The researchers found that about 108,000 people live downstream of the glacial lake, where the ice could break off and send pulses of flood water downstream.
Flood waters rising 164 feet and traveling 31 miles may sound like an extraordinary thing. However, flooding in the Botkohi River and its tributaries in August moved about 120 miles downstream, causing water levels to rise by about 330 feet in some communities, said Dipesh Chapagain, a senior scientist at the United Nations University Institute for Environment and Human Security.
“The risk may even be higher than the paper suggests,” said Chapagan, a Nepalese scientist who studies climate change and flooding. “This event sets a new level, a new threshold.”
Unlike the United States, there are no detailed flood hazard maps for much of Nepal. Watson said previous efforts to understand how much of Nepal’s population faces dangerous flooding from high mountains had only created a 1-kilometer (0.62 mile) buffer zone around mountain rivers.
Watson said new research will tell us where flood hazard mapping should be a priority.
“We’re collecting very high-resolution elevation models to run some detailed flood hazard maps so we can focus on the most exposed hot spots,” Watson said. “To understand this kind of detail, we have to use this screening tool to say where are the hot spots?”
More than a month after searching for missing people in Botkosi floods, Nepal has called off the search. Chapagan said the focus now is on building temporary shelters for thousands of flood victims. There are no concrete plans yet on how permanent resettlement will take place, but Chapagan said new construction must prioritize safe areas.
“Future planning should consider all these risks when planning new infrastructure projects, new settlements, and flood recovery projects,” Chapagan said, adding that flood warning systems should prioritize areas identified as high risk in this study.
Watson said research shows too much construction and development in Nepal goes unregulated. Nepal’s booming hydropower investment and tourism have brought paved roads to the high mountain basin. The study found that from 2016 to 2023, the construction area within a vertical 50 meters of the river bank increased by approximately 46%.
“Once these communities start to form, the amenities quickly follow, your schools, your health care — you quickly get a growing community in an area that’s actually very close to the waterway,” Watson said.
At the same time, he said thousands of glacial lakes are forming upstream as climate change causes glaciers to disappear and permafrost melts, many of which could send pulses of floods downstream without any notice or warning.