Rain bombs

Sonia Awale

·  At least 400 people are dead or missing after heavy rainfall lashed arid regions of northwestern Pakistan. Walls of mud and boulders engulfed entire villages in Khyber Pakhtunkhwa Province. More than 700 people have been killed in floods in Pakistan since June. 

·  Over in India’s Jammu and Kashmir at the same time, multiple slurry floods on 17-18 August killed more than 80 people

· A mudslide possibly caused by a glacial collapse upstream in Uttarakhand on 5 August nearly wiped the village of Dharali off the map. At least 70 people, 23 of them Nepalis are dead or missing.

· On the night of 8 July, glacial lakes in Tibet overflowed. By dawn the mudflow had demolished the Nepal-China border bridge killing at least 11 people, destroyed hydropower plants, swept away bridges, roads, over 100 cargo trucks and newly imported electric vehicles.

All these disasters have a common thread: a cascade of hazards caused by record-breaking rainfall on fragile Himalayan slopes destabilised by melting glaciers and permafrost, seismicity and poorly engineered infrastructure.

They are called ‘rain bombs’ — a heavy localised downpour measuring more than 100mm/h on a steep mountainside. As global warming heats the atmosphere, warmer air can hold more water vapour. The heavy rain then falls where there should be snowfall: on moraines and slopes that have lost the permafrost adhesive that held the debris together.

“Increasingly, there is now more rainfall at higher altitudes where there used to be snowfall previously, and this directly impacts loose material in that fragile landscape which moves down,” explains Mohan B Chand, a Himalayan Glaciologist at Kathmandu University. “Moreover, the rains these days are heavy and fall within a few hours. So even a small burst has bigger impact.”

The 2021 Melamchi flood and the Sikkim disaster in 2023 have been attributed to record-breaking torrential rain on a glacial moraine that collapsed, unleashing a catastrophic debris flow. 

The Bhote Kosi flood this year happened even without torrential rain, and originated at rapidly expanding supraglacial lakes that overflowed, similar to what happened on the Mendenhall Galcier in Alaska last week. In Uttarakhand, the trigger could be in the upper catchment cryosphere, or a landslide dammed lake.

Basanta Raj Adhikari of the Centre for Disaster Studies at the Institute of Engineering also calls them ‘sediment bombs’. He clarifies: “There is lots of sediment up there from landslides or glacial activity, and when there is heavy rainfall and hotter temperature, they are unleashed downstream. One thing is for sure, these extreme events are getting more frequent and more destructive.”

Researcher Alton C Byers who has studied melting glaciers in the Himalaya and Andes names the eight most potentially dangerous glacial lakes in Nepal: Lower Barun, Chamlang North Tso, Chamlang South Tso, Dig Tso which burst in 1984, Lumding Tso, Thulagi, Tso Rolpa and Imja Tso (see map).

Of these, Tso Rolpa and Imja have been lowered by 3m each, while Thulagi, Lower Barun, Lumding Tsho, and Hongu 2 glacial lakes have been targeted for lowering by the Department of Hydrology and Meteorology as part of the recent Green Climate Fund grant of $36.1 million awarded to Nepal.

“These lakes have changed over the years as their debris-covered glaciers continue to recede,” Byers told Nepali Times. “When water volume in the lake basin increases, they become more vulnerable to flood triggers such as overhanging ice as glaciers recede further to their headwalls, and permafrost on their moraines continues to melt.”

These are not ‘natural disasters’

Scientists have given the current planetary epoch a name: the Anthropocene. And anthropogenic causes in this era heighten the risk of disasters.

Indeed, an analysis of recent Himalayan calamities (page 1) shows that there is much more at play here than climate breakdown. Haphazard encroachment of floodplains, quarrying and crusher activity exacerbated the effect of extreme rainfall on 28 September around Kathmandu Valley that killed 200 people.

The human cost of the 5 August catastrophe in Uttarakhand would have been high in any case, but the fact that the tiny village had expanded across a vulnerable alluvial fan to cater to a tourism boom meant more death and destruction.

