Chriselyn Kavanagh, David Stephens, Peter Hill
Two-dimensional hydraulic models are now widely used to simulate flooding downstream of dams as part of dambreak assessment studies. These models provide high resolution information on velocity distribution across the floodplain, which is of paramount importance to accurate estimation of the depth-velocity product required when undertaking loss of life assessments. In addition, the outputs from these models are much more readily presented as maps and animations, which can be an important tool in the dam safety emergency planning process.
Recently, the United States Army Corps of Engineers released a new version of the popular hydraulic model HEC-RAS which includes the ability to conduct two-dimensional simulations. Other widely used two-dimensional models include DHI’s MIKE suite and TUFLOW. This paper presents a review of the capability, functionality and useability of these models for the specific purpose of dambreak modelling. Key features considered as part of the review include model stability, run times, methods of simulating dam breaches, outputs and the ability to link to loss of life simulation models. A case study comparing the performance of three commonly applied models is presented and discussed, and advice is provided on model selection.
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Jason Needham, John Sorensen, Dennis Mileti, Simon Lang
The potential loss of life from floods, including those caused by dam failure, is sensitive to assumptions about warning and evacuation of the population at risk. Therefore, the U.S. Army Corps of Engineers engaged with social scientists to better understand the process of warning and mobilizing communities that experience severe flooding. This improved understanding enables dam owners to better assess the existing risk posed by their assets and investigate non-structural risk reduction measures alongside structural upgrades.
In this paper, the U.S. Army Corps of Engineers research is summarised to provide general guidance on the warning and mobilization of populations at risk for practitioners assessing the potential loss of life from dam failure. This includes commentary and quantification of three primary timeframes: warning issuance delay, warning diffusion, and protective action initiation. A questionnaire for estimating these parameters is also introduced, alongside a case study application for an Australian dam.
This paper also summarises the current understanding of how to reduce delays in determining when to issue warnings, increase speed at which warnings spread through communities, and decrease the time people spend before taking the recommended protective action. These insights will help all people involved with emergency management, including those tasked with developing Dam Safety Emergency Plans.
Alan P. Jeary, James O’Grady, Thomas Winant
Mainmark are introducing the STRAAM system of full scale non-destructive testing for dams into Australia and New Zealand. Advances in measuring extremely low amplitude vibrations combined with methods for extracting the unique dynamic signature have now enabled the rapid measurement of the response of earthen and concrete dams. This ability allows the quick calibration of Finite Element Models that can be used to accurately assess the strength of a dam. Furthermore, this information allows dam owners to efficiently track changes in the capacity of their dams due to aging, earthquake or flood activity through changes in the dynamic.
The STRAAM system measures the vibration of the dam structure to establish the natural frequencies, mode shapes and associated damping ratios of the dam. The field measurements are correlated with a three-dimensional finite element model to fine tune the effects of abutments and foundations on the three dimensional model. Because of the sensitivity of the instrumentation and the novelty of the analysis techniques, the information available to dam managers allows information-based decisions to be made in a way that optimizes the financial implications. In addition, the techniques are non-invasive and non- destructive and they give additional information about the connectivity of the dam with the surrounding terrain, and whether that connectivity is compromised by water seepage.
This paper discusses the results obtained from field measurements from four dams located in Switzerland, USA and Scotland.
This paper reviews methods used to estimate the MCE in Australia and New Zealand. In the ICOLD (2016), NZSOLD (2015) and proposed ANCOLD (2016) guidelines, the deterministic approach is applicable only to fault sources, whereas the probabilistic approach is applicable to both fault sources and distributed earthquake sources. Although ICOLD (2016) states that the use of a deterministic approach to develop the SEE “may be more appropriate in locations with relatively frequent earthquakes that occur on well- identified sources, for example near plate boundaries,” the proposed ANCOLD (2016) guidelines retain the use of the deterministic approach for critical active faults which show evidence of movements in Holocene time (i.e. in the last 11,000 years), or large faults which show evidence of movements in Latest Pleistocene time (i.e. between 11,000 and 35,000 years ago). In Australia, active faults make a significant contribution to the probabilistic MCE only at near-fault sites, and even in those cases most of the hazard comes from distributed earthquake sources. However, some sites may be close enough to nearby or even more distant identified active faults that a Deterministic Seismic Hazard Analysis (DSHA) produces MCE ground motions that are far larger than those obtained probabilistically even for very long return periods. Conversely, the deterministically defined MCE may be lower than the probabilistically defined MCE for very long return periods at near fault sites in New Zealand, requiring the probabilistic approach.
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.
Matthew Ind, Kate Brand and Mark Ferrier
The framework for undertaking a dam breach analysis for water dams is reasonably well established with a depth of information and software available to guide practitioners on a consistent approach to undertaking failure impact assessments. In contrast, dam breach modelling for tailings dams is currently a developing field with a wide range of modelling approaches taken and an inconsistency in the quality of the failure impact assessments undertaken. Recent tailings dam failures at the Mt Polley Mine in British Columbia, Canada and the Fundäo and Santarém dams at the Samarco iron ore operation in Minas Gerais, Brazil have provided a sobering reminder of the hazards presented by tailings dams and the clean-up challenges that are significantly more complex than a similar failure of a water dam.
Current guidelines and approaches to dam breach modelling are often done assuming the run-out material from the breach is just water without due consideration of the impact from tailings loss. There is limited analysis undertaken on credible failure modes of tailings dams with an assumption that the embankment just “breaks” at some random point without appreciation of the failure mechanism. The misunderstanding of failure modes leads onto inconsistencies with application on whether a ‘sunny-day’ or extreme flood event modelling should be applied, with one or the other selected without explanation.
This paper outlines a framework that can be applied when undertaking a dam breach study for tailings dams to enable a consistent and credible assessment of potential failure impacts. The following tasks are discussed in detail in support of this framework: