The UN has warned that in 2050, dams will be a big …

Research by the United Nations University Institute on Water, Environment and Health (UNU-INWEH) predicts that by 2050 the majority of the world’s population will be threatened by tens of thousands of people. large dams built in the 20th century that are already functioning beyond their useful life.

According to the study, most of the 58,700 large dams in the world were built between 1930 and 1970 with a lifespan of between 50 and 100 years. The problem is that by the age of 50, a large concrete dam would “probably start to show signs of aging.”

These signs of deterioration can be seen in the increasingly frequent cases of dam failures, increased repair and maintenance costs, increased sedimentation of reservoirs and loss of functionality and efficiency of a dam. , a set of “strongly interconnected” events.

The report in turn indicates that well-designed, constructed and maintained dams can “easily” reach 100 years of service, although it predicts an increase in their “dismantling”. This phenomenon, which is gaining ground in the United States and in Europe, is due to economic and practical limitations which prevent dams from being improved, either because they are old or because their original use has become obsolete.

The most affected dams

The volume of water stored behind large dams around the world is estimated to be between 7,000 and 8,300 cubic kilometers. The document also gives an overview of dams by region and their main function: water supply, irrigation, flood control, hydropower and recreation.

It also details the increasing risk of older dams, such as increased maintenance costs, decreased functionality due to sedimentation, benefits of restoring or redesigning natural environments and social impacts, which must be evaluated by decision-makers. “The nature of these impacts varies considerably between low and high income countries,” he notes.

At another level of analysis, we include case studies of dismantling or aging dams in the United States, France, Canada, India, Japan, Zambia and Zimbabwe.

“This report aims to draw worldwide attention to the progressive problem of aging water storage infrastructure and to spur international efforts to address this emerging risk. and water culture, ”explains Vladimir Smakhtin, research co-author and director of UNU-INWEH.

“This highlights the fact that the increasing frequency and severity of flooding and other extreme environmental events can exceed the design limits of a dam and accelerate the aging process of a dam. Therefore, decommissioning decisions must be made in the context of a changing climate, ”he adds.

For his part, Duminda Perera, lead author of the report, declares that “this problem of the aging of large dams concerns a relatively small number of countries today: 93% of all large dams in the world are located in just 25 nations ”.

“The construction of large dams began in the mid-twentieth century and peaked in the 1960s and 1970s, especially in Asia, Europe and North America, while in Africa the peak occurred in the 1980s. The large dams built thereafter diminished continuously and gradually, ”he adds.

The environmental and social impact

According to research, the world is unlikely to become the scene of another great revolution in dam building as it was in the mid-20th century, but those that were built during this time will inevitably face the end of their useful life.

Currently, China has 23,841 large dams (40% of the world total), while another 32,716 (55% of the world total) are found in just four Asian countries: China, India, Japan and Korea. South. Most of these dams will reach the 50-year threshold relatively soon, as will many of those found in Africa, South America and Eastern Europe.

The rate of construction of large dams has slowed considerably over the past four decades and continues to decline in part because “the best locations for such dams around the world are steadily declining, as nearly 50% of the world’s river volume is fragmented. or regulated by dams ”, underlines the report.

In addition, the environmental and social impacts of dams, especially large ones, are of great concern, as are the ideas and practices that are emerging on alternative types of water storage, nature-based solutions and types of dams. energy production beyond hydropower.

The main reasons for decommissioning dams are public safety, increased maintenance costs, sedimentation of reservoirs and restoration of a natural river ecosystem.

Despite this, most of the dams that have been removed to date are small. The dismantling of large dams (defined by ICOLD as 15 meters or more from the lowest foundation to the top, or 5 to 15 meters, storing more than 3 million cubic meters) is “still in its infancy, with only a few known cases. in the last decade ”.

“Some case studies of large, aging and dismantled dams illustrate the complexity and length of the process often required to organize the safe removal of the dam,” says Allen Curry, co-author of the research.

“Even removing a small dam requires years, often decades, of continued expert and public participation, as well as extensive regulatory reviews. With the massive aging of dams underway, it is important to develop a framework of protocols that will guide and accelerate the process of dam removal, ”Curry analyzes.

The expert warns that decommissioning will also have several positive and negative economic, social and ecological impacts that must be considered in a local and regional social, economic and geographic context “essential to protect the broader objectives of sustainable development of a region.

“In general, the decommissioning of dams should be considered as important as the construction of dams in the overall planning process of water storage infrastructure developments,” says Curry.

“Ultimately, value judgments will determine the fate of many of these large water storage structures. It is not an easy process and therefore learning lessons and sharing experiences of dam decommissioning should be a common global goal. Lack of this knowledge and lack of reflection on relevant regional / national policies / practices can gradually and negatively affect the ability to properly manage water storage infrastructure as it ages, ”the report concludes.