The researchers found about 108,000 people downstream of glacial lakes with ice that could fail and send a pulse of floodwater racing downstream.
Floods that rise 164 vertical feet and travel 31 miles might sound like an extraordinary occurrence. However, the August flooding in the Bhote Koshi river and its tributaries traveled about 120 miles downstream and raised water levels by roughly 330 feet in some communities, said Dipesh Chapagain, a senior scientist at the United Nations University Institute for Environment and Human Security.
“The risk could even be higher” than the paper suggests, said Chapagain a Nepali scientist who studies climate change and flooding. “This event set a new level, a new threshold.”
Unlike the U.S., detailed flood hazard mapping is not available for much of Nepal. Watson said previous efforts to understand what portion of Nepal’s population is exposed to dangerous flooding that originates in the high mountains simply drew 1-kilometer (.62-mile) buffers around alpine rivers.
Watson said the new research will inform where flood hazard mapping should be prioritized.
“We’re collecting very high-resolution elevation models to run some of the detailed flood hazard maps so we can focus on the most exposed hot spots,” Watson said. “To get into that level of detail, we kind of had to have this screening tool to say, you know, where are the hot spots?”
After more than a month of searching for missing people in the Bhote Koshi flooding, Nepal called off its search. The focus now, Chapagain said, is to build temporary settlements for thousands of flood victims. There is not a concrete plan yet for how to approach permanent resettlement, but Chapagain said new construction must be prioritized in safe areas.
“Future planning should consider all these risks when planning for new infrastructure projects, new settlements and also for the flood recovery projects,” Chapagain said, adding that flood early-warning systems should be prioritized in areas this study identified as high-risk.
Watson said the study shows that too much construction and development in Nepal has not been regulated. Hydropower investment and tourism are booming in Nepal, which has brought paved roads into high mountain catchments. The study found that the amount of area built within 50 vertical meters of riverbanks increased by approximately 46% from 2016 to 2023.
“Once those communities start to form, the amenities soon follow, your schools, your healthcare — you can quite quickly end up with a growing community in an area that’s actually very close to the river channel,” Watson said.
Meanwhile, he said thousands of glacial lakes are forming upstream as glaciers disappear and permafrost thaws due to climate change — and many could send a pulse of floodwater downstream with little notice or warning.
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