation or tree roots and narrowed embankment sections owing to trees being blown over
- Application of the International Levee handbook (CIRIA 2013) and the “Piping Toolbox” (USACEet al 2008
- Evaluation of risk reduction with upgrade options
This paper will present the levee design and the process applied for the analysis of the levee and the upgrade options selection
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Papers 2017
2017 – Breach Development Relationships for Small Earthen Dams
Learn moreElaine Pang, Robert Fowden
There are numerous established methods available for assessing the consequences of failure for earthen water dams.The estimation of breach dimensions and failure times remains the greatest common area of uncertainty, particularly for dams under 10m in height, where the number of historic records behind the established methods reduces considerably.Also, various factors can have a significant impact on the strength of small dam embankments, potentially contributing to the likelihood of failure.Consequently, failure impact assessments for smaller dams may rely more heavily on the engineering judgement of the responsible engineer. Although the consequences of failure may indeed be lower for smaller dams, the large number of unknown or unregulated dams in some locations means that it can be difficult to quantify their overall contribution in terms of dam safety risk. This paper presents an on-going project to compile and analyse observed small earthen dam failures with the intent of refining existing statistical breach relationships for smaller dams.Context is provided through an overview of DEWS’ investigative program, including the presentation of several case studies which highlight field data collected throughout the program.
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Papers 2017
2017 – Can They Jump Puddles? A Discussion of the Design Features of a Weir to Limit Impacts on Aquatic Species
Learn moreLesa Delaere, Dr Natalie Clark, Dr Shayan Maleki
Waterway barriers, such as dams and weirs, have the potential to impact aquatic fauna species through the restriction of fauna movement and direct injury and mortality of individuals. Without suitably designed aquatic fauna passages and features to minimise injury and mortality, these barriers may adversely affect the viability of local and regional populations, through disruption to critical behaviours (e.g. breeding, dispersal).
The Lower Fitzroy River Infrastructure Project comprises of two weirs on the Fitzroy River in central Queensland. Two threatened turtle species, the Fitzroy River turtle and the white-throated snapping turtle, and a range of fish species needed consideration of species-specific requirements and development of targeted design solutions.
This paper discusses the ecological needs of these species as well as features incorporated into the design to reduce the impact of the weirs. The design incorporated modular fishlocks, gate, spillway and stilling basin features, an innovative turtle passage, special considerations for outlets and operational aspects. The design was further subject to complexity due to the variation in river flows, zero flow to approximately 9,000m3/sat bank full, and needed to account for a wide range of operational scenarios with respect to the species impacts.The paper also includes a discussion on computational fluid dynamics modelling (CFD) which was used to validate the design of fish passage structures.
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Papers 2017
2017 – Certainty in Uncertainty: Navigating Environmental Impact Assessment Regulatory Processes
Learn moreGeraldine Squires
There is increased pressure from stakeholders for projects to include evaluation of emerging broader development issues within the environmental assessment process. These emerging issues are not well documented or understood and at the forefront of untested preliminary government policy positions.
Agencies expect proponents to invest in evaluating these matters outside of typical assessment practices. Requests are made late in the evaluation and approval process.Assessmen involves matters not directly related to the project or within the proponent’s control and occurs late in the project development cycle.
The Lower Fitzroy River Infrastructure Project (LFRIP) was identified through the Central Queensland Regional Water Supply Study in 2006, as a solution to secure future water supplies for the Rockhampton, Capricorn Coast and Gladstone regions. The Gladstone Area Water Board and SunWater Limited, as proponents, propose to raise the existing Eden Bann Weir and construct a new weir at Rookwood on the Fitzroy River in Central Queensland.
The LFRIP environmental impact statement (EIS) was approved, subject to conditions, by the Queensland Coordinator-General in December 2016 and the Commonwealth Minister for the Environment and Energy in February 2017. Achieving conditions that will realise positive environmental outcomes while simultaneously achieving project objectives, particularly with regard to timeframes and costs, was not without its challenges.
