Rapid warming in the Himalayas is increasing the threat of catastrophic floods like the ones that killed thousands in Nepal and Tibet this summer. Yet in recent years, people have continued to build in flood-prone areas, putting more lives and property at risk, according to a new analysis of satellite data.
The total footprint of buildings along vulnerable river corridors in Nepal and the adjoining areas of India and China increased by nearly half between 2016 and 2023, scientists reported in a study published Thursday. Almost 300,000 people now live in these exposed zones, the researchers estimated.
The study, published in the journal Scientific Reports, didn’t try to classify the new buildings by use. But it identified tourism and hydropower construction as two of the likely drivers of recent development in the high mountain valleys.
The tragic floods in August have focused attention on flood hazards in the Himalayas, where the vast and complex terrain makes disasters a huge challenge to predict.
The potential triggers there are many: monsoon downpours, earthquakes, avalanches that send boulders tumbling into lakes formed by melting glaciers. As the water races down the valleys, it can mix with sediment and debris to form a slurry that crashes into towns and cities below with incredible, and deadly, momentum.
Scientists are using satellites and other instruments to try to better understand these hazards. But for a long time, it was tough to estimate how many buildings and structures in the Himalayas were in harm’s way, said Scott Watson, a researcher at the University of Leeds and the lead author of the new study. Satellite images don’t always provide complete coverage in the mountains.
But a recently released Google data set on building footprints around the world between 2016 and 2023 presented Dr. Watson and his colleagues with an opportunity.
After checking that Google’s data matched up well enough with other satellite imagery of Nepal, the researchers combined it with topographic maps to assess how much development had taken place in the steep-sided valleys where floodwaters can pour through.
They found that the total area of buildings sitting up to 50 meters, or roughly 160 feet, above the valley floors increased by 46 percent during that eight-year period. Based on recent population trends, the researchers estimated that the number of people living in these areas could grow by 6 percent between 2015 and 2030.
Few floods in the Himalayas are likely to raise the water by 50 meters, Dr. Watson said. Still, August’s floods appear to have reached such heights or more in some places. And it isn’t just the water that can wreck homes and buildings. Land eroded around a river can continue slumping and sliding for days or weeks after a flood rushes through.
In other regions where flooding is a problem, building codes are based on estimates of the likelihood of floods of different sizes: a so-called 100-year event, a 500-year event, and so on. But producing such estimates for the Himalayas remains a “big challenge,” Dr. Watson said. Floods there unfold over too unpredictable a landscape for scientists to say with confidence how much water communities should expect, and how often, he said.
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