Abstract

Abstract. The Himalayas of South Asia are home to many glaciers that are retreating due to climate change and causing the formation of large glacial lakes in their absence. These lakes are held in place by naturally deposited moraine dams that are potentially unstable. Specifically, an impulse wave generated by an avalanche or landslide entering the lake can destabilize the moraine dam, thereby causing a catastrophic failure of the moraine and a glacial lake outburst flood (GLOF). Imja-Lhotse Shar Glacier is amongst the glaciers experiencing the highest rate of mass loss in the Mount Everest region, in part due to the expansion of Imja Tsho. A GLOF from this lake may have the potential to cause catastrophic damage to downstream villages, threatening both property and human life, which prompted the Nepali government to construct outlet works to lower the lake level. Therefore, it is essential to understand the processes that could trigger a flood and quantify the potential downstream impacts. The avalanche-induced GLOF process chain was modeled using the output of one component of the chain as input to the next. First, the volume and momentum of various avalanches entering the lake were calculated using Rapid Mass Movement Simulation (RAMMS). Next, the avalanche-induced waves were simulated using the Basic Simulation Environment for Computation of Environmental Flow and Natural Hazard Simulation (BASEMENT) model and validated with empirical equations to ensure the proper transfer of momentum from the avalanche to the lake. With BASEMENT, the ensuing moraine erosion and downstream flooding was modeled, which was used to generate hazard maps downstream. Moraine erosion was calculated for two geomorphologic models: one site-specific using field data and another worst-case based on past literature that is applicable to lakes in the greater region. Neither case resulted in flooding outside the river channel at downstream villages. The worst-case model resulted in some moraine erosion and increased channelization of the lake outlet, which yielded greater discharge downstream but no catastrophic collapse. The site-specific model generated similar results, but with very little erosion and a smaller downstream discharge. These results indicated that Imja Tsho is unlikely to produce a catastrophic GLOF due to an avalanche in the near future, although some hazard exists within the downstream river channel, necessitating continued monitoring of the lake. Furthermore, these models were designed for ease and flexibility such that local or national agency staff with reasonable training can apply them to model the GLOF process chain for other lakes in the region.

Highlights

  • The Hindu Kush–Himalayan region contains more glacial ice and perennial snow than any other region on Earth outside the polar regions, and supplies water via its rivers to over a fifth of the earth’s population (Qiu, 2008; Matthew, 2013)

  • Since avalanche-induced glacial lake outburst flood (GLOF) are a chain of individual events, there are generally two options for characterizing them in the absence of true integrative modeling: modeling each component and using their outputs as inputs for the component in the chain, or approximating components so that the chain can be simulated in a single model run (Worni et al, 2014)

  • The methodology used in this study presents a hybrid approach using two models: modeling the avalanche in a single model, and using its output as the input for environmental flow modeling software that takes into account the subsequent wave, moraine erosion, and downstream debris flow and inundation

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Summary

Introduction

The Hindu Kush–Himalayan region contains more glacial ice and perennial snow than any other region on Earth outside the polar regions, and supplies water via its rivers to over a fifth of the earth’s population (Qiu, 2008; Matthew, 2013) While these glaciers are undeniably significant in sustaining the populations of South and East Asia, they provide some of the best gauges for understanding regional and global climate change, since temperatures in high altitudes are increasing faster than in lower elevations (Wang et al, 2017; Kraaijenbrink et al, 2017). These small ponds can coalesce and become the large glacial lakes found throughout the Hindu Kush–Himalayan region (Benn et al, 2012)

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