The EIS was developed in accordance with the requirements of the State Development Public Works Organisation Act 1971 (Qld) and the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999, including an extensive stakeholder consultation programme. These regulatory requirements are well understood and applied to projects as normal accepted practice. They ensured that potential project impacts and benefits were identified, that appropriate levels of effort were applied to investigations to establish baseline conditions and that risks to and impacts on environmental (including social and cultural) matters were adequately mitigated and managed.
The environment is not static. Emerging issues and perceptions results in regulation and policy changes in response to political and social drivers. During the development of the EIS both new legislation and new policies were imposed on the project.New legislation resulted in additional assessment around matters previously considered mitigated and managed (fish passage). New legislation introduced new matters for assessment (connectivity). Collaboration and engagement with stakeholders were key to understanding the applicability of these elements to the project and for developing an approach to address the legislative requirements late in the project’s development and assessment process.
In Queensland,policy is emerging to mitigate and manage impacts of development on the Great Barrier Reef World Heritage Area’s universal values. The EIS was required to address the direct project impacts on water quality and the impacts arising because of the LFRIP (facilitated development). Water secured by the LFRIP is for urban, industrial and agricultural purposes. Urban and industrial developments are well regulated and subject to specific environmental approvals processes. Use of water for agricultural purposes, intensive irrigated agriculture in particular,is less regulated. Policies developed are reactive and require individual projects to address these impacts.In the absence of regulatory guidelines for assessment of consequential impacts, the project adopted a collaborative approach. The proponents established a working group, including State and Commonwealth technical agencies. This allowed for robust and scientifically defendable methodologies to be developed and agreed upfront. Streamlining the approach by including key decision makers assisted in managing expectations and focused the assessment on realistic and achievable outcomes relative to the project. The result was defendable outcomes allowing timely decision making and avoided rework as much as possible.
This paper describes developments in environmental assessment relating to new and augmented weirs.
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Papers 2017
2017 – CFD Simulation of Pressure Fluctuations in Plunge Pools: In Search of a New Method
Learn moreShayan Maleki, James Apostolidis, Tom Ewing, Virgilio Fiorotto
The stability analysis of dam spillways and stilling basin chutes requires the knowledge of the spatially fluctuating pressure at the bottom of the structure with reference to the large vortex system with dimensions comparable with the structure characteristic length of the order O (0.1 –1 m). In this context only the small frequency pressure fluctuations (smaller than 1 –10 hz in prototype) must be analyzed in Large Eddy Simulation (LES)context; while the higher frequency pressure fluctuations could be filtered given their negligible importance in relation to stability computations with reference to the spatial Taylor macroscale and fluctuating pressure variance evaluation. These two quantities allow us to define the variance of the force acting on the structure, and as a consequence via statistical analysis, the design force on the structure. This procedure is historically performed via.physical hydraulic modelling (PHM)where these quantities are measured in a laboratory setup. Considering the limits of.current industry approach to Computational Fluid Dynamics (CFD), the use of Detached Eddy Simulation (DES) could become a valid low cost solution and could potentially be a valid method to perform preliminary studies in order to refine the design while avoiding expensive physical model modifications. In this paper, the pressure field at the base of a rectangular impinging jet is measured in laboratory flume setup and is compared with the numerical results obtained via equivalent DES simulations conducted in CFD.Maximum values and the structure of spatial correlation of the anisotropic field of fluctuating pressures are described in view of their relevance to the structural design of the lining of spillway stilling basins and other dissipations structures,as well as in view of their relevance to rock stability analysis. The comparison of the laboratory study with DES simulations presented in this paper shows a good agreement indicating.that this approach may eventually provide a lower.cost substitute for physical model studies in the design of stilling basins and plunge pools.However,it is acknowledged that virtually all stilling basins and plunge pools present a three-dimensional hydraulics complexity, and numerous.further studies need to be done.
