Assessment of hazard associated with mass movements in the Himalayan region

Abstract

The collision between Eurasian and Indian plates led to the formation of the Himalayas, the northward movement of the Indian plates develops continuous stress on the surface rendering them friable and weak. Hilly terrain along with high seismic vulnerability and recurrent extreme precipitation events create a fragile system, augmenting the mass movement susceptibility. Mass movement events are natural phenomena causing the downward flow of slope material such as rocks, ice and debris under the influence of gravity. Additionally, global warming alters glacier melt patterns, enhancing glacier melt rates in many locations. Glacier mass loss supports the formation of terminus glacial lakes, these lakes are hazardous owing to the phenomenon of GLOF, which occurs when damming boundaries are either overtopped or breached due to the generation of instability. Triggering of initial instability in the damming structure (consequently leading to GLOF) are usually caused because of mass movement intrusion into the lake in the form of avalanche or rockfall. Such events are common in glacial proximity due to steep topography and the presence of retreated glaciers which are characterized to support the development of new avalanche starting zones. GLOF is the sudden discharge of a large amount of stored water from glacial lakes, caused majorly due to dynamic failure (mass entering the lake in the form of avalanche, rockfall or GLOF in the upstream portion of the lake) and minorly due to self-destructive failures (presence of ice cored moraine or unstable moraine structure), seismic activities, earthquake and extreme climatic events. The remote location, infrequent occurrence and interconnection between these triggering mechanisms makes it difficult to assess their cause. Nonetheless, the hazard from these triggering mechanisms needs to be quantified, especially for the Himalayas, where over a billion people live in critical susceptible zones. Documentation of hazardous areas and application of remedial measures can prevent events such as the ones that occurred at Chamoli in 2021, Chorabari in 2013, Ayaco Lake in 1969 and 1970, Nare Lake in 1977, Dig Tsho in 1985, Sabai Tsho in 1998, and so on; these are some of the examples of calamitous events that have caused extensive damage to lives and socioeconomic condition of inhabitants. In the light of this discussion; In this thesis, initially, I model the mass movement trajectories (representative of all possible avalanche, rockfall, mud-flow, landslides) in the Hindu Kush Karakoram Himalayas (HKKH) considering various depth scenarios. The developed trajectories were then used to assess the susceptibility of buildings, roads and rivers in the Himalayan Mountain ranges against mass movements. The analysis was further extended to assess the susceptibility of existing and future populations obtained from socio-economic scenarios representing two distinct futures. Later, I created deglaciated topography for the HKKH to develop future mass movement trajectories rendering the possible shift of critical zones of glacial hazards in the Himalayas. Finally, the domain of the analysis was narrowed to the Indian Himalayas, where I carried out a detailed analysis on hazard, downstream impact and risk associated with glacial lakes. Lastly, I altered the glacier boundaries used in the assessments against minimum snow cover extent data (produced using a novel proposed algorithm) in order to assess the sensitivity of avalanche trajectories. Major scientific advancements from this thesis include (1) Development of novel algorithms to trace mass movement trajectories; (2) susceptibility estimation of ~7 million buildings, ~3.9 million km of river network and ~ 1 million km of road network; (3) susceptibility estimates of existing and future glacial lakes to mass movements; (4) development of new framework to assess GLOF susceptibility; (5) new equations to determine the uncertainties in potential flood volumes; and (6) development of novel algorithms to compute snow cover changes. Based on these scientific advancements, I report that over 5 people/km2 are susceptible to mass movements, less expenditure on education and v healthcare will lead to higher mass movement casualties, I also report that regions subjected to higher GLOF hazard will shift from Central Himalayas and Eastern to Karakoram Himalayas. Based on the framework developed to assess GLOF susceptibility, I identified 23 critical glacial lakes in Indian Himalayas that needs further attention. Additionally, newly proposed method of snow cover extent estimation shows a gradual increase in spatial distribution of maximum snow cover extent in the Indian Himalayas. The findings from this study will lay a foundation for quantifying present and future losses resulting from landslides, avalanches and GLOFs in the alpine regions and thereby aiding in the deployment of appropriate adaptation measures.