Over 85% of dams in US operating at or beyond life expectancy, study shows why they are a growing threat

Older dams combined with poor maintenance represent a higher risk to public safety, particularly for downstream areas


                            Over 85% of dams in US operating at or beyond life expectancy, study shows why they are a growing threat
(Getty Images)

Aging dams are an emerging risk, according to scientists. They project that most people globally will live downstream of large dams by 2050, which were built in the 20th century and many of them are already operating at or beyond their design life, say scientists. In the US, for example, over 85% of the dams (over 90,000 of all sizes) in 2020 were operating at or beyond their life expectancy. Dam failure risks the lives of people living downstream, and aging dams should be investigated to assess the threat, recommend investigators. 

Water storage infrastructure, particularly large dams in the last 100 years, has traditionally been used to regulate river flow worldwide. Large dams are defined by the International Commission on Large Dams (ICOLD) as having a height of 15 meters or greater from lowest foundation to crest, or a dam between 5 meters and 15 meters impounding more than 3 million cubic meters.

The new analysis by United Nations University (UNU) says that most of the 58,700 large dams worldwide were constructed between 1930 and 1970 with a design life of 50 to 100 years, adding that at 50 years, a large concrete dam “would most probably begin to express signs of aging.”

The median age of large dams by country (UNU-INWEH))

The aging signs may include increasing cases of dam failures, progressively increasing costs of dam repair and maintenance, increasing reservoir sedimentation, and loss of a dam’s functionality and effectiveness.

“These manifestations are strongly interconnected. Age per se is not a decisive variable for action. Yet, age is the simplest ‘macro’ metric by which dams can be characterized and compared, in the context of their diminishing effectiveness, increasing risks, and impacts for the economy and the environment – in time,” write authors. 

The aging signs may include increasing cases of dam failures, progressively increasing costs of dam repair and maintenance, increasing reservoir sedimentation, and loss of a dam’s functionality and effectiveness (Getty Images)

Well-designed, well-constructed, and well-maintained and monitored dams can reach 100 years, but many of today’s large dams were built long before the risks of the climate crisis became clear. Extreme weather events, especially floods, are expected to become more severe and frequent with the changing climate. Consequently, these events increase the threat to aging large dams designed using historical hydrological data. Put simply, aging increases the vulnerability of a dam to changing climate due to exposure to more frequent and extreme floods and/or increasing evaporation from the reservoir that can lead to accelerated loss of its function, they caution.

“This report aims to attract global attention to the creeping issue of aging water storage infrastructure and stimulate international efforts to deal with this emerging, rising water risk. Underlined is the fact that the rising frequency and severity of flooding and other extreme environmental events can overwhelm a dam’s design limits and accelerate a dam’s aging process. Decisions about decommissioning, therefore, need to be taken in the context of a changing climate,” advises co-author Vladimir Smakhtin, director of UNU’s Institute for Water, Environment and Health (INWEH) in Canada.

What are the findings?

The report reveals that many large dams worldwide have reached or are approaching the lower bound (50 years) of their anticipated lifespan. North America and Asia together hold approximately 16,000 large dams in the range of 50-100 years old and about 2,300 large dams over 100 years old. In the US, the average age of all the 90,580 dams (of all sizes) is 56 years. About 75% of US dam failures have occurred after 50 years of age.

Large dams by country (UNU-INWEH)

“Canada, Mexico, and the US are among the global leaders in large dam numbers but aging water storage infrastructure is most prominent in the US. It has greater than 90,000 registered dams, of which over 9,000 are large dams. Approximately 80% of all dams are over 50 years old as of 2020,” the findings state.

The team says that The American Society of Civil Engineers` (ASCE) infrastructure report card has repeatedly assigned the country a ‘D’ grade (implying poor/at risk) for the dangerous state of its dams, citing the need for an estimated $64 billion to adequately refurbish the nation’s dams. 

“This emerging issue was accentuated by the Oroville dam incident in California in February 2017, where the partial collapse of a spillway forced the evacuation of 200,000 people. This 50-year-old dam, the highest in the US at 235 meters, is critical to California’s water supply, and repairs are estimated at $500 million. The incident has been blamed on human error, specifically inadequate inspection and maintenance,” states the report. 

Most dams in the US are privately owned, and this leaves owners fully responsible for the costs of upkeep, leading many dams to be left abandoned due to unmanageable costs.

North American dams’ most common function is flood control, while the oldest dams, on average, are those used for hydropower. However, in nearly all functional categories, large dams’ average age in North America exceeds 50 years. In South America, large dams have not yet faced the same issue of widespread aging seen in other regions, but the average age in some functional categories is close to 50 years. 

Age of large dams by main geopolitical regions (UNU-INWEH)

What about public safety?

Scientists emphasize that dams, and large dams in particular, even if structurally sound, are considered to be “high hazard” forms of infrastructure because of the potential loss of human life that would be a consequence of failure, besides triggering forced displacement and the destruction of livelihoods. Development downstream of dams is persistent and thus elevates the magnitude of the consequences of dam failure. 

The experts emphasize that delaying the removal of certain aged dam structures could lead to “catastrophic consequences” with millions of people and their economies affected.

A time series of recorded dam failure accidents from 1950 to 2019 (UNU-INWEH)

“Dam failures, whether from excessive seepage (piping), cracking, overtopping, or structural failure, are frequently the result of poor design or construction, lack of maintenance, or operational mismanagement. Older dams combined with poor maintenance represent a higher risk to public safety, particularly for downstream areas. Therefore, the challenge is to reduce the probability of avoiding the potential consequences; this requires an effort to conduct risk assessments for aging dams,” they explain. 

Emerging trends

Dam decommissioning is not particularly new and yet it is still a relatively recent phenomenon, say investigators. Public safety, escalating maintenance costs, reservoir sedimentation, and restoration of a natural river ecosystem are among the reasons driving dam decommissioning.

Even if structurally sound, dams are considered to be “high hazard” forms of infrastructure because of the potential loss of human life that would be a consequence of failure, besides triggering forced displacement and livelihood destruction (Getty Images)

However, most dams removed to date have been small. Decommissioning large dams is “still in its infancy, with only a few known cases in the last decade.”

“A few case studies of aging and decommissioned large dams illustrate the complexity and length of the process that is often necessary to orchestrate the dam removal safely. Even removing a small dam requires years (often decades) of continuous expert and public involvement and lengthy regulatory reviews. With the mass aging of dams well underway, it is important to develop a framework of protocols that will guide and accelerate the dam removal process,” explains co-author and UNU-INWEH adjunct professor R Allen Curry, based at the University of New Brunswick.

Dam removal in the US (UNU-INWEH)

Decommissioning will also have various positive and negative economic, social, and ecological impacts to be considered in a local and regional social, economic, and geographic context “critical to protect the broader, sustainable development objectives for a region,” the study suggests. 

The authors note that overall, dam decommissioning should be seen as equally important as dam building in the overall planning process on water storage infrastructure developments. 

“Ultimately, value judgments will determine the fate of many of these large water storage structures. It is not an easy process, and thus distilling lessons from and sharing dam decommissioning experiences should be a common global goal. Lack of such knowledge and lack of its reflection in relevant regional/national policies/practices may progressively and adversely affect the ability to manage water storage infrastructure properly as it is aging,” they conclude.

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