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 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.

Mass Loss From Calving in Himalayan Proglacial Lakes

“…Along the glacier centreline, Thulagi Lake expanded by 203 m (2008-2018) compared to 614 m for Lower Barun and 729 m for Imja (Haritashya et al, 2018). Imja Lake was reportedly lowered by 3 m in 2016 to reduce its hazard but a corresponding change in lake area was unclear (Lala et al, 2018).…”

“…It is clear that Imja Lake has the largest potential to expand and is doing so at the fastest rate (Figure 9), which could increase the future hazard due to the potential for avalanches to directly impact the lake (Rounce et al, 2016(Rounce et al, , 2017. The role of engineering works to lower the lake level by 3 m is not clear and requires further investigation (Lala et al, 2018). Consideration of glacier flotation is critical when considering GLOF mitigation FIGURE 9 | Projected glacial lake expansion for (a) Thulagi, (b) Lower Barun and (c) Imja, using mean expansion rates (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) (Haritashya et al, 2018) and modelled ice thickness data (Farinotti et al, 2019).…”

Modeling of Extreme Hydrological Events in the Baksan River Basin, the Central Caucasus, Russia

“…In the presented study, an actual GLOF occurred in the Baksan River basin on 1 September 2017, possible scenarios of a potential re-outburst in the case of rock avalanche impact and expansion of the existing slit, as well as calculations of the outburst flood propagation downstream from the Adylsu valley were implemented using only the STREAM_2D software package. A number of hydrodynamic models, or "process chains" are used frequently for modeling outburst floods, as they allow for a complete description of the process of the outburst, from the beginning to the end, including assessing the potential hazard [85,86]. The results of each process are used to simulate subsequent processes in the chain.…”

A Perspective of the Cumulative Risks from Climate Change on Mt. Everest: Findings from the 2019 Expedition

“…These lakes risk catastrophic breakthrough of this impoundment and subsequent flooding. Outburst floods of glacial lakes are most often triggered by mass movements into the lakes [29], including avalanches, landslides, or rockfall (Figure 1) [30,31]. Evidence from sediment coring of proglacial Lake Gokyo demonstrates that mass movements into lakes have occurred, even at high elevations (4750 m) [32].…”