Climate change is stressing large reservoirs, recent failures and near misses are encouraging countries to update their regulatory approaches. A comprehensive risk assessment can help dam owners and operators in identifying, quantifying and managing associated risks, enabling analytically informed decisions and increasing confidence with regulators and insurers.
Dams help to regulate water levels, protect land from flooding, and provide water for drinking, industrial processes and irrigation. They are also commonly used to facilitate navigation and generate hydroelectricity. Dams are of considerable benefit to society, but they are capable of wreaking great destruction should they fail.
Globally there over 200,000 dams. Of these, around 50,000 are above 15m high or store more than 3,000,000 m3 of water and are therefore considered by the International Commission on Large Dams (ICOLD) as large[1].
The paradigm of a high-risk dam is one which is tall, poorly constructed, operated and maintained, and holds a large volume of water close to a community and infrastructure. In 2023 storm Daniel caused two dams in Libya to fail, killing over 8,000 people and destroying a quarter of the city of Derna.
Source : Alamy
Because of these risks, most nations have developed their own versions of dam safety legislation and risk classification. Owing to recent dam failures and near misses and the impacts of climate change, several countries are updating their regulations.
It is fair to say that once a dam is constructed and filled, it will start the agony of failure. However, through their actions, the operator can prevent failure in most circumstances. This article looks at how a responsible owner might manage one or a whole fleet of water storage reservoirs safely.
Dams may be built using various materials and methods. This ranges from earthen or rockfill embankments to concrete dams of several types, each having its own advantages and disadvantages. The key features of a reservoir are the water retaining structure itself, a channel or conduit to pass normal flows including most floods, a means of drawing the reservoir down, and an emergency spillway to pass exceptional flood flows.
Since the great acceleration of the 1950s, the number of dams, in particular large ones, has increased significantly, with construction peaking in the 1970s. During that decade, the failure rate was around 15 per year, a number which reduced to single digits by the turn of the twenty first century. Recently there has been a slight increase both in the number of reservoirs constructed and, in the number failing[2]. In both cases this is because one of the most hazardous times in a dams life is in the first few years after first filling. This is because construction and design defects are revealed pretty quickly once the water is added. Recent analysis of dam failures by the Association of State Dam Safety engineers revealed that since the year 2000 an average of 5 high and significant consequence dams in the U.S. have failed each year so far this century[3]. This has often been caused by a shift in the clustering of rainfall events, with sequential events working together to cause spillways to be exceeded.
Some reservoirs are exceptionally old, having been in continuous operation for nearly two thousand years. Though age is not the defining characteristic for a structure’s risk profile, it is important to consider that an older structure will not have been designed to contemporary safety standards. Also, they may not be able to cope with increasing frequency and intensity of rainfall events driven by climate change[3]. A peculiar feature of older dams is a tendency for post construction development downstream to increase the risk profile.
In a 2021 report[4] the American Society of Civil Engineers assessed infrastructure and ranked the nation’s dams as D grade, that is in poor condition and at risk of failure. Furthermore, the U.S. Association for State Dam Safety Engineers[5] estimated that non-federal owned dams require over $75 billion in investment, $24 billion of which is for high hazard dams. This shows that even the most advanced economies can struggle to keep assets in tip top condition.
So, in a changing climate and with ageing assets, what steps should a dam owner take to ensure the safety of their asset or portfolio of assets?
A structured approach to reservoir safety can enhance the benefits accruing to these assets and greatly reduce the risk they pose to society. The first step is to draw up a list of what might cause each reservoir to fail. In some locations this could be earthquakes, it might be structural deterioration or the slow seepage of water eroding the dam from the inside. There are many drivers of failure, but assessing the reservoir’s capacity to pass extreme flows in the case of an extreme precipitation event will be a given.
Once a realistic list of failure modes has been identified, the likelihood and consequence of failure should be assessed. Modeling can assess the impacts of a sudden release of water to property, people, infrastructure and the environment and the likely loss of life. In some cases emergency draw down scenarios designed to save a dam from catastrophic failure may also be damaging and require assessment.
The operator will now have sufficient information to take a risk-based approach to managing a single dam or a portfolio of structures. There are a wide range of options, perhaps through enhanced or specific maintenance, or with mitigating measures like emergency planning and preparedness. Such plans should include outreach to local authorities and communities to allow them to prepare appropriate actions and should be tested, exercised and continuously improved.
A programme of inspections and maintenance should be drawn up. The inspections might be visual or through instrumentation to allow timely identification of likely failure modes. They could also include enhanced upstream hydrological monitoring or instigation of satellite or land-based monitoring of the assets. In some circumstances they include physical investigations or excavation.
The dam failure modeling can also be used to quantify the third party Property Damage and Business Interruption (PDBI) losses, as well as Third Party Liability losses. Due to the unique nature of assets being modeled, a more bespoke methodology may be required to accurately quantify a dam operator’s risk profile.
To manage damage to the structure and associated infrastructure it is also be prudent to use natural catastrophe models to understand potential first party losses. These are heavily used within the insurance industry to forecast PDBI losses from natural catastrophe events and may need to be adjusted to reflect an understanding of dam design tolerances. These adjustments should capture the likelihood of scenarios like overtopping in the case of extreme inflow rates and potential structural damage caused by earthquake events.
“We have recently collaborated with a dam operator in New Zealand to quantify their PDBI first party losses where insurance market recognised catastrophe vendor models were adjusted to provide a more accurate and tailored view of risk taking into account the designed flood return period of the dams.”
All this detailed modeling will help dam operators ensure they have appropriate insurance arrangements in place in the case that dams fail or there is substantial business interruption. Insurance arrangements should cover losses incurred from direct damages to the dam infrastructure, as well as potential liabilities from property damage and loss of life from downstream flooding.
With a thorough risk assessment and effective control measures in place—through engineering, operations, or insurance—risk managers can sleep more soundly at night.
A comprehensive dam break modeling assessment will help dam operators to understand the risks and associated losses these catastrophic events can cause. This will help inform and refine maintenance regime as well as emergency response plans and business continuity plans. In addition, and from the risk management perspective, this will also help to make informed decisions around appropriate insurance purchasing and the impact these losses can have on our clients balance sheet and reputation.
