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.
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This paper describes taking the data from the transducer recording of dynamic fluctuations at 300 Hz in the physical hydraulic model of the stilling basin of Fairbairn dam and analysing the response of the proposed design solution to these loads. The analysis not only looked at the direct time history loading, but reviewed the response of the anchoring system to the inertial and damping loads. A further extension of the analysis allowing for the stiffness of water has come up with some findings that verify what has intuitively been believed about the design of spillway stilling basin slabs.
Recent advances in communication technologies have made available an array of new systems and functionalities that dam operators can use to improve automation and centralisation in the daily surveillance tasks of their portfolios. These functionalities include real-time monitoring, target-oriented video surveillance and the remote management of PLCs and data loggers.
The present paper aims to outline some integration possibilities using TCP/IP technologies for remote operations and video surveillance.
The case study features a comprehensive dam instrumentation upgrade, in which the acquisition systems were complemented with a series of IP cameras designed to be triggered by local and remote events.
The notion of probability and its various interpretations brings numerous opportunities for errors and misunderstandings. This is particularly true of contemporary risk analysis for dams that mostly consider geotechnical, hydraulic, and structural capacities subjected to extreme loads considered as independent evets. In these analyses subjective “degree of belief” probability has a major role, both in the modelling of the risk in the system by means of event trees based on inductive reasoning and in the assignment of probabilities to events in the event tree. There are numerous situations where physically possible conditions are eliminated from consideration in a risk analysis on the basis of probabilities that are judged to be too low to be of relevance. This is despite the fact that the assignment of a probability to a condition means that the occurrence of the event or condition is inevitable sometime, with the added complication that the time of occurrence is unknown and unknowable. Although there is no relationship between a remote probability and the possibility (or credibility) of the occurrence of the event in the event tree, it is quite common for physically feasible conditions to be either eliminated or their importance discounted on the basis of low probability in a risk assessment of a dam. Twenty five years ago, this elimination process might have been referred to as “judicious pruning of the event tree”. In more modern parlance, the elimination process is based on consideration of whether or not the condition or sequence of events is clearly so remote a possibility as to be non-credible or not reasonable to postulate. In contrast to the consideration of extreme loads vs. structural or geotechnical capacities, experience has shown that many dam failures and perhaps the majority of dam incidents do not result from extreme geophysical loads, but rather from operational factors. These incidents and failures occur because an unusual combination of reasonably common events occurs, and that unusual combination of events has a bad outcome. For example, a moderately high reservoir inflow occurs, but nowhere near extreme; the sensor and SCADA system fail to provide early warning for some unanticipated reason; one or more spillway gates are unavailable due to maintenance, or an operator makes an error, or there is no operator on site and it takes a long time for one to arrive; and the pool was uncommonly high at the time. This chain of reasonable events, none by itself particularly dangerous, can in combination lead to an incident or even a failure. This leads to the unnerving conclusions that; our estimates of risk made in terms of best available practice using the best available estimates will be underestimates of the actual risk, and the extent to which we underestimate the risk is unknowable. This paper examines why these improbable events occur and what can be done to prevent them. Some implications with respect to the endeavour of risk evaluation are also considered.
Earthquakes are a well-known threat to the safety of dams. While this threat is subdued for Australian Dams, the potential for earthquake induced failure of a dam requires risk minimisation in the downstream community through monitoring and emergency response procedures. This paper details WaterNSW’s approach to their development of a Seismic Monitoring Strategy which was to align the business and ensure an appropriate post-seismic response.
The strategy also identifies that a proactive approach to seismic instrumentation can be taken to reduce business risk by aiding decision making should a dam be in a damaged post-seismic state.
The interim outcome of implementing the Seismic Monitoring Strategy resulted in a fast emergency
response time and less overreaction/distraction of dam safety resources in insignificant seismic events. There is opportunity for other Australian dam owners to implement similar systems to = WaterNSW and achieve similar results.
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.