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Papers 2017
2017 – Comparing the New Rare Design Rainfalls with CRCFORGE Estimates
Learn moreC.Jolly and J.Green
New rare design rainfalls were released for Australia in February 2017, for durations from one to seven days and probabilities from 1in 100Annual Exceedance Probability (AEP) up to 1 in 2000 AEP.The differences between the previous rare design rainfalls using estimated Cooperative Research Centre –FOcussed Rainfall Growth Estimation (CRC-FORGE) method and the new rare design rainfall estimates vary with location, duration and probability. In this paper, these differences are explored spatially through the use of national maps, comparing percentage change between the two datasets for selected durations and probabilities. Before this comparison with the new rare design rainfalls could be completed, the State-basedestimates had to be resampled and aggregated to form a national data set for Australia.For rare design rainfalls, it is often the catchment values that are required to determine the gross rainfall for design purposes. The impact of the revised areal reductions factors and rare design rainfalls is explored through case study catchments in Tasmania.
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Papers 2017
2017 – Conditions Under Which Identified Faults Contribute Significantly to Seismic Hazard in Australia
Learn morePaul Somerville, Andreas Skarlatoudis and Don Macfarlane
The 2017 draft ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake specify that active faults (with movement in the last 11,000 to 35,000 years) and neotectonic faults (with movement in the current crustal stress regime, in the past 5 to 10 million years) which could significantly contribute to the ground motion for the dam should be identified, and be accounted for in the seismic hazard assessment. The purpose of this paper is to provide guidance on the conditions under which these contributions could be significant in a probabilistic seismic hazard analysis (PSHA)and a deterministic seismic hazard analysis (DSHA).We consider five primary conditions under which identified faults can contribute significantly to the hazard: proximity, probability of activity, rate of activity, magnitude distribution, and return period under consideration
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Papers 2017
2017 – Construction Flood Risk Strategies for Dam Upgrades
Learn moreColleen Baker, Sean Ladiges, Peter Buchanan, James Willey, Malcolm Barker
Dam Owners and Designers are often posed with the question “what is an acceptable flood risk to adopt during the construction of dam upgrade works?” Both the current ANCOLD Guidelines on Acceptable Flood Capacity (2000) and the draft Guidelines on Acceptable Flood Capacity (2016) provide guidance on the acceptability of flood risk during the construction phase. The overarching principle in both the current and draft documents is that the dam safety risk should be no greater than prior to the works, unless it can be shown that this cannot reasonably be achieved.Typically with dam upgrade projects it is not feasible to take reservoirs off-line during upgrade works, with commercial and societal considerations taking precedent. It is therefore often necessary to operate the reservoir at normal levels or with only limited drawdown. The implementation of measures to maintain the risk at or below that of the pre-upgraded dam can have significant financial and program impacts on projects, such as through the construction of elaborate cofferdam arrangements and/or staging of works. This is particularly the case where upgrade works involve modifications to the dam’s spillway.The use of risk assessment has provided a reasonable basis for evaluating the existing and incremental risks associated with the works, such as the requirement for implementation of critical construction works during periods where floods are less likely, in order to justify the As Low As Reasonably Practicable (ALARP) position. This paper explores the ANCOLD guidelines addressing flood risk, and compares against international practice. The paper also presents a number of case studies of construction flood risk mitigation adopted for dam upgrades on some of Australia’s High and Extreme consequence dams, as well as international examples. The case studies demonstrate a range of construction approaches which enable compliance with the ANCOLD Acceptable Flood Capacity guidelines
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Papers 2017
2017 – Construction of Borinquen Dam 1E for the Panama Canal Expansion
Learn moreJames Toose, Lelio Mejia, Jorge Fernandez
The recently completed Panama Canal Expansion project required construction of a new, 6.7-km-long channel at the Pacific entrance to the Panama Canal, to provide navigation access from the new Post-Panamax locks to the existing Gaillard Cut section of the Canal. The new channel required construction of four new dams adjacent to the existing canal, referred to as Borinquen Dams 1E, 2E, 1W, and 2W. The dams retain Gatun Lake and the Canal waterway approximately 11 m above the level of Miraflores Lake and 27m above the Pacific Ocean.The largest of the dams, Dam 1E, is 2.4km long and up to 30 m high. The dam abuts against Fabiana Hill at the southern end, and against the original Pedro Miguel Locks at the northern end. This paper provides an overview of the key challenges in construction of Dam 1E including the foundation, seepage cut-offs and embankment.
