A small glacier lake known as Nagma Pokhari sits nestled in a valley near Mount Everest in Nepal, surrounded by steep walls of sediment that hold the icy waters in place. Back in June 1980, one of Nagma Pokhari’s dams failed—and the resulting floodwaters inundated and destroyed farms and houses for more than 70 kilometers downstream.
In a warming world, the threat of such “glacial lake outburst floods” is becoming increasingly urgent. As glaciers in most parts of the Himalayas melt, floods caused by the bursting of rapidly expanding glacial lakes pose an increasing risk to mountain communities. Predicting which lakes will burst and how much damage they might cause to downstream settlements is a challenge. But a new study finds that a simple satellite index based on the surrounding topography can provide a way to identify which lakes are most at risk of causing a damaging flood to those communities.
“There is an urgent need for a reliable way to target potentially dangerous lakes,” says Pradeep Mool, a remote-sensing expert at the International Centre for Integrated Mountain Development (ICIMOD) in Kathmandu.
There are ways to monitor when the dam of a glacial lake might break: Scientists track the lake’s depth, the geological composition and geometry of its dam, and other factors, such as melting rate and steepness of adjacent glaciers. But with more than 20,000 glacier lakes in the Himalayas, most in remote areas and difficult to access, “it’s impossible to manually monitor them all,” says Koji Fujita, a glaciologist at Nagoya University in Japan, who has been working in Nepal and Bhutan for decades.
Another way to assess a lake’s overall danger to downslope communities is to estimate how much floodwater might pour out of it, should a dam burst, Fujita says. He and colleagues thought that the answer to the floodwater question might also lie in the lakes’ moraines—piles of sediments bulldozed by glaciers into high ridges that act as dams. Using now-declassified data from a 1970s- to 1980s-era U.S. spy satellite called Hexagon KH-9, the team estimated changes in the elevations of five moraine dams of lakes that have burst between 1984 and 1994.
In all five cases, the moraine dam was flushed out and lowered by the outburst of water, the team found, and the dam’s slope continued to decrease during the flood. The process continued until the slope was about 10o, which the researchers suspect might be a critical threshold for a flood to end, they report in a study in press in Natural Hazards and Earth System Sciences. Changes in the slope of moraine dams are thus “one of the most important parameters when deciding which lakes are dangerous,” Fujita says.
Using a modern version of the Hexagon satellite called the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), the team screened 2276 large glacial lakes in the Himalayas and found that 49 of them have potential flood volumes of over 10 million cubic meters, which are generally considered to be major floods. Most of these lakes are in the eastern Himalayas, where glacier lakes are expanding more rapidly than those in other parts of the mountain range mostly due to rising temperatures and decreasing snowfall during the summer monsoon as a result of climate change.
The technique, Fujita says, could be used in the initial screening of thousands of large glacial lakes in the Himalayas and continuous monitoring of topographical changes around the lakes as they expand and new moraine dams develop.
“It’s a valuable study with important implications for assessing potential hazards of glacial lakes,” says Tobias Bolch, a glaciologist at the University of Zurich in Switzerland. This is the first time that a satellite index is verified by past glacier lake floods, he says. Bolch stresses, however, that elevation estimates from either Hexagon or ASTER are “notoriously tricky” and, therefore, should be confirmed with field investigations.
The new satellite index provides an additional criterion for assessing risks of glacier lake floods, Mool says. Fujita’s list of dangerous glacial lakes, however, is quite different from the one made by ICIMOD in 2001. For instance, 23 of the 44 dangerous lakes identified by ICIMOD do not have steep moraine dams, and only five of them have a potential flood volume over 10 million cubic meters.
Mool says that ICIMOD took a more integrated, albeit qualitative, approach while making its assessment—taking into account factors such as the distance between the lake and the glacier, the stability of moraine dams, potential landslides, and downstream population and infrastructure. “It’s good to have a quantitative measure,” he says. “But glacier environment is very complicated and no single index is enough.”