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Glacial lake outburst floods (GLOFs) are a major concern throughout High Mountain Asia, where societal impacts can extend far downstream. This is particularly true for transboundary Himalayan basins, where risks are expected to further increase as new lakes develop. Given the need for anticipatory approaches to disaster risk reduction, this study aims to demonstrate how the threat from a future lake can be feasibly assessed alongside that of worst-case scenarios from current lakes, as well as how this information is relevant for disaster risk management. We have focused on two previously identified dangerous lakes (Galongco and Jialongco), comparing the timing and magnitude of simulated worst-case outburst events from these lakes both in the Tibetan town of Nyalam and downstream at the border with Nepal. In addition, a future scenario has been assessed, whereby an avalanche-triggered GLOF was simulated for a potential large new lake forming upstream of Nyalam. Results show that large ( > 20 × 10 6 m 3 ) rock and/or ice avalanches could generate GLOF discharges at the border with Nepal that are more than 15 times larger than what has been observed previously or anticipated based on more gradual breach simulations. For all assessed lakes, warning times in Nyalam would be only 5–11 min and 30 min at the border. Recent remedial measures undertaken to lower the water level at Jialongco would have little influence on downstream impacts resulting from a very large-magnitude GLOF, particularly in Nyalam where there has been significant development of infrastructure directly within the high-intensity flood zone. Based on these findings, a comprehensive approach to disaster risk management is called for, combining early warning systems with effective land use zoning and programmes to build local response capacities. Such approaches would address the current drivers of GLOF risk in the basin while remaining robust in the face of worst-case, catastrophic outburst events that become more likely under a warming climate.

Allen, S. K., Sattar, A., King, O., Zhang, G., Bhattacharya, A., Yao, T., and Bolch, T.: Glacial lake outburst flood hazard under current and future conditions: worst-case scenarios in a transboundary Himalayan basin, Nat. Hazards Earth Syst. Sci., 22, 3765–3785, https://doi.org/10.5194/nhess-22-3765-2022, 2022.

Widespread retreat of glaciers has accelerated over recent decades in the Himalaya as in most other mountain regions worldwide as a consequence of global warming (Bolch et al., 2019; King et al., 2019; Maurer et al., 2019; Zemp et al., 2019). A main consequence has been the rapid expansion and new formation of glacial lakes (Gardelle et al., 2011; Nie et al., 2017; Shugar et al., 2020), which has large implications for both water resources and hazards (Haeberli et al., 2016). When water is suddenly and catastrophically released, glacial lake outburst floods (GLOFs) can devastate lives and livelihoods up to hundreds of kilometres downstream (Carrivick and Tweed, 2016; Lliboutry et al., 1977). This threat is most apparent in the Himalaya, where glacial lakes have been increasing rapidly in both size and number (Chen et al., 2021; Wang et al., 2020; Zhang et al., 2015), and where a frequency of 1.3 GLOFs per year has been recorded since the 1980s (Veh et al., 2019). The fact that GLOFs can extend across national boundaries exacerbates the challenges for early warning or other risk reduction strategies, particularly in politically sensitive regions (Allen et al., 2019; Khanal et al., 2015a).

Lakes can develop either underneath (subglacial), at the side of, in front of (proglacial), within (englacial), or on the surface of a glacier (supraglacial), with the dam being composed of ice, moraine, or bedrock. In Asia, most scientific attention has focussed upon the hazard associated with the catastrophic failure of moraine-dammed lakes and particularly those trapped behind proglacial moraines (e.g. Fujita et al., 2013; S. Wang et al., 2015). Such lakes can be very large, with volumes larger than 100×10 6 m 3 and depths exceeding 200 m (Cook and Quincey, 2015), and are susceptible to a range of failure mechanisms owing to the low material strength of the dam structure (Clague and Evans, 2000; Korup and Tweed, 2007). In Asia, as elsewhere in the world, displacement waves generated from large impacts of ice or rock have contributed to the majority of moraine dam failures, occurring predominantly over the warm summer months (Emmer and Cochachin, 2013; Liu et al., 2013; Richardson and Reynolds, 2000). At least 17 GLOF disasters (causing loss of life or infrastructure) have been documented in Tibet since 1935, mostly originating in the central-eastern section of the Himalaya (Nie et al., 2018). Coupled with rapidly increasing population and infrastructural development in the region, an urgent need for authorities to take action and implement timely risk reduction measures has been acknowledged (Wang and Zhou, 2017), considering the best available knowledge on existing threats (e.g. Allen et al., 2019; S. Wang et al., 2015; Wang et al., 2018) but also with a view to the future (Furian et al., 2021; Zheng et al., 2021a).

Despite no clear trend observed in GLOF activity over recent decades in the Himalaya (Veh et al., 2019), the ongoing expansion of lakes towards steep and potentially destabilised mountain flanks is expected to lead to new challenges in the future with implications for hazards and risk (Haeberli et al., 2017). Based on approaches to model the possible future expansion and development of new lakes (Linsbauer et al., 2016), several studies have aimed to quantify the possible implications for GLOF frequency and/or magnitude for different regions (Allen et al., 2016; Emmer et al., 2020; Magnin et al., 2020). For example, in the Indian Himalayan state of Himachal Pradesh, Allen et al. (2016) demonstrated a 7-fold increase in the probability of a GLOF being triggered and a 3-fold increase in the downstream area affected by potential GLOF paths under future deglaciated conditions. Meanwhile, Zheng et al. (2021a) have elaborated such analyses for the entire High Mountain Asia, revealing that the number of lakes posing a transboundary threat within border areas of China and Nepal could double in the future. While such large-scale, first-order studies are important for raising general awareness of the future challenges that mountain regions will face (Hock et al., 2019), there are limitations in the extent to which these studies can directly inform planning and response actions at the ground level.

The need for forward-looking, anticipatory approaches to hazard and risk modelling, including attention to possible worst-case scenarios, is clearly recognised within international guidelines on glacier and permafrost hazard assessment (GAPHAZ, 2017). However, practical examples on how to account for worst-case scenarios and future lake development in local GLOF hazard assessment and risk management have been rarely demonstrated. International best practice is framed by both a first-order assessment undertaken at large scales (to identify potentially critical lakes), followed by a detailed assessment for these lakes using numerical models to simulate downstream flood intensities as a basis for hazard mapping (GAPHAZ, 2017). This is a common approach for existing threats, for which the time, data, and expertise needed to invest in comprehensive hazard modelling and mapping can be well justified for a lake that is known to be critical; yet worst-case scenarios are often neglected and may far exceed historical precedence. For future lakes, where the timing of lake formation is typically highly uncertain, there remains a methodological gap in the hazard assessment process, as authorities are unlikely to undertake sophisticated hazard mapping for a threat that may not even eventuate. In this study we aim to address these gaps by providing an illustrative example of how a worst-case outburst scenario from a potential future lake can be systematically assessed alongside the threat posed by current lakes, before discussing the relevance of such an assessment for disaster risk management in a transboundary context.