Modeling the Glacial Lake Outburst Flood Process Chain in the Nepal Himalaya: Reassessing Imja Tsho's Hazard

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 10 the moraine dam, thereby causing a catastrophic failure of the moraine and a glacial lake outburst flood (GLOF). ImjaLhotse Shar glacier is amongst the glaciers experiencing the highest rate of mass loss in the Mount Everest region, which has contributed 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. 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 15 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 RAMMS. Next, the avalanche-induced waves were simulated using BASEMENT 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 20 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 25 hazard exists within the downstream 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.Hydrol. Earth Syst. Sci. Discuss., https://doi

The State of Remote Sensing Capabilities of Cascading Hazards Over High Mountain Asia

“…Potential outburst flood volume Lake bathymetry map derived from sonar points interpolated within the lake boundary Use empirical area-volume relationships to estimate lake volume. Lake bathymetry fused into a DEM needed for outburst flood modeling Section "Changri Shar and Lhotse Glacier Outburst Floods" and "Langmale GLOF"; Cook and Quincey, 2015;Haritashya et al, 2018;Lala et al, 2018 Presence of permafrost Preconditioning of slopes Global Permafrost Zonation Index (PZI) Decreased slope stability at the transition from permafrost to non-permafrost zones, Spatial coincidence between lower most rock glacier distribution and the presence of permafrost Sections "Landslides"3.4, and "Open Data"; Stanley and Kirschbaum, 2017;landslides in Nepal, Petley et al, 2007;Gorkha Earthquake inventories images are also being used to generate time series of highresolution (2-8 m) Digital Elevation Models (DEMs) that can be used to quantify elevation and volume change of landforms and glaciers. Composite mosaics of these high-resolution DEMs are also fundamental for modeling landslides or outburst flood runouts and producing accurate landslide inventories.…”

“…Given the unpredictable nature of glacier outburst floods, remote sensing has primarily been used to analyze floods following the events (e.g., Rounce et al, 2017a;Byers et al, 2018) or to conduct a first-pass hazard assessment for proglacial lakes (e.g., Worni et al, 2014;Rounce et al, 2017b). The latter typically utilize optical imagery and DEMs to identify potentially dangerous glacial lakes that require further investigation, which may include fieldwork and detailed flood modeling with a high-resolution DEM (e.g., Lala et al, 2018).…”

“…DEMs are fundamental inputs when modeling downstream flood propagation, either in first-pass GIS-based assessments (e.g., Huggel et al, 2004;Mergili and Schneider, 2011;Watson et al, 2015;Rounce et al, 2017b) or in physically-based hydrodynamic models (e.g., Westoby et al, 2014aWestoby et al, , 2015Worni et al, 2014;Lala et al, 2018). Spatial and temporal resolution is a limitation in the HMA region where only 30 m resolution global products are widely available, such as the Shuttle Radar Topography Mission (SRTM) or ALOS World 3D (AW3D30) DEMs.…”

Modelling glacial lake outburst flood impacts in the Bolivian Andes

“…Somos-Valenzuela et al 2016), the PRO version of which comes at a cost, but which can be used to model debris flows, and BASE-MENT (e.g. Worni et al 2012Worni et al , 2013Lala et al 2017), which is downloadable free of charge, but cannot model debris flows. A direct comparison of the outputs of each of these modelling tools is beyond the scope of this study, but it would be worthwhile undertaking such work in the future for the three lakes investigated here.…”

“…Further, recent GLOF studies have attempted to model complex process chains in a multi-hazard proglacial lake context (e.g. Lala et al 2017;Mergili et al 2018), and it is suggested here that investigation of process chains for GLOFs from proglacial lakes in Bolivia would be a valuable undertaking in the future.…”