Glacial and geomorphic effects of a supraglacial lake drainage and outburst event, Everest region, Nepal Himalaya

Evan S Miles,Patrick Wagnon,Etienne Berthier,Fanny Brun,Bryn Hubbard,Duncan J Quincey,C Scott Watson,Michel Esteves,Katie E Miles

doi:10.5194/tc-12-3891-2018

Abstract

Abstract. A set of supraglacial ponds filled rapidly between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ∼180 000 m2 lake before sudden and complete drainage through Changri Shar and Khumbu glaciers (15–17 July). We use PlanetScope and Pléiades satellite orthoimagery to document the system's evolution over its very short filling period and to assess the glacial and proglacial effects of the outburst flood. We also use high-resolution stereo digital elevation models (DEMs) to complete a detailed analysis of the event's glacial and geomorphic effects. Finally, we use discharge records at a stream gauge 4 km downstream to refine our interpretation of the chronology and magnitude of the outburst. We infer largely subsurface drainage through both of the glaciers located on its flow path, and efficient drainage through the lower portion of Khumbu Glacier. The drainage and subsequent outburst of 1.36±0.19×106 m3 of impounded water had a clear geomorphic impact on glacial and proglacial topography, including deep incision and landsliding along the Changri Nup proglacial stream, the collapse of shallow englacial conduits near the Khumbu terminus and extensive, enhanced bank erosion at least as far as 11 km downstream below Khumbu Glacier. These sudden changes destroyed major trails in three locations, demonstrating the potential hazard that short-lived, relatively small glacial lakes pose.

Highlights

Outburst floods occur due to the sudden release of water from glaciers
The two Pléiades digital elevation models (DEMs) were 3-D-coregistered using off-glacier terrain (Berthier et al, 2007), differenced to produce a map of surface elevation change spanning the 2017 monsoon period. This geodetic difference encompassed the majority of the ablation season, so for the glaciers we focused on zones of enhanced surface lowering not solely attributable to ice cliffs and supraglacial ponds, which are known hotspots of melt for Himalayan debris covered glaciers (e.g. Sakai et al, 2002)
The geomorphic evidence from this study suggests that supraglacial lake outburst floods of this magnitude are not common in the Khumbu catchment

Summary

Introduction

Outburst floods occur due to the sudden release of water from glaciers This water can be stored within topographic lows at the glacier surface (Benn et al, 2012; Chu, 2014); internally along englacial conduits, crevasses, and voids (Fountain and Walder, 1998); or at the glacier’s bed (Jansson et al, 2003). Walder and Driedger, 1995; Wadham et al, 2001; Garambois et al, 2016), or adjacent to ice margins (Huss et al, 2007; Steiner et al, 2018) These storage components are interlinked: water retained at the surface can reach englacial and subglacial systems through hydrofracture or exploitation of zones of permeabil-

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Discussion

Conclusion