Extreme flood analyses are routinely used as inputs to dam risk assessments, spillway adequacy assessments and spillway designs. Estimation methods applied in Australia using rainfall-runoff models in combination with a Probable Maximum Precipitation (PMP) estimate are consistent with the current best practice applied around the world. The estimation methods can, however, result in substantial variability in peak flow estimates depending upon the practitioner and the methods used to quantify model parameters. Around the world, validation procedures are commonly applied to combat this variability, but no such techniques are routinely applied in Australia. A method is proposed for application across Australia which may variously be applied to validate and constrain extreme flood estimates and also provide quick estimates.
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Population at Risk (PAR) estimation involves quantification of people who could be exposed to flooding in the event of a dam failure. Conventionally, estimates of PAR involve manual and subjective assessment of individual structures located downstream of dams. To reduce the reliance on subjective judgement and better leverage publicly available population datasets, an automated method of PAR assessment was developed. This approach used the Geoscape dataset of building representations to disaggregate Australian Bureau of Statistics 2016 Census data for a study area around Gawler, South Australia.
Representative day and night spatial distributions of PAR were constructed to characterise the diurnal movement of people between homes and workplaces or other day activities. Flows of people were directly quantified to reduce reliance on high level assumptions regarding exposure. A Random Forest model was used to filter sheds and other unpopulated structures from the Geoscape dataset.
The largest deficiency in this approach is the lack of high detail data to classify building usage. It is recommended that the potential for automation of PAR assessment be continually revisited as more datasets become available.
There are a number of software packages that have been developed to conduct Probabilistic Seismic Hazard Assessments (PSHA’s). Each one has advantages and disadvantages. Two such programs are compared; the licenced subscription-based EZ-FRISK software package developed by Fugro USA Land, Inc. and the open-sourced OpenQuake-engine (OQ) software package by the Global Earthquake Model (GEM) Foundation. Both of these packages use the classical PSHA methodology as described by Cornell (1968) and modified by McGuire (1976). Each of these packages offers different advantages; OQ is freely distributed, code based and provides easy access to a number of tools. EZ-FRISK doesn’t rely on command-line tools and instead provides an easy user interface with quick access to plots to check results. EZ-FRISK is computationally faster than the OQ program.
A simple rectangular source model with four sites was used to investigate the degree of agreement between these two software packages. Results indicate that hazard estimates from the two packages agree to within 4% for the two closest sites. At long return periods for the two furthest sites, the difference is larger.
Many Australian and international dam owners use risk assessments to understand and manage the societal risks posed by their dams. This requires estimates of dam failure consequences, particularly the potential loss of life (PLL). The methods used to assess PLL have become more varied and sophisticated in recent times. This paper summarises the current status of the methods most relevant to the Australian dams industry (i.e. RCEM, HEC-LifeSim, the Life Safety Model), and comments on their applicability for Australian PLL assessments. This commentary is based on material presented by dam owners, regulators, researchers and consultants from the United States, Canada, United Kingdom and the Netherlands, at workshops on estimating dam failure consequences held in Denver in 2016 and Toronto in 2018.
Since publication in 2003, the ANCOLD Guidelines for Risk Assessment have reached broad acceptance and use in Australia. In practice, dam owners use the principles of risk assessment to drive business investment decisions. As the guidelines undergo revision, it is timely to assess whether our practices need to evolve to more holistically consider all types of consequences, rather than our current focus on loss of life, in decision-making. This paper aims to prompt dam owners and consultants alike to re-assess our focus on loss of life in risk assessment decision-making, and whether we should more meaningfully consider alternative or broader indicators.
An industry survey was undertaken which found that large dam owners are generally happy with the current system of dam safety decision making. However, the survey responses did identify difficulties in relation to justifying investment below the limit of tolerability that are subject to ALARP principles. In a small number of cases, dam owners found it difficult to justify investment when life safety was not important.
Building on the industry survey and subsequent discussions with practitioners, this paper discusses how the current approach to risk based decision making may result in sub optimal decision making. Further it is discussed how there is an important role that economics should play in providing a universally accepted framework for assessing trade-offs and providing consistent evidence to support decision making.
Deformation Survey is a simple and widely implemented technique to identify the early signs of dam failure and is regularly undertaken on many dams. Thanks to advances in equipment and more accurate survey records, there is now a better understanding of measurement and movement of embankments and previous records.
However, the “expected” range of transverse deformation and implications for failure modes of dams is not particularly well researched or understood.
This paper collates a case history of transverse deformation for a number of Tasmanian dams and examines the relative behaviour of the embankment dams. From this the “expected behaviour” of an embankment dam can be estimated and related to key influencing factors, such as observed settlements, height and age of the dams, and thereby providing guidance on when transverse deformation is considered unusual for similar dams.