D Stephens, S Lang, P Hill, M Scorah
Robust estimates of the duration of flood overtopping are a key input into the dam safety risk assessment process. For embankment dams, the likelihood of erosion of the dam crest, downstream face and eventual unravelling of the embankment are heavily dependent on the duration of water flowing over the crest. Similarly, the chance of erosion of the abutments of concrete dams is strongly linked to the duration of floodwaters overtopping the dam. Previously, it has been difficult to define the annual exceedance probability (AEP) of the flood required to cause overtopping of a certain depth for a certain duration, and coarse assessments have typically been made based on critical storm durations of the dam crest flood (DCF). This approach carries significant uncertainty, particularly for structures on smaller catchments where the critical storm duration on outflow may be relatively short. In these cases, it has been difficult to confirm with any reliability that the flood required to achieve a significant duration of overtopping has an AEP close to that of the DCF. This paper describes a new algorithm that has been incorporated into the RORB hydrological model which allows for a frequency curve of flood overtopping duration to be determined within a Monte Carlo framework. The results of this analysis are presented for a case study of a quantitative risk assessment, to demonstrate how the outcomes influenced numerous aspects of the risk analysis process.
— OR —
Now showing 1-12 of 47 2981:
Dr Andy Hughes
On Hampstead Heath in Central London, just 3 kilometres from the centre of the city, there are more than 20 dams and reservoirs set within the landscape setting of Hampstead Heath. A number of dams were built in the 16th century and formed the original water supply to the City of London. They are set in a landscape laid out by the world renowned Humphrey Repton.Three of the embankments which are laid out in two chains of reservoirs across the Heath are subject to safety legislation in the UK. As such they were identified as being deficient in spillway capacity and thus fairly significant works were required to be carried out in this sensitive setting.The Heath is protected by the Hampstead Heath Act of 1870 which seeks to prevent significant changes to the Heath and thus it was quite clear that there would be opposition to any works on the Heath, even though they were required by law to protect persons and property downstream. In fact a significant lobby group formed which challenged the need for the works and also the legislation of the UK via a judicial review. This paper will describe the process by which significant stakeholder consultation was undertaken (costing more than £2M), the judicial review that took place in the Royal Courts of Justice, the option study and the major engineered elements carried out on the Heath.
Zivko 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.
Colleen 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
Alberto 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.
Barton Maher and Michael Peel
The Queensland Bulk Water Supply Authority (Seqwater) manages up to $12 billion of bulk water supply infrastructure and the natural catchments of the region’s water supply sources to ensure a reliable, quality water supply for more than 3million consumers across the region. Seqwater was formed on 1 January 2013 through a merger of three State-owned water businesses, the SEQ Water Grid Manager, LinkWater and the former Seqwater. Seqwater delivers a safe, secure and reliable water supply to South East Queensland, as well as providing essential flood mitigation services and managing catchment health. Seqwater also provides water for irrigation to about 1,200 farmers and offers community recreation facilities enjoyed by more than 2.5 million people each year.Seqwater owns and operates 26 referable dams which fall under the dam safety regulation in Queensland, 51 weirs, and two bore fields across the region. Twelve key dams across the region supply as much as 90% of South East Queensland’s drinking water.In 2011, Seqwater engaged a consultant team of URS (now AECOM) and SKM (now Jacobs) to undertake a portfolio risk assessment of the 26 referable dams and Mount Crosby Weir. At the completion of the project in December 2013 there were 12 dams with life safety risks assessed as being above the ANCOLD and DEWS Limit of Tolerability. A $6.2 million investigation was approved in 2014 to commence planning for the recommended dam safety upgrades and reduce uncertainties in the risk assessment.This program of work was completed in late 2016. The estimated costs of the identified dam safety upgrades exceed $900 million.Confronted with such a large capital program, Seqwater has instigated a number of key actions including:-benchmarking capital investment and rates of risk reduction achieved by other dam owners through a dam owners group-developing a dam safety investment policy to provide a clear guidance on the framework for prioritising and scheduling upgrades-undertaking targeted investigations to reduce uncertainty in the risk assessments including the use of detailed consequence assessment-preparing a prioritised schedule of planned upgrades to gain endorsement from Government and the Dam Safety Regulator. This paper presents the outcomes of the Portfolio Risk Assessment and key changes to the initial risk assessment following further studies. The basis for the dam safety investment policy is presented and the proposed prioritisation tools.The impacts of the risk assessment provisions in the most recent revision of Queensland Acceptable Flood Capacity Guidelines for Water Dams are also discussed. In particular,the application of the economic criteria for determining the minimum upgrade required by the Queensland Dam Safety Regulator and its relevance to other dam owners.