Modeling and risk assessment of glacial lake outburst floods (GLOFs) : a case study of Imja Tsho in the Nepal Himalayas
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The Himalayas of South Asia are home to many glaciers, which are retreating because of climate change and leaving in their wakes large lakes. These lakes are held in place by naturally deposited and inherently unstable moraine dams, which are at risk of overtop by impulse waves, commonly generated when avalanches or landslides enter the lake. When an overtopping event causes catastrophic failure, a glacial lake outburst flood (GLOF) occurs. The Imja glacier is experiencing the highest mass loss rate in the Mount Everest region, contributing to the expansion of Imja Tsho, a lake with several villages downstream. A GLOF from the lake can bring catastrophic damage to these villages, threatening both property and human life and creating a need to understand the processes that trigger these floods. The process chain for an avalanche-induced GLOF was modeled numerically, using the output of one subprocess as input into the next. First, the volume and momentum of various avalanches entering the lake were calculated using RAMMS. Next, the subsequent waves were simulated using BASEMENT software and validated with empirical equations to ensure the proper transfer of momentum from the avalanche to the lake. BASEMENT was then used to model moraine erosion and downstream flooding, which yielded a map of risk areas downstream. Moraine erosion was calculated for two geomorphologic scenarios: a site-specific scenario using field data, and a worst-case scenario based on past literature and applicable to lakes in the entire region. Neither case resulted in flooding outside the river channel at downstream villages. The worst-case scenario resulted in some moraine erosion and increased channelization of the lake outlet, which yielded greater discharge downstream but no catastrophic collapse. The site-specific scenario generated similar results, but with very little erosion and a smaller downstream discharge. Results indicated that Imja Tsho is unlikely to produce a catastrophic GLOF in the near future; however, some risk exists within the river channel, necessitating continued monitoring of the lake. Furthermore, these models were designed for ease and flexibility so that they can be adopted by a wide range of stakeholders and appropriated for other lakes in the region.