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Papers 2017
2017 – Dam and Foundation Repsonses to the 2016 MW7.8 Kaikōura Earthquake in New Zealand
Learn moreAndrew Balme, Dan Forster, Tim Logan
The MW7.8 Kaikōura earthquake on 14 November 2016, ruptured over 20 faults during the initial shaking,which lasted nearly two minutes. A complex series of fault ruptures propagated northeast for nearly 180 km from the initial rupture location. Instrumentation from dams across New Zealand shows that whilst most dams did not suffer physical damage, piezometric responses were measured in dams and their foundations. Earthquake related changes in seepage regimes are not unusual and depend on the characteristics of the ground motions,and site specific characteristics that influence how a dam and its foundation respond to ground motions. The ability to measure a piezometric response in a dam or foundation is heavily influenced by the instrumentation network and method of monitoring. Data collected during events such as the Kaikōura earthquake provides valuable information for both characterising performance of a dam during the event, and assisting future analysis such as failure mode assessments. Careful consideration must be given to the scope of installed instrumentation and the frequency of monitoring in order to provide these benefits,and the robustness of the system to ensure it adequately survives the event.
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Papers 2017
2017 – Decommissioning of Roaring Meg Dam and Cutten Street Dam No 3 – Creating a New Legacy
Learn morePaul Southcott,Suraj Neupane and David Krushka
TasWater owns and operates the water supply in Queenstown on the west coast of Tasmania. Anew water treatment plant was constructed downstream from one of the seven small dams that made up the original supply system, making the remaining six dams redundant.Two of these dams hada very high annual probability of failure and unacceptable societal life and financial risk due to their poor condition.Both dams required urgent attention (upgrade) to retain them as a lasting asset and legacy for the community or decommissioning to create a new ecological legacy.Roaring Meg Dam (6m high with a 9ML capacity) was constructed on Roaring Meg Creek around 1963. The Cutten Street Dam No 3 (10m high with a 2.4ML capacity) was constructed on Reservoir Creek around 1902to supply water to a growing mining community and had been in use since then. From a heritage perspective, the dam had some value as a timber crib and rockfill dam and its historical context as a key factor in the development of the town.There is limited guidance in the ANCOLD (2003) Dam Safety Management Guidelines on decommissioning and a process had to be developed in cooperation with the Regulator in this relatively new area of dam engineering. Detailed design of the decommissioning including diversion work during decommissioning, channel design to align with the original creek to help restore its ecological function and rehabilitation work on the exposed reservoir soils to stabilise them were undertaken. Aboriginal and historic heritage studies, flora & fauna studies and fluvio-geomorphological study at the dam sites were also undertaken to ensure that the decommissioning work did not interfere with the heritage, threatened species and riparian processes. The community were consulted to ensure acceptance of the changes to their town. Dam safety emergency management plans for the decommissioning of these dams was were also prepared. A significant issue in the decommissioning work was frequent and high rainfall due to the location of these dams on the west coast of Tasmania. The entire dam removal work had to be planned within the window of dry weather or very little rainfall. This paper presents the process, activities and lessons learned in successfully decommissioning these dams,to eliminate the unacceptably high risks posed by these dams and to restore the normal riparian processes.The general approach adopted for this project has applicability for other damsandis proposed as a starting point for an ANCOLD practice note in this area
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Papers 2017
