GoKawiilFor Jialongco, Galongco, and the potential future lake, we focus below on results relating to the susceptibility of the lakes to produce an outburst event, as well as the potential magnitude of downstream impacts, as simulated under worst-case scenarios. A full hazard and risk assessment, including a complete range of outburst scenarios and vulnerability mapping, is beyond the scope of this study.
4.1 Lake susceptibility and scenario development The susceptibility component of GLOF hazard assessment establishes the likelihood of an event from a given lake, considering the wide-ranging factors that can condition or trigger an outburst. The likelihood (which can be both qualitative or quantitative for some hazards) is always specific to a given magnitude and valid for a given time frame, recognising that susceptibility can evolve over time (Allen et al., 2022). Based on this assessment, scenarios for hazard modelling can be established, including worst-case outburst scenarios as we focus on here. Taking a systematic approach (after GAPHAZ 2017), we compare the relative susceptibility of the three lakes considered in this study, considering also how this susceptibility might evolve in the future (Table 2). The table distinguishes those factors that condition and/or trigger an outburst event while also linking to those factors that inform about possible outburst magnitudes. Located in a transitional zone to the north of the main Himalayan divide, the upper Poiqu basin is subject to heavy rainfall during the Asian summer monsoon. With a significantly larger watershed area, Galongco is considered more susceptible to heavy rain and/or snowmelt leading to high lake water levels, and under future deglaciated conditions the lake may become fed by a well-developed paraglacial stream network. However, even under these conditions, the relatively favourable dam geometry (large width to height ratio and 15 m dam freeboard) suggests that the likelihood and magnitude associated with an outburst via this triggering mechanism is low. Similarly, self-destruction via warm temperatures and melting of ground ice within the moraine dam is extremely unlikely. Creeping permafrost features visible in the vicinity of Galongco, modelled mean annual ground surface temperature (MAGST) (after Obu et al., 2019), and a partially hummocky appearance of the lake dam suggest a strong likelihood of a partially ice-cored moraine, but the huge width (> 200 m) and gentle downstream slope of the dam would make a catastrophic failure in the case of thawing extremely unlikely. As with the majority of large glacial lakes across the Himalaya (Liu et al., 2013; Richardson and Reynolds, 2000; Sattar et al., 2021), the main triggering threat is considered to come from large slope instabilities, impacting the lake. Under current conditions, Jialongco is assessed to be most susceptible to ice avalanches, given the presence of a steep, highly crevassed tongue positioned directly behind the lake (Fig. 2a). With an average slope of 35∘, large transverse crevasses marking a sharp break in topography, and likely temperate conditions at the bed, a full collapse of the glacier tongue (∼ 18 × 106 m3) is considered a feasible worst-case scenario (Table 1). The mass would impact the lake in a direction parallel to the longitudinal axis of the lake, leading to maximum overtopping wave heights and a swashing effect. Smaller ice avalanches from this glacier triggered GLOFs from Jialongco in 2002, at a time when the lake was less than half of its current size (Chen et al., 2013). While atmospheric warming is expected to increase temperatures and meltwater at the glacier bed (Kääb et al., 2021), potentially reducing the stability of the glacier, warming-driven retreat of the tongue will see a reduction in the potential avalanche volume over time, and eventually, this threat will be eliminated completely as the ice retreats to a flatter plateau. In comparison, the partially debris-covered parent glacier tongue of Galongco has a gentle mean slope (18∘) and uniform gradient. Potential unstable ice masses threatening Galongco, from steep ice cliffs and hanging glaciers, are found higher up on the mountain (Fig. 2b), with estimated maximum volumes in the range of 0.1–1 × 106 m3. Avalanches from the larger of these starting zones would strike the lake perpendicular to the longitudinal axis of the lake (from the west), meaning most of the energy from a displacement wave would be dissipated on the opposing side of the lake. It is a similar situation above the future lake, where small and comparatively thin hanging glaciers are restricted to the slopes southwest of the potential lake (Fig. 2c). Hence, a large