There are similar alluvial fans across Nepal, where small towns have expanded because new highways follow rivers, like Beni located at the confluence of the Myagdi Khola and Kali Gandaki. Other towns like Kagbeni or Damauli are located on the banks of rivers and tributaries that are directly downstream from rapidly expanding glacial lakes or unstable mountains.

“In Butwal, people are building houses on the floodplain of the Tinau River, and yet people blame climate change or rainfall when there is a disaster,” says Basanta Raj Adhikari at the Institute of Engineering. “This is happening all over the country. Imagine the kind of devastation Beni would suffer if there was a big flood.”

RISKY CONFLUENCE: Beni has expanded rapidly after the highway to Mustang was built. But the town is situated on the alluvial fan of past flash floods on both the Kali Gandaki and Myagdi Khola which have dangerous glacial lakes in their upper catchment. Photo: BENI TOURISM

Traditionally, Nepal’s villages and settlements were located high above rivers because our forebears learnt lessons from recurring flash floods. Now, the road network follows rivers, and towns have sprung up along these highways. Even when roads are not on river banks, many gouge out slopes above, unleashing massive landslides that block rivers.

Experts say Nepal’s hazard map should now be updated for glacial outbursts so municipalities can plan risk-sensitive land use zoning. This  must go hand-in-hand with measures to reduce the risk by structural mitigation, such as engineering and construction, and then with risk awareness and early-warning systems.

Says Himalayan researcher Alton C Byers: “Sooner or later, communities will realise that certain sites are simply no longer safe for a village or city, especially because of climate change in the mountains. The Himalaya has always been high-risk with its steep slopes, thin soil, monsoon rainfall regime, and frequent seismic activity. So, some places are no longer safe to live in.”

Mohan B Chand of Kathmandu University adds that hazard mapping is especially crucial for expensive hydropower plants: “Most of these are on snow-fed rivers, but environmental risk assessment is not taken seriously.”

He says that because Nepal is set to generate 28,500MW by 2035 to meet its carbon neutral target, the government must work to minimise upstream risk by integrating disaster mitigation components in new energy investments.

Few know that Nepal’s first hazard mapping was done 40 years ago in the Khumbu with a United Nations University study titled ‘Highland Lowland Interactive Systems’. It is hard to find that document anywhere.

Today, hazard mapping technologies, satellite remote sensing, drones, synthetic aperture radar (SAR) and modelling with AI have made it much easier to integrate multi-hazard risk assessment. That data needs to be made available to local governments so they can plan.

Cell phones and new apps that monitor real-time glacial lake levels, live precipitation and hydrological data, and local weather forecasts can make early warning systems more effective.

Recent disasters have shown that even the smallest of glacial lakes that are rarely considered in risk assessments can cause much damage. Langmale Lake in the Barun Valley was less than 0.1 km2 in size, but the flood in 2017 caused extensive downstream damage.

The 2012 Seti flood in Pokhara, the Chamoli disaster in Uttarakhand in 2021, the South Lhonak proglacial lake burst in Sikkim in 2023 that washed away the $1.7 billion Chungthang Teesta III Project, and the August 2024 floods above Thame in Khumbu are other recent examples. 

Byers thinks some of these floods may have been initially triggered by massive rockfall or moraine collapse related to melting permafrost.

The size of a glacial lake alone does not determine the severity of destruction. What makes all the difference is downstream development, and if there pre-warning. Dig Tso in Khumbu which burst in 1984, was much larger than the lake that caused the Bhote Kosi flood in July, but the economic cost to the country of the trans-boundary disaster this year was much higher.

Says glaciologist Chand: “We have to monitor the lakes, not just via remote sensing but with field visits. These may be costly, but mitigating the risk is less expensive than the disasters themselves.”

Nepal has the technology and trained personnel to do hazard mapping, monitoring, and to design early warning apps. That data and information now needs to be available to all three levels of government to improve preparedness and risk reduction.

“The cascading and compounding hazards are increasing in the Himalaya, this must be the subject of our research now,” says Adhikari at the Centre for Disaster Studies. “Recent events are a wake-up call, if we do not take this seriously things may be more dangerous in future.”