Nepal disaster shows why Himalayas need multi-hazard risk assessment: GSI expert

The Himalayas need to be assessed for cascading, multi-hazard risks rather than individual threats in isolation, a senior Geological Survey of India (GSI) official has said, pointing to Nepal’s recent disaster as an example of how a high-altitude event can trigger a chain of processes and magnify destruction downstream.

Pradeep Singh, Additional Director General of Policy Support System at the GSI, said a single high-altitude instability could lead to an avalanche, landslide or glacier failure, followed by river blockage, sudden water release, debris flows and downstream flooding.

“Such processes can interact and amplify one another,” Singh told PTI.

His comments come after flash floods near the Nepal-Tibet border on 26 August devastated towns and villages in northern and central Nepal. The Bhotekoshi river carried floodwaters downstream, destroying homes, roads and bridges and killing more than 1,300 people.

Satellite analysis of the Langtang Lirung ice-rock avalanche site also identified an impoundment, with imagery from 28 August showing an approximately 11-hectare area of water accumulation. Singh said this indicated that the initial ice-rock failure was associated with temporary obstruction and water accumulation in the river system.

The episode, he said, demonstrated why Himalayan hazards need to be examined as connected processes rather than separately.

Focus on high-risk source areas

Singh said the immediate priority should be systematic, science-based screening of potentially unstable mountain systems, particularly areas where a failure at high altitude could quickly enter a river and threaten communities and infrastructure downstream.

Such assessments should focus on rapidly deforming glacier margins and glacier-rock systems, unstable or moraine-dammed glacial lakes, steep and fractured rock slopes, areas showing persistent or accelerating deformation in satellite observations, and valleys where failures could rapidly propagate downstream.

Satellite observations can provide useful information on changes preceding a failure, particularly where progressive deformation or acceleration is detected. But Singh cautioned that increased movement alone cannot determine when a slope will fail.

“A detected increase in movement indicates that the particular slope is in distress,” he said, adding that failure would occur only after the system reaches a limit-equilibrium condition, which could be difficult to predict.

He said the most effective strategy would therefore combine multiple forms of satellite data, including multi-temporal InSAR, optical feature tracking and digital elevation model differencing, with field investigations, ground instrumentation and direct observations.

The resulting data could be used to develop a regional inventory of anomalous or rapidly deforming glacier, rock, moraine and slope systems. Authorities could then concentrate detailed investigation and monitoring on sites where instability could have 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.

India faces site-specific risks

The concern is particularly relevant to the Indian Himalayas, where glaciers, steep and fractured slopes, permafrost-affected terrain, glacial lakes, unstable moraines and fast-flowing rivers coexist with settlements and critical infrastructure in downstream valleys, Singh said.

He called for a shift from “hazard-by-hazard assessment” to a “source-to-impact multi-hazard assessment”.

“This 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.

Singh, however, cautioned against interpreting the Nepal disaster as evidence 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.

He nevertheless identified several areas requiring continued vigilance, including changing glacier conditions, expanding or unstable glacial lakes and moraines, possible degradation of permafrost at high elevations, steep and fractured slopes, intense rainfall, landslides and debris flows, and rising infrastructure, settlement and tourism in mountain valleys.

Glacial Lake Outburst Floods (GLOFs), in particular, remain a concern. Such floods can occur when water accumulated in glacial lakes is suddenly released because of factors including excessive water accumulation or earthquakes. A GLOF in Sikkim in October 2023 killed at least 60 people and caused extensive damage across four districts.

The GSI is conducting landslide susceptibility assessments and glaciological studies in the Himalayas. Singh said these datasets 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 an overall negative average annual specific mass balance, indicating substantial glacier mass wastage under changing climatic conditions.

Singh said the response should focus on preparedness rather than alarm, with scientific monitoring, improved data sharing, land-use planning, resilient infrastructure and community preparedness helping reduce exposure and improve response time.

“For India, the appropriate response is not alarm, but science-based preparedness,” he said.