2017 – Detailed Seismic Hazard Assessment of Mt Bold Area: Comprehensive Site-Specific Investigations on Willunga Fault
Learn moreZivko R. Terzic, Mark C. Quigley, Francisco Lopez
The Mt Bold Dam, located in the Mt Lofty Ranges in South Australia, is a 54m high concrete arch-gravity dam that impounds Adelaide’s largest reservoir. The dam site is located less than 500m from a suspected surface rupture trace of the Willunga fault.Preliminary assessments indicate that Mt Bold Dam is likely to be the dam with the highest seismic hazard in Australia, with the Flinders Ranges-Mt Lofty region experiencing earthquakes of sufficient magnitude to generate shaking damage every 8-10 years on average. Prior evidence suggests that the Willunga Fault is likely capable of generating M 7-7.2 earthquakes.As part of the South Australia Water Corporation (SA Water) portfolio of dams, Mt Bold Dam is regularly reviewed against the up-to-date dam safety guidelines and standards. SA Water commissioned GHD to undertake detailed site-specific geophysics, geotechnical and geomorphological investigations, and a detailed site-specific Seismic Hazard Assessment (SHA) of the Mt Bold Dam area. The results of this investigation will be used to inform decisions related to planned upgrade works of the dam.Geomorphological mapping of Willunga Fault, detailed geological mapping, analysis of airborne lidar data, geophysical seismic refraction tomography and seismic reflection surveys,and paleoseismic trenching and luminescence dating of faulted sediments was conducted to obtain input parameters for the site-specific SHA.Discrete single-event surface rupture displacements were estimated at ~60 cm at dam-proximal sites. The mean long-term recurrence interval (~37,000 yrs) is exceeded by the quiescent period since the most recent earthquake (~71,000 yrs ago) suggesting long-term variations in rupture frequency and slip rates and/or that the fault is in the late stage of a seismic cycle. The length-averaged slip rate for the entire Willunga Fault is estimated at 38 ± 13 m / Myr. Shear wave velocity (Vs30) of the dam foundations was estimated based on geotechnical data and geological models developed from geophysical surveys and boreholes drilled through the dam and into the foundation rock. The nearest seismic refraction tomography (SRT) lines were correlated with the boreholes and those velocity values used in the Vs30 parameter determination. All relevant input parameters were included into seismic hazard analysis with comprehensive treatment of epistemic uncertainties using logic trees for all inputs.Deterministic Seismic Hazard Analysis (DSHA) confirmed that the controlling fault source for the Mt Bold Dam site is Willunga Fault, which is located very close to main dam site (420m to the West).For more frequent seismic events (1 in 150, 1 in 500 and 1 in 1,000 AEP), the probabilistic analysis indicates that the main seismic hazard on the dam originates from the area seismic sources (background seismicity).Based on deaggregation analysis from the site specific Probabilistic Seismic Hazard (PSHA), the earthquakes capable of generating level of ground motion for the 1 in 10,000 AEP event can be expected to occur at mean distances of approximately 22km from the dam site(with the mean expected magnitude atMt Bold Damsite estimated at Mw >6).For less frequent (larger) seismic events, the contribution of the Willunga Fault to the seismic hazard of Mt Bold Dam can be clearly noted with Mode distance in the 0-5 km range, which indicates that most of the seismic hazard events larger than the 1 in 10,000 AEP comes from the Willunga Fault. The Mode magnitudes of the events are expected to be Mode Magnitude at Mw= 6.6 for a segmented Willunga Fault scenario, and Mw= 7.2 for a non-segmented fault scenario.Consideration was also given to the upcoming update of the ANCOLD Guidelines for Earthquake, which calls for the determination of the Maximum Credible Earthquake (MCE) on known faults for the Safety Evaluation Earthquake (SEE) of “Extreme” consequence category dams. The MCE for Mt Bold Dam was estimated from the DSHA; in terms of acceleration amplitude, the MCE event approximately equals the 1 in 50,000AEP seismic events.
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