Dam safety: protecting lives and driving efficiencies
March 26, 2015
Humans have been damming rivers since 3000 BC for water retention and supply.
People can fish in the lakes formed by dams, sail on them, swim in them. They are important to life, providing water supply, irrigation for crops and power generation. But if a dam fails, causing devastating flooding downstream, the results can be catastrophic.
The worst dam failure in history occurred in China in 1975 when the Banqiao Reservoir Dam and other dams in Henan province failed, killing an estimated 171 000 people, with 11 million people losing their homes.
Safety is the key
Dam safety is a crucial aspect of modern dam building. Ensuring the safety of existing dams is a key concern of any dam owner, whether the dam is for generating power, protecting a nearby population from potential flooding or for the age-old reason of water supply.
Specialist power and water consulting firm Entura provides dam safety programs for Gordon Dam, the largest concrete arch dam in Australia, and other assets owned by Entura’s parent company, Hydro Tasmania, and for government and corporate clients around Australia, south-east Asia and southern Africa.
Hydro Tasmania is Australia’s largest water manager, so it needs to ensure that the risk of a failure across the portfolio is very, very small. With 55 large dams and many smaller ones, Hydro Tasmania needs to be able to identify the highest risk dams, so it is able to prioritise work on maintenance and refurbishment. This is achieved using a process undertaken by Entura called a portfolio risk assessment (PRA).
Hydro Tasmania is committed to the safety of its assets for the people of Tasmania and its PRA enables the business to prioritise where it focus its financial resources, management and dam safety efforts.
What’s the level of risk?
Energy and water asset owners are increasingly choosing to implement a dam safety program to quantify and manage operational risks, evaluate high risks and find solutions for identified risks or deficiencies. Entura believes that a risk-based framework like a PRA is the best way to achieve an efficient and effective dam safety program.
Older dams or dams located upstream from populated areas are often high risk. The inspection regime for high-risk dams is understandably more robust and rigid than for low risk dams. However, it’s not just ageing that creates risks for dams. In Australia, the methods used to predict and anticipate floods are equally important, as is the increased knowledge that we’ve gained of the various causes and mechanisms through which dams can fail.
Entura’s PRA reviews the consequences of failure of a dam by looking at the impact that it may have on downstream populations, and also looks at the chance of failure occurring by considering extreme events such as floods and earthquakes, taking into account the specific site conditions. It is the combination of the chance of failure with the resulting consequence that determines the level of risk.
A world-wide issue
A number of dam owners across the Asia-Pacific region are now looking at Entura’s PRA program, especially when they have a significant dam portfolio already developed. Our clients in the region are exploring how they can best use their resources and how to prioritise expenditure on their assets.
Having undertaken safety reviews and dam safety work in the Philippines, China, Indonesia, Malaysia, New Zealand, Papua New Guinea, Fiji and India, Entura understands the typical safety needs and drivers of dam owners across the world.
Australia has well-developed dam safety legislation in which Malaysia, for example, with its increasing number of dams, is very interested. Malaysia has experienced the tragic results of a dam safety incident, with the death of a number of people due to water releases through the spillway gates of Sultan Abu Bakar Dam.
On behalf of the Ministry of Public Utilities, Sarawak, Malaysia, Entura used its dam risk framework, similar to that used in a PRA, to assess whether the Bakun and Murum dams were ready to be impounded as construction reached conclusion. This dam risk framework looked at the potential scenarios that could lead to the sudden release of reservoir water, the likelihood of these scenarios occurring, and the corresponding consequences. Through this process the key dam safety risks were identified, and actions were determined to either eliminate or reduce the risks to an acceptable level. These actions needed to be addressed prior to impoundment.
Dam safety is an ongoing concern
The safety and risk-level of a dam can change with time. As dams or assets age, the way they are managed needs to change and they often need upgrading to ensure continued safe operation and desired life expectancy. But undertaking upgrades can potentially increase the risk of the dam during the construction phase – due to the removal of shoulder material from an embankment, or restricting the discharge capacity in a spillway chute. To contain the risks during the construction phase, operational practices might need modification, for example, operating a dam at a lower lake level during the period of construction to ensure the overall risk is not significantly increased. This is why it is so important to have a plan to manage the safety of the dam during its entire lifecycle.
