Safer dams are a matter of priority
February 14, 2019
Examples from around the world demonstrate the devastating consequences of dam failures. Safety must be every dam owner’s key concern, but how should action be prioritised across a large portfolio of dams?
To prioritise effort and resources to achieve the best safety result across a whole portfolio of dams, you need a portfolio risk assessment (PRA). A PRA determines the risk position of the dams based on known information, identifies any information gaps, develops a strategy to close these gaps, and then determines the most effective actions to decrease any risks.
APPLYING PRA TO A LARGE AND COMPLEX PORTFOLIO
Entura has supported dam owners and water managers across the Indo-Pacific region with PRAs, but our most extensive application of the PRA process has involved the 54 large dams of our parent company, Hydro Tasmania.
Hydro Tasmania is Australia’s largest water manager and is committed to ensuring that the risk of a dam failure is very, very low across the entire portfolio. Across so many dams, clear priorities are needed to focus dam safety efforts and human and financial resources.
It has now been 20 years since Hydro Tasmania’s PRA journey began in 1999, so it’s timely to reflect on its outcomes.
With so many dams of greatly varying types, ages and heights, the PRA across Hydro Tasmania’s dams was always going to be complex, and needed to be staged. The first step was a small pilot study on five selected dams that represented the range of potential risks within the broader portfolio.
During the pilot study, the five steps of Entura’s PRA process were defined:
This methodology was applied across Hydro Tasmania’s dams portfolio, and an average of eight dams were added to the review each year.
By 2005, the initial ‘baseline’ assessment of the full portfolio was complete. The focus of the dam safety program has now moved to investigation and implementation of upgrades, and the communication of outcomes to senior management.
The PRA process has increased the focus on potential failure modes and risk as drivers of the dam safety program and as the basis for deciding priorities for allocating operational and capital resources.
DETERMINING PRIORITIES THROUGH A RISK FRAMEWORK
Entura’s PRA process reviews the consequences of failure of a dam by looking at the impact that it may have on downstream populations and infrastructure. The engineering assessment considers the effects on dams of extreme events such as floods and earthquakes, taking into account the specific site conditions. Combining the chance of failure and the resulting consequence determines the level of risk.
Hydro Tasmania assesses, prioritises and mitigates risks across the business using an integrated business risk management program, and the dam PRA feeds into this overall risk management approach. A dam’s assessed risk rating across common tolerance criteria drives the risk management response. The assessed dam risks are plotted together on a chart to provide a risk profile for the whole portfolio. This allows dam safety risks to be compared, understood and communicated readily throughout the business in a similar way to all other business risks.
The initial objective of the dam safety program is to reduce all the risks categorised as ‘high’ or ‘extreme’ as soon as practical , and then to continue with a program of investigations and capital works to diminish risks even further. Actions for dams lying in the higher risk zones did not wait for completion of the PRA, but were initiated as soon as risks were identified.
Some cost-effective and expedient risk-mitigation was achieved by identifying and implementing ‘quick wins’. These early actions reduced the overall portfolio risk while more complex mitigation plans were being developed. In some cases the ‘quick win’ actions have even provided the ultimate solution. In other cases, more major works have been required.
PROGRESSING THE DAM SAFETY JOURNEY
The PRA process has substantially benefited Hydro Tasmania’s dam safety program, by improving understanding of the dam portfolio, underpinning a strong strategic plan for addressing risks, improving surveillance and monitoring, and considerably strengthening dam safety emergency planning and warning.
However, this isn’t the end of the dam safety journey. Knowledge of any dam is never complete, and it is critical for dam owners to remain aware that not every failure mode may necessarily have been identified in a baseline study that relies on existing information. There may still be a level of uncertainty about the ‘unknowns’.
For Hydro Tasmania’s PRA, identifying these uncertainties enabled development of a prioritised list of investigations necessary across the portfolio. These detailed investigations have been critical to the development of the dam safety program, by confirming any potential failure modes identified in the PRA.
The list of potential failure modes of a dam portfolio must be rigorously and regularly reviewed, and investigations to reduce uncertainty about the portfolio should be ongoing. New methods and techniques for analysis are being developed all the time, and it is important to understand how these may change existing risk assessments. As well, the safety and risk-level of a dam can change as dams age, or when there are changes to the way the dam is managed.
It is also important to realise that the capital works program for dam safety risk reduction across a portfolio must remain flexible and be actively managed to respond to new or changed risks, new developments in the field of dam engineering, shifts in business priorities, delays to projects, and new developments in risk management.
The sheer number and variety of types, ages and consequence categories of Hydro Tasmania’s dams made Hydro Tasmania’s PRA a challenging process, but the benefits are substantial. The baseline study completed in 2005 is not the end of this journey, which continues to prioritise actions, reduce risks and enhance safety across the portfolio.
If you would like to discuss how we can assist you with assessing your dam risks, developing a resource-effective and comprehensive dam safety program, or applying the same PRA process to other key assets, please contact Paul Southcott, Richard Herweynen or Phillip Ellerton.
About the author
Paul Southcott is a specialist civil engineer at Entura. He has more than 32 years of professional expertise in civil and dam engineering, as well as expertise in geotechnical, foundation, structural, hydraulic and hydropower engineering. Paul’s dam engineering experience spans geotechnical and hydrological investigation; feasibility and options studies; concept, preliminary and detailed design; engineering assessment, consequence assessment and risk assessment; safety reviews; monitoring and surveillance; and emergency planning. He has extensive experience in dam risk assessment including as project manager for Hydro Tasmania’s, Taswater’s and SAWater’s portfolio risk assessment projects. He was a member of the ANCOLD committee that issued the Guideline on Consequence Categories for Dams in 2012 and is currently a member of the ANCOLD committee drafting the new Guideline on Geotechnical Investigations for Dams.