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.
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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.
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.
Extending the useful life of a dam to an extent well beyond what was envisaged by the original designer poses diverse challenges. In this paper, three case studies are described, one involving strengthening of two similar dams and two cases involving raising. In all three cases, the dams continue to provide a reliable source of supply in a water scarce country.
The Woodhead and Hely-Hutchinson Dams have substantial historical significance which guided the selection of restressable post-tensioned anchors as the preferred method of strengthening.
The Stettynskloof Dam was almost doubled in height by constructing a clay core rockfill embankment abutting the downstream face of the existing concrete gravity dam. The new structure was well instrumented to cover areas of concern but the dam was found to perform as largely predicted by the designers.
Keerom Dam faced both technical and regulatory challenges that were eventually overcome and the raising of the dam was able to proceed. A further raising will increase the utilisation of this valuable resource still further.
Kelly Maslin, Richard Rodd
As an industry there have been many advances in the assessment of the probability of failure associated with a range of failure modes including embankment piping and stability. However, little work has been done on the development of a meaningful tool to assist in the assessment of probabilities of failure for embankment breach due to overtopping.
In the development of this paper a number of embankment overtopping case studies were reviewed and these were used to anchor the suggested probabilities of failure. The case studies assessed were all low to medium height, homogeneous earthfill embankment dams. Consideration has been given to a range of factors including embankment material and construction, embankment geometry, duration of overtopping and the presence and condition of vegetation on the embankment face.
The results of the analysis of the case studies indicate that the probability of breach due to overtopping, particularly for short duration events, is actually relatively low compared to the typical values being adopted within the industry.
It is the intended purpose of this paper that it provides guidance to the industry on the assignment of the probability of embankment breach due to overtopping to allow more consistent, robust and defensible estimates for dam safety risk assessments.
Robert Shelton, Jako Abrie, Matt Wansbone
The Mahinerangi dam – arguably the most valuable in Trustpower’s portfolio of 47 large dams – is over 80 years old and needs a plan of work to confirm it meets current design standards.
The dam was completed in 1931, subsequently raised in 1944-1946, and strengthened with steel tendon anchors in 1961.
A comprehensive safety review (CSR) in 2007 noted a potential deficiency in the fully grouted anchors and a program of work commenced to re-evaluate the overall stability of the dam.
A potential failure mode assessment revealed that the dam may need upgrading to meet the criteria for maximum design earthquake (MDE). Areas of uncertainty were identified and a significant programme of survey, geological mapping, concrete testing and site specific seismic assessments have been carried out to reduce risk and uncertainty in design.
The paper discusses the dam’s history, current condition, and describes the ongoing programme of work planned to extend the life of the dam for another 80+ years.