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:
Rory Nathan, Peter Hill
This paper provides an overview of the different simulation frameworks used for the estimation of design floods.. For small events the behaviour of many flood modifying factors is highly variable and chaotic, whereas as the magnitude of the event increases so does the organising influence of the dominant meteorologic conditions. The approach to design flood estimation will depend upon the availability of data and the exceedance probabilities of interest. The techniques can vary from frequency analysis of the data recorded at a site to rainfall-runoff modelling with design rainfall inputs derived from regional frequency analysis. For extreme floods, which are of relevance for assessing flood loadings for dams and the assessment of spillway adequacy, the stochastic (Monte Carlo) approach offers a number of advantages over the traditional deterministic approach. Although there has been significant progress in design flood estimation practice in Australia over the last couple of decades there remains many significant research and training needs.
Tommie Conway, Katherine Miller, Peter Hill
The ‘Black Saturday’ fires of the 7th of February 2009 and the continuation of fires over the following weeks had devastating human, environmental and financial costs for Victoria. Many of Melbourne’s water supply catchments and assets were burnt and the major harvesting catchments were seriously threatened. This paper highlights the need for owners and managers of catchments, dams and associated infrastructure to better understand and plan for the potential impacts of fire, given its predicted increased likelihood and severity due to climate change.
The paper will share Melbourne Water’s recent experiences of the fire, the scale of the impact to the business in terms of assets damaged and catchments affected, the extent of the burn and the threat that was faced. It will describe Melbourne Water’s experience with the United States Burnt Area Emergency Response (BAER) team to expediently map the severity of the fires, to identify areas of concern and prioritise fire recovery works. Of interest to those involved in risk management will be the discussion of the construction flood risk analysis at Tarago Reservoir which was revisited due to severe fire damage to the catchment.
Keywords: fire impact, Melbourne’s water catchments, BAER team, hydrology, Tarago, construction flood risk analysis
Simon Lang, Chriselyn Meneses, Kelly Maslin, Mark Arnold
It is now common practice for dam owners in Australia to take a risk based approach to managing the safety of their large dams. Some dam owners are also using risk based approaches to manage other significant assets. For example, Melbourne Water manage the safety of their retarding basins in a manner similar to their water supply dams.
Assessing the risks posed by retarding basins using methods developed for larger dams can raise challenges. For example, the Graham (1999) approach to estimating potential loss of life (PLL) is generally applied when estimating the consequences of dam failure. However, Graham (1999) may not be the most suitable model for estimating PLL downstream of structures with relatively low heights and storage volumes (e.g. retarding basins), given the characteristics of the case histories used to develop the method.
In this paper six potential methods for estimating PLL are tested on four retarding basins in Melbourne. The methods are Graham (1999), the new Reclamation Consequence Estimating Methodology (RCEM), the UK risk assessment for reservoir safety (RARS) method, a spreadsheet application of HEC-FIA 3.0, and empirical methods developed by Jonkman (2007) and Jonkman et al. (2009). Results from the methods are compared, and comment is made about which is most suitable.
Keywords: potential loss of life, dam safety, risk analysis, retarding basins.
David Stephens, Kristen Sih, Peter Hill, Rory Nathan, David Dole
The spring and summer of 2010-11 were characterised by severe flooding affecting much of Victoria. In a number of cases, communities downstream of large dams developed to supply water for irrigation and critical human and stock needs were significantly impacted. Following the floods, the Victorian Government commissioned the Victorian Floods Review (VFR) to consider the total warning and response to these floods. Whilst dam operations were not specifically included in the terms of reference, overwhelming community interest lead to the VFR commissioning a high level review of the way a number of key dams were operated during the floods. This review identified some of the inherent tensions in the legislative framework for water harvesting, storage and dam safety in Victoria. These tensions were often matched by the conflicting expectations of the public living immediately downstream of the dams versus those dependent on the water resource stored in the dams. The final report of the VFR was handed down in December 2011 and contained a number of recommendations specifically for dam owners. These recommendations are reviewed and discussed in light of both the legal and public relations ramifications for owners and operators of large water supply dams. An overview is also given of the operational constraints to downstream flood mitigation facing many dam owners. Such constraints are typically imposed by the type of dam (i.e. fixed crest), relatively small storage and outlet capacities when compared to flood volumes and limitations on the reliability of forecast rainfall information. Some possible ways of overcoming these constraints are identified and discussed.
Keywords: Flood, mitigation, Victorian Floods Review