Rockfall hazard in the Imja Glacial Lake, eastern Nepal

Durga Khatiwada,Ranjan Kumar Dahal

doi:10.1186/s40677-020-00165-9

Abstract

In Nepal, rockfall related studies are rarely conducted and are limited to the landslide study along with few case studies on rockfall events. Rockfall problems in Nepal are more frequent in the Higher Himalayan region than Midland and Lesser Himalayan regions. In the glaciated valley and glacial lakes, rockfall and associated tsunami like huge wave are a recently initiated research. In this context, a glacial lake of the Himalaya, named as Imja Glacial Lake situated in eastern Nepal has been selected to understand the rockfall problems and their possible consequences. The lake was formed at the end part of the Imja glacier. The northern valley slope of Imja Glacier Lake, i.e. southern slope of the Island Peak has serious problem of rockfall into the lake. Rockfall simulation was performed during this research with field data. Three different Plots were defined for simulation of rockfall. Among them, Plot III seems to be the most hazardous since the detached boulders on the higher elevation can enter into the lake with the maximum velocity of 40 m/s with pre-impact energy more than 3500 kJ. This can develop a surge in the lake water that can break moraine dam. This leads to a serious threat to the downstream communities. Moreover, this research confirms that the rockfall hazard in the higher mountain region of Nepal is critical to creating flash floods due to tsunamis like surge in the lake water and breaching of the dam.

Highlights

Rockfall is a fragment of rock detached by sliding, toppling, or falling, that falls along a vertical or subvertical cliff, proceeds down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes (Varnes 1978)
Desk study During desk study, initially different possible areas of rockfall in the southern face of Island Peak area were identified through the visual analysis of Imja Glacial Lake in Google Earth images and topographical map of the area
Total three Plots, namely, Plot I, Plot II and Plot III (Fig. 4) were extracted as main rockfall area based on the topography of the Island peak located on the northern side of Imja Glacial Lake

Summary

Introduction

Rockfall is a fragment of rock (a block) detached by sliding, toppling, or falling, that falls along a vertical or subvertical cliff, proceeds down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes (Varnes 1978) These rock blocks can be detached by different mechanisms such as natural mechanisms like freeze - thaw cycles (McCarroll and Pawellek 1998; Matsuoka and Sakai 1999) seismic activities (Abebe et al 2010) and by anthropogenic activities along the slope cuts or due to earth moving mechanism (Dorren 2003; Vijayakumar et al 2011) which are slow time-dependent deformation that occur in the slope materials.

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