2017 – Observations from Five Australian Applications of HEC-LifeSim to Estimate Potential Loss of Life from Dam Failure
Chriselyn Kavanagh, Simon Lang, Andrew Northfield, Peter Hill
The U.S. Army Corps of Engineers have recently releasedHEC-LifeSim1.0, a dynamic simulation model for estimating life loss from severe flooding (Fields, 2016). In contrast to the empirical models that are often used to estimate life loss from dam failure, HEC-LifeSim explicitly models the warning and mobilisation of the population at risk, and predicts the spatial distribution of fatalities across the structures and transport networks expected to be inundated. This capability provides additional insights to dam owners that can be used to better understand and reduce the life safety risks posed by large dams. In this paper, we demonstrate the use of HEC-LifeSim to model the potential loss of life from failure of five large Australian dams. Particular attention is paid to how the predicted life loss varies with warning time, in a manner that depends on human response and the transport network’s capacity for mass evacuations, and the modelled severity of flooding. We also examine how the HEC-LifeSim estimates of life loss compare with those from the empirical Reclamation Consequence Estimating Methodology (RCEM).
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2017 Papers
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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2017 Papers
2017 – Ridge Park Dam: An Innovative Design for an Urban Setting
Learn moreLisa J Neumann, Rod Westmore
In Australia construction of a new dam on a greenfield site is relatively uncommon and construction of a new dam on a brownfield site is even more unusual.This paper presents an innovative design solution to address the challenges associated with such a project.Ridge Park Dam is a new flood retarding dam located in a suburban recreation park, less than 10km south east of Adelaide, South Australia.The dam was constructed in 2014/15 and was designed to limit the peak flows in the creek downstream of the park under the 1 in 100 ARI event and to impound water as a component of the infrastructure required for the Managed Aquifer Recharge (MAR) scheme located in Ridge Park.The expectations of both the client and community and the technical issues encountered in the early stages of the project resulted in some unique design criteria. At the outset the client and community expectation was that the dam would improve the overall amenity of the park without impacting the existing vegetation or functionality of the park, including public access and safety.Identifying a dam type to suit the client and community expectations and address the technical issues was not straightforward.Typical dams types such as embankment dams, mass concrete gravity or concrete buttress structures, were found to be not suitable.A less typical, innovative solution was sought.The outcome was to construct a dam comprising a concrete core wall supported by rock filled gabion baskets.
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2017 Papers
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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2017 Papers
2017 – Hardfill Dams and Geomembrane Facings
Learn moreAlberto Scuero, Giovanna Lilliu, Marco Scarella, Gabriella Vaschetti
Hardfill dams present technical and cost advantages. Placement is like in embankment dams, thus construction is fast. The typical trapezoidal shape makes possible use of local aggregates and low cement content. Despite the low strength material, these dams can be built on weak foundation, and resist earthquake and overtopping. However, being the material semi-pervious, they require an impervious facing. Until 2014 this was typically made with conventional concrete slabs with waterstops, or grout enriched hardfill. Concrete facings require heavy and costly equipment, long construction time, are expensive, frequently require maintenance.Construction of the facing can have a big impact on the overall construction costs of the dam. Replacing the concrete facing with a geomembrane lining is a cost-effective solution. This paper describes two hardfill dams’ projects with an exposed geomembrane as upstream liner: Filiatrinos (Greece, 2015), 55.6 m high,and Ambarau(Democratic Republic of the Congo, 2017), 19.30 m high.
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2017 Papers
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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