Five steps to assess dam safety risks
Risks can be reduced in a range of ways and, in many cases, a number of risk treatment options are used together:
- reducing the likelihood of the risk occurring (e.g. strengthen a dam)
- reducing the consequences if the risk does occur (e.g. upgrade emergency planning)
- staging risk treatments (e.g. address most cost-effective risk mitigation works first)
- balancing the resources required to treat a risk with the benefits of reducing the risk (e.g. examine various levels of investment and their respective risk reductions).
Entura’s portfolio risk assessment approach has five key steps:
|1 – Flood hydrology||Flood hydrology is a key input as it is used in both the engineering and consequence assessments. Generally the hydrology is based on current best practice. For some low-hazard dams, an innovative regional flood hydrology approach has been developed that provides sufficiently robust answers at minimal cost.|
|2 – Engineering assessment||The engineering assessment is a critical part of the process as it requires all potential failure modes to be identified and then the probability of failure to be estimated for each mode. The engineering assessment gathers all the information about the dam including design, construction, geology and operations that may assist in defining the potential failure modes. This is presented as response curves (i.e. conditional probability of failure). The level of detail varies with the hazard category of the dam (i.e. a high-hazard-category dam receives more intense scrutiny than a low-hazard dam). The engineering assessment also ensures an assessment of compliance with current engineering standards.|
|3 – Consequence assessment||Key steps in the consequence assessment method include developing a dam break model; mapping the inundation zones; assessing the population at risk and probable loss of life; assessing the financial and economic losses; and qualitatively assessing the environmental, social and intangible consequences.|
|4 – Risk assessment||The risk assessment uses event trees that evaluate societal and individual life risk and financial risks. Risks can be aggregated across a range of load conditions and failure modes to give a total risk position, retaining visibility of the scenarios that contribute to the total risk. Where the risks are unacceptable, upgrade concepts are developed and their impact on reducing risk is plotted.|
|5 – Reporting||The results of the risk assessment are reported, summarising the potential failure modes, consequences and risk profile for each individual dam. Summary data across the portfolio is also presented, including information on the failure modes, and risk of all the dams against societal life risk and financial risk criteria in a simple but meaningful format.|
All businesses need to share resources across many activities besides risk mitigation works. For a hydropower company, a significant reinvestment in generating plant is required to both manage risk and maintain asset capability. Resources are also needed for investment for growth and diversification.
Within the dam portfolio, it is important to ensure that resources, both human and financial, are not applied to risk mitigation projects at the expense of the fundamentally important surveillance and monitoring activities. A balance is needed between reducing risk and maintaining and enhancing surveillance and monitoring instrumentation.
An ongoing effort
Finally, it is important to realise that the capital works program for dam safety risk reduction is not a static program, locked in and then implemented without change. Rather, it is an active program that is reviewed regularly, actively managed and needs to remain responsive to new or changed risks, developing understanding of dams engineering, shifts in business priorities, delays to projects, and new developments in risk management processes.
The same process can be applied to other assets
The PRA process can also be used for other assets that have a low chance of failure but high consequences if failure were to occur. At Entura, we have successfully applied our PRA process to penstocks and pipelines, and to canal systems, identifying key risks and prioritising the actions needed in a way that most effectively manages resources.
If you would like to discuss how we can assist you with assessing your dam risks or developing a resource-effective and comprehensive dam safety program, or apply the same PRA process to other key assets, please contact Richard Herweynen on +61 3 6245 4130.
About the author
Richard Herweynen is Entura’s Principal Consultant in Civil Engineering. Richard has 25 years of experience in dam and hydropower engineering, and has worked throughout the Asia-Pacific region on both dam and hydropower projects. In recent years, Richard has led the design of three roller-compacted-concrete (RCC) dams within Australia and a number of significant dam upgrades. Richard was part of the ANCOLD working group which updated the guidelines for concrete gravity dams, and is the Chairman of the ICOLD Technical Committee on Engineering Activities in the Planning Process for Water Resources Projects. Richard has won many engineering excellence and innovation awards, and has published over 30 technical papers on dam engineering including dam safety and risk assessment, RCC dam design and the unique challenges of older style post-tensioned anchors.