GSI official warned that avalanches, landslides and river blockages can combine to trigger flash floods, calling for integrated satellite monitoring and ground-based assessment
The Himalayas face the threat of a cascading disaster, where a single high-altitude calamity can trigger avalanches, block rivers and unleash flash floods, rapidly magnifying destruction downstream — a danger laid bare by Nepal’s recent catastrophe, said a senior Geological Survey of India official.
Therefore, Himalayan hazards cannot be assessed in isolation, he pointed out and stressed the need for combining satellite-based observations with field investigations and ground instrumentation.
A case in point is the Nepal disaster that appears to have involved cascading processes.
Satellite analysis identified an impoundment near the Langtang Lirung ice-rock avalanche site, with imagery of August 28 showing an approximately 11-hectare water impounded area.
This provides evidence that the initial ice-rock failure was associated with temporary obstruction and water accumulation in the river system, said Pradeep Singh, the ADG of Policy Support System at the Geological Survey of India.
The flash floods near the Nepal-Tibet border on August 26 devastated towns and villages in northern and central Nepal. The Bhotekoshi river carried the floodwaters downstream, sweeping away homes, roads and bridges and killing more than 1,300 people.
Singh stressed the need to identify unstable mountain systems before they turn into catastrophic events.
“A single instability at high elevation can initiate a sequence involving an avalanche, landslide or glacier failure, river blockage, dam-break type release, debris flow and downstream flooding. Such processes can interact and amplify one another,” he told PTI here.
The geologist said the priority should be systematic scientific screening of potentially high-risk areas, particularly locations where a high-altitude failure could rapidly enter a river and affect populations and infrastructure downstream.
“The immediate priority should be a systematic, science-based screening of potentially high-risk source areas, rather than assuming that a similar catastrophic event will occur everywhere.” He pointed out that such screening should prioritise rapidly deforming glacier margins and glacier-rock systems, unstable and moraine-dammed glacial lakes, steep and fractured rock slopes, locations showing persistent or accelerating deformation in satellite observations, and valleys where a high-altitude failure could rapidly propagate downstream.
Glacial lakes are formed by melting glaciers accumulating in depressions on or near the glacier’s surface. Glacial Lake Outburst Floods occur when these lakes suddenly burst open due to factors such as excessive water accumulation or earthquakes. One such outburst in Sikkim in October 2023 resulted in at least 60 fatalities and extensive damage in four districts.
Singh said, “Satellite observations can potentially provide valuable precursor information, particularly where progressive deformation or acceleration occurs before failure. However, a detected increase in movement indicates that the particular slope is in distress; however, the failure will occur on reaching the limit equilibrium condition, which may be difficult to predict.” The answer lies in combining satellite-based observations with field investigations and ground instrumentation.
Singh asserted that the most effective approach was not to rely on a single satellite indicator, but to integrate multi-temporal InSAR, optical feature tracking, DEM differencing, field instrumentation and ground observations.
This would allow authorities to build a regional inventory of anomalous or rapidly deforming glacier-rock, moraine and slope systems and focus detailed investigations and monitoring on locations with potentially serious downstream consequences.
“The objective should be to identify potentially unstable systems early, understand how they may fail, estimate their possible downstream effects and establish an appropriate level of monitoring and preparedness.” Singh said the concern was particularly relevant for the Indian segment of the Himalayas because the region combines glaciers, steep and fractured rock slopes, permafrost-affected terrain, glacial lakes, unstable moraines and rapidly flowing rivers, while settlements and critical infrastructure are often concentrated downstream.
“The key shift required is from hazard-by-hazard assessment to source-to-impact multi-hazard assessment. It allows authorities to understand not only where instability may occur, but also what could happen next and who or what may be exposed downstream,” he said.
He, however, cautioned against interpreting the Nepal disaster as proof that every Himalayan town in India faces an imminent glacier-related catastrophe. “Risk in India is highly site-specific and depends on local geology, slope stability, glacier and glacial-lake conditions, hydrology, climate, terrain and patterns of human development.” He said there were nevertheless reasons for vigilance on changing glacier conditions, expanding or unstable glacial lakes and moraines, possible permafrost degradation at high elevations, steep and fractured slopes, intense rainfall, landslides and debris flows, as well as increasing infrastructure, settlement and tourism in valleys.
GSI is undertaking landslide susceptibility assessments and glaciological studies in the Himalayas, Singh said, adding that these data should be integrated with satellite deformation monitoring, glacial-lake inventories, geophysical studies and flood and debris-flow modelling.
GSI studies of nine Himalayan glaciers have recorded overall negative average annual specific mass balance, indicative of substantial glacier mass wastage under changing climatic conditions.
“For India, the appropriate response is not alarm, but science-based preparedness. The objective is not to eliminate the mountains’ natural processes, but to understand them better, identify highly vulnerable areas, reduce exposure and improve available response time.
“With sustained scientific monitoring, improved data sharing, appropriate land-use planning, robust infrastructure and community preparedness, risks can be better managed,” Singh added.
