More than one in five dams in the Himalayas are likely to experience overwhelming floods caused by the failure of rock embankments that impound glacier-fed lakes, according to the first systematic analysis of current and planned hydropower projects in the region.

With few downstream sites left, hydropower dams are rapidly spreading up Himalayan valleys, closer and closer to rivers’ headwaters, to meet rising electricity demand. Improvements in transportation and communication in otherwise remote and inaccessible regions have facilitated the expansion, which put more projects within reach of so-called glacial lake outburst floods (GLOFs). “The dams are getting closer to glacial lakes, making them more vulnerable to GLOFs,” says Wolfgang Schwanghart, a geologist at the University of Potsdam in Germany who is lead author of the analysis published July 8 in Environmental Research Letters.

GLOFs have wrecked havoc in the past. In 1981 the moraine ridge of Zhangzangbo, a glacier-fed lake in southern Tibet, collapsed, unleashing nearly 20 million cubic meters of floodwater. The torrent flowed across the border into Nepal where it destroyed the Sunkoshi hydropower dam and affected the highway connecting Kathmandu, Nepal, with Tibet for three years, with total losses close to $4 million.

Earlier research looked only at case studies of individual dams, which do not provide a big-picture assessment of the overall risk situation in the Himalayas. For their analysis, Schwanghart and his colleagues mined published papers, reports and official documents of dams in operation, under construction or planned in the Himalayas. They got sufficient information to evaluate 257 dams—more than a quarter of the total number in the region.

They then mapped those projects against the flood risk from more than 2,300 glacial lakes, revealing that in the event of a breach many dams could experience a deluge greater than they are designed to withstand. The analysis could not provide site-specific probabilities, however, but as hydroelectric projects push upstream, driven by strong demand, the risks and uncertainties grow. “The study provides an important bridge between science and social demand,” says Koji Fujita, a glaciologist at Nagoya University in Japan, who has been working in Nepal and Bhutan for decades. “[It] is critical to all stakeholders, especially communities that have to live with the risk.”

Schwanghart’s team was surprised by the high density of dams, especially in the states of Uttarakhand and Sikkim in India. “In some places there is a dam every 30 kilometers or so along the river,” he says. In extreme cases, planned projects place them only several kilometers away from glacial lakes whose waters are held back by rocky embankments.

Using computer simulations the team assessed how much floodwater might pour out of such lakes should the embankments fail and how far the water would likely travel. Estimated flood volumes varied by 100-fold, largely due to uncertainties in factors such as lake depth, breach depth and breach rate. For 56 dams, the volume of floodwater could exceed the limit they are designed to withstand—and some floods could affect sites as far as 100 kilometers downstream.

The analysis found that most hydropower projects pay little attention to GLOF risks in their hazard assessments and focused mostly on so-called meteorological floods, which are those caused by heavy rainfall. “This is worrying because in the headwaters GLOFs are a greater threat than meteorological floods,” Schwanghart says.

Pradeep Mool, a geologist and remote-sensing expert at the International Center for Integrated Mountain Development in Kathmandu, says the study is a good first attempt to address the issue on a regional scale but stresses that it is extremely challenging to determine the likelihood of individual lakes causing a damaging flood. In contrast, the methods for assessing the risks of meteorological floods are well established. “This is mostly due to a lack of field measurements, which are crucial for computer simulation,” says Mool, who has been involved in assessing GLOF risks of hydropower projects. Only a handful of studies, for instance, contain data on lake depths. “This has led to great uncertainties,” and so most companies resort to a statistical method based on previous GLOF events that is rather simplistic, he says. “A much more rigorous approach is urgently needed.”

Schwanghart’s study did not factor in climate change, which will make things worse, Fujita says. As global warning tightens its grip, glacial lakes will become bigger and more numerous due to rising ice melt. A study published last year in the journal Hydrological Processes found that the area of glacier-fed lakes in the central Himalayas expanded by 122 percent between 1976 and 2010. As hydropower projects are normally designed to operate for several decades, “glacial lakes will be an increasing danger to [headwater] dams in a warming world,” Fujita says.

Moreover, extreme rainfall events, which have become increasingly frequent in the Himalayas, could also trigger GLOFs. In June 2013 unusually intense and prolonged downpours breached the embankment of the Chorabari Lake in the Indian state of Uttarakhand, causing a devastating flood that killed over 6,000 people in the town of Kedarnath, an important destination for Hindu pilgrims.

“Developers must closely monitor glacial lakes upstream of the dams,” Mool notes. “And mitigation strategies, such as attempts to reduce water levels, and early-warning systems must be in place for those that are expanding rapidly.”