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
Mark Pearse, Peter Hill
Risk assessments for large dams and the design of upgrades are often dependent on estimates of peak inflows and outflows well beyond those observed in the historic record. The flood frequencies are therefore simulated using rainfall-runoff models and design rainfalls. The recent update of Australian Rainfall and Runoff (ARR) has revised the design rainfalls used to model floods that are of interest to dam owners. This will change the best estimate of flood frequencies for some dams. However, for most dams the impact of revised design rainfalls on flood frequencies is small compared to other factors that can change (independent of dam upgrades). These include model re-calibrations to larger floods, changes to operating procedures that affect the drawdown distribution and improvements in how the joint probabilities of flood causing factors are simulated. In this paper, we look at how the design flood frequencies for some of Australia’s large dams have changed, the reasons for this and then identify five key questions for dam owners to ask to aid assessment of whether the hydrology for a dam should be reviewed
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.
Andrew Northfield, Simon Lang, Peter Hill
Melbourne Water currently manages more than230retarding basins (RBs). A large portion of these are less than 4 metres high, and traditionally structures of this size have not been subject to intermediate or detailed ANCOLD Consequence Assessments. However, the need to understand the failure consequences for smaller structures has increased over time, as risk based approaches to managing safety have expanded from large dams to other water retaining assets.
Undertaking detailed consequence assessments for all Melbourne Water’s RBs would not be practical, given the costs and time involved. Therefore, this paper describes a method for assessing the level of ANCOLD Consequence Assessment that is justified, based on the structure’s attributes. It also presents an equation that was used to estimate peak outflows from RB failure. The peak outflow estimates can be used to model approximate failure inundation extents downstream of small dams and RBs.
The paper draws on work that HARC have recently undertaken for Melbourne Water to assess the failure consequences for 88 RBs. The outcomes are relevant to other organisations that own or manage significant numbers of small water dams or RBs.
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
This paper presents a number of innovative hydrologic investigations undertaken for the recent
detailed design of upgrades for Ross River Dam in North Queensland. A key issue for estimating
extreme floods in the tropics is the estimation of flood events of long critical durations. The
implication is that there is an increased focus on estimating the correct volume (not only the peak
flow). This paper describes the regional analysis of flow volumes that was used to validate the
estimated flood volumes.
Another issue of considerable importance is the assumed relationship between inflows and initial
reservoir level. The analyses described in this paper showed that inflows are independent of reservoir levels for the more frequent events but for more extreme events they are correlated. This has important implication on how the initial reservoir level is incorporated in the hydrologic analysis. The final aspect covered by the paper is the derivation of seasonal flood frequency curves. This is particularly important given the highly seasonal nature of rainfalls in the tropics and the results are important for assessing risks during construction and scheduling the upgrade works.