D Stephens and P Hill
Dambreak modelling and consequence assessment is a key component of many dam safety related studies. The outputs from these assessments can be used to inform the consequence category, dam safety emergency planning, risk-based surveillance and dam safety risk assessment. These studies are complex, intensive and expensive to complete, and all too often there is a need to manipulate or extrapolate the results of these assessments to fit a purpose other than what they were intended for. This issue is particularly prevalent for risk assessment, where the likelihood calculations are directly tied to analysis of the key failure modes, but consequences may be taken from previous studies which were not informed by failure mode selection. The result of this mismatch may lead to inefficiencies and uncertainties in preparing the risk estimates. Subtle changes to the timing or scope of the original dambreak modelling and consequence assessments, at relatively small incremental cost, may help to prevent these issues arising for future studies. Advice is provided on specific issues such as the determination of the downstream extent of the dambreak modelling, selection of the dambreak modelling scenarios and reconciliation of the consequence assessment results with flood and seismic loading partitions for risk assessment. It is hoped that the advice provided will lead to an overall increase in the efficiency and value for money of these studies.
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The key differences between probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard analysis (DSHA, preferably referred to as a scenario-based analysis) are that, unlike DSHA, PSHA takes account of all magnitudes on all earthquake sources that may affect the site, including the frequency of occurrence of each earthquake scenario that is considered, and fully considers the random variability (epsilon) in ground motion level. The result of a DSHA is the ground motion at the site resulting from a single earthquake scenario (or a few scenarios) having a preselected value of epsilon (usually 0 or 1), and the annual frequency of exceedance (or return period) of this ground motion level is undefined. In contrast, the hazard curve produced by PSHA yields the mean annual rates of exceedance (or return period) for each ground motion level. The complementary nature of PSHA and DSHA is manifested in the fact that practical application of PSHA, especially using ground motion time histories, results in scenario earthquakes that resemble the products of DSHA. Application of the period dependence of epsilon using the conditional mean spectrum (CMS) avoids the inaccurate and overconservative representation of the hazard by the uniform hazard spectrum (UHS) obtained in PSHA.
Sean Ladiges, Robert Wark, Richard Rodd
The use of permanent, load-monitorable post-tensioned, anchors for dam projects has been in place for approximately 35 years in Australia. Since then, over 30 large Australian dams have been strengthened using this technology, including the world record for anchor length (142 m – Canning Dam, WA) and size (91×15.7 mm strands – Wellington Dam, WA and Catugunya Dam, TAS).
In order to achieve the design life of 100 years expected of these anchors, an ongoing program of monitoring, testing and maintenance is required, to identify and rectify the initiation of corrosion or loss of pre-stress. Guidance for maintenance and testing regime for post-tensioned anchors in dams is provided in the ANCOLD Guidelines on Dam Safety Management (2003). The various conditions which may affect the performance of the anchor with time, such as anchor type, ground condition and loading fluctuations are not covered in the Guideline.
This paper reviews the implementation and results of anchor monitoring programs by Australian dam owners. The first part of this paper provides a summary of the testing and monitoring programs currently being implemented. The second part of the paper reviews the aggregated anchor load test results from a number of Australian dam owners, and identifies trends in anchor response over time following installation.
The paper aims to assess whether the recommended anchor testing regime proposed in ANCOLD (2003) is appropriate and cost effective, using evidence from recent load test data which has become available following the writing of the guideline. The lessons learnt from anchor maintenance programs will also be discussed.
Peter Woodman, Andrew Northfield, Hench Wang
Current empirical approaches assume different fatality factors for the ‘fail’ and ‘no fail’ scenarios even when the same hazard is experienced by a property. This approach can lead to some inconsistencies particularly for small dams and retarding basins. This paper looks at the base data behind the current fatality factors and explores possible alternatives to the current approach. The paper will rely on a number of examples from a recent investigation undertaken by GHD for Melbourne Water on a number of their retarding basins.
Woodrow Lee Fields
Although flooding can lead to many types of severe consequences, the primary objective of the US Army Corps of Engineers (USACE) dam and levee safety programs are to manage risk to the public who rely on those structures to keep them reasonably safe from flooding. Thus, reducing the risk associated with loss of life is paramount. This paper discusses new methods that have been developed for estimating life loss with uncertainty from flood events.
HEC-LifeSim is a dynamic simulation system for estimating life loss with the fundamental intent to simulate population redistribution during an evacuation in conjunction with flood wave propagation. The population redistribution process has been revised from the ground up as an agent based model. In addition to the agent based model, uncertainty analysis has been enhanced. Through Monte Carlo sampling, the natural variability of warning and mobilization timing and likelihood of fatality varies delivering a range of potential life loss from a hazard. Knowledge uncertainty about parameters, such as warning issuance time, can also be defined. To accommodate the new HEC-LifeSim computation engine, an innovative GIS interface has been developed to quickly summarize and animate results. The methods that are discussed in the following provide new tools to estimate life loss and educate local authorities.
J.H.Green, C.Beesley, C.The, S.Podgerand, A.Frost
The ability to estimate design rainfalls for probabilities rarer than 100 years or 1% Annual Exceedance Probability (AEP) is an essential part of dam hydrology. The earliest means of estimating rare events consisted of a pragmatic curve fitting procedure between the 50 and 100 year design rainfalls and the Probable Maximum Precipitation. In the 1990s a more rigorous method of estimating design rainfalls as rare as 2000 years was developed – the Cooperative Research Centre – FOcussed Rainfall Growth Estimation (CRC-FORGE) method. CRC-FORGE estimates were derived for Victoria in 1997 followed progressively by each of the other states. Over the subsequent two decades CRC-FORGE estimates were an integral part of the risk assessment of large dams – being used to determine the AEP of the Dam Crest Flood.
The Bureau of Meteorology will soon release new rare design rainfall estimates for probabilities to 2000 years. The new rare design rainfalls are a significant improvement on the CRC-FORGE estimates as they have been derived using up to date data; contemporary analytical techniques and a method that is consistent across Australia.
However, there are differences between the CRC-FORGE estimates and the new rare design rainfalls. These differences do not constitute a systematic change to the CRC-FORGE estimates but rather vary with location; duration and probability. The results of a detailed comparison between the CRC-FORGE estimates and the new rare design rainfalls are presented together will an assessment of the possible impacts on previous estimates of the AEP of the Dam Crest Flood.