Dambreak & Consequences (September 2013) – MODULES 1 to 5
An understanding of the consequences of dam failure is essential in dam safety emergency planning and as an input to risk assessment. In recent years there has been significant advances in hydraulic modelling and access to high quality elevation data which has revolutionised dambreak modelling. The advent of risk based approaches has increased the focus on estimating the consequence of dam failure and particularly the potential loss of life. The method developed by the USBR in 1999 has had widespread application in Australia and in recent years a number of more sophisticated simulation approaches have been developed. This session will cover the latest developments in dambreak modelling and the estimation of potential loss of life from dam failure.
This course is designed to present the state of practice on these matters for dam safety risk management. The 2 days are designed for both experienced and less experienced dam owners, regulators and consultants.
Includes access to the following videos:
$0.00 - $80.00
Peter Hill, David Stephens, Kelly Maslin, Rachel Brown, Simon Lang, and Chriselyn Meneses
There has been a growing awareness of the potential dam safety risks associated with hydraulic structures in urban environments such as retarding basins, water quality detention basins and recreational lakes. This has required estimates of rare and extreme floods for urban catchments and there are a number of important characteristics of urban catchments which distinguish them from rural catchments such as impervious areas, lack of streamflow data, blockage of structures and complex hydraulics. This paper describes the key considerations for flood estimation in urban catchments and draws examples from a number of current flood studies for urban catchments in Canberra.
Peter Hill and Rory Nathan
The ANCOLD Acceptable Flood Capacity (AFC) guidelines were published in 2000 and provide guidance on the selection of design flood capacities for dams and specifically a deterministic fallback provision for spillway capacities. Since the guideline was published, there has been a continual evolution in dam safety management practices and related guidelines, including the 2003 ANCOLD guidelines on risk assessment and the current revision of Australian Rainfall and Runoff by Engineers Australia. This paper describes the scope of the current AFC guidelines and perceived opportunities for refinement. A survey of users was used to test and identify issues and gauge the need for the guideline to be updated. A number of topics were identified that would benefit from clarification or further guidance. These topics include consistency with other ANCOLD guidelines, clarity on the selection of the AFC, definition of the dam crest flood, freeboard and application to gated structures.
Simon Lang, David Stephens, Peter Hill, Mark Arnold and Tommie Conway
Considerable thought has been given in recent years to managing the risks associated with floods during the construction of new dams and dam upgrades. Both ANCOLD and the NSW DSC provide some limited advice on how this risk should be managed, with many dam owners aiming for societal risk during construction to be no higher than pre-construction. One approach to do this is to draw down the reservoir such that sufficient airspace is created to reduce the probability of overtopping the construction works to be equal to that of overtopping the dam crest pre-construction. However, this frequently leads to very large releases of valuable water resource being required. This approach also fails to consider that the conditional probabilities of failure may be quite different during construction than during normal operation. A risk-based approach was applied for the recent upgrade of Tarago Reservoir. Existing event trees from a failure modes analysis were adjusted to reflect the construction conditions. In some cases, the event probabilities increased (for example as a result of excavation of the dam embankment), however some also decreased (for example as a result of more rapid means of detecting and intervening in breach formation during construction). The conditional probabilities of failure during construction were then used to estimate the overall seasonal probability of failure, and it was found that a limited draw down of the reservoir would be sufficient to ensure that risks were no higher during construction than pre-construction. To reinforce this, the cost-to-save-a-statistical life was estimated for further drawdown of the reservoir and used to demonstrate that the risks were as low as reasonably practicable.
Simon Lang, Peter Hill, Wayne Graham
The empirical method developed by Graham (1999) is the most widely used in Australia to estimate potential loss of life from dam failure. It is likely to remain that way while spatially based dynamic simulation models are not publicly available (e.g. LIFESim, HEC-FIA and LSM). When the Graham (1999) approach was first developed the prevalence of spatial data and the speed of computers was much less. In addition, most people did not have mobile phones, social media was in its infancy, and automatic emergency alert telephone systems were 10 years from being used in Australia. Graham (1999) was intended to be applied to populations at risk (PAR) lumped into a discrete number of reaches. The selection of fatality rates for the PAR in each reach was based on average flood severity and dam failure warning times. Today, there is typically much more spatially distributed data available to those doing dam failure consequence assessments. Often a property database is available that identifies the location of each individual building where PAR may be, along with estimates of flood depths and velocities at those buildings. News of severe flooding is likely to be circulated by Facebook, Twitter and e-mail, in conjunction with official warnings provided by emergency agencies through radio and television and emergency alert telephone systems.
This raises the question of how Graham (1999) is best applied in today’s digital age. This paper explores some of the issues, including the estimation of dam failure warning time, using Graham (1999) to estimate loss of life in individual buildings and the suitability of Graham (1999) for estimating loss of life for very large PAR.
Keywords: loss of life, dam safety, risk analysis.