rock or combined ice–rock avalanche is considered to be the most feasible mechanism capable of triggering a worst-case GLOF from either Galongco or the potential future lake. The northeast-facing slopes of Shishapangma rise nearly 3000 m above Galongco and are likely to be mostly underlain by cold permafrost conditions. This is inferred both from the distribution of rock glaciers in the region, extending down as low as 4000 m a . s . l . (Bolch et al., 2022), and modelled MAGST (Obu et al., 2019) (Table 2). However, the presence of ice cliffs and hanging glaciers can lead to thermal perturbations and even melt conditions in otherwise very cold environment (Shugar et al., 2021). Based on close examination with high-resolution imagery, a large potential starting zone extending from 6550–7340 m a . s . l . was identified on a heavily fractured slope beneath the south ridge of Shishapangma (Fig. 2b). Here, as in the surrounding peaks, layered leucogranite sits above sillimanite gneisses with a gentle northerly dipping schistosity (Searle et al., 1997). The slope has been eroded and is potentially oversteepened by the glacier below. Based on structures outcropping on the face, a 40 m maximum bedrock depth was assumed, while steep ice cliffs and firn covering the slope is estimated to not exceed 10 m, resulting in a combined starting volume of 23 × 106 m3 (20 % ice and 80 % rock). The potential future lake is positioned directly beneath the ice-free ∼ 2000 m high eastern face of Ramthang Karpo Ri (Fig. 2c), where MAGST is in the range of −3 to −6∘C. The face is dissected by numerous vertical structures, and there is evidence of several scarps from previous instabilities. A large potential source area was identified, comparable to the Galongco scenario, with scarps on the face suggesting similar maximum depths of up to 40 m, leading to a total rock avalanche volume of 20.6 × 106 m3 (Table 1). Download Print Version | Download XLSX Even on a global scale, ice and/or rock avalanche volumes of the magnitude included in the scenarios here are rare (Kääb et al., 2021; Schneider et al., 2011), although they have occurred recently (Shugar et al., 2021) and prehistorically (Stolle et al., 2017) in the Himalaya. While Poiqu basin is located within a high seismic hazard zone (Shedlock et al., 2000), it is notable that the 2015 Gorkha earthquake did not cause any large ice and/or rock avalanches in the Poiqu basin despite significant damage in Nyalam and along the highway to Nepal (Kargel et al., 2016). Hence, given a lack of historical large instabilities in the basin, ice and/or rock avalanches of the magnitude included in this study are assessed to be low to very low likelihood events (see also Sect. 5.2). Geologically there is little basis for distinguishing between the likelihood of bedrock failures above the three lakes, and permafrost conditions are comparable (Table 2). Owing to the position of Jialongco directly beneath a steep glacier tongue, the history of ice-avalanche-triggered outburst events, and more unfavourable dam conditions (low freeboard, narrow width), we assess a worst-case outburst from this lake to be more likely than from Galongco under current conditions. Finally, all three lakes are or will be susceptible to instantaneous or progressive landslides occurring from the adjacent lateral moraines, most notably for Jialongco where active instabilities are clearly evident (Fig. 2a). Recent studies have shown that large lateral failures, either instantaneous or progressive, can be sufficient to initiate catastrophic process chains where dam geometries are sufficiently prone to erosion (Zheng et al., 2021b).
4.2 GLOF modelling Worst-case outburst scenarios for the three lakes were simulated until the border between China and Nepal (town of Zhangmu). The modelled flow does not extend beyond the border owing to the limited coverage of the required high-resolution Pléiades DEM. Of the two current lakes assessed, the modelled peak discharge from Galongco is more than 5 times larger than that from Jialongco, leading to flow depths of up to 14 m higher impacting the town of Nyalam (Table 3, Figs. 3 and 4). At the border, 20 km downstream, inundation depths are up to 17 m higher for the Galongco simulation, as the large volume of water becomes constricted in the narrow topography of the valley, with discharge values remaining above 100 000 m3 s−1 even after 1 h (Fig. 4). The simulated worst-case outburst from the potential future lake has a calculated peak discharge at the dam of 359 628 m3 s−1, resulting in flow depths (27 m) and discharge (163 667 m3 s−1) in Nyalam that would exceed those of Jialongco but are an order of magnitude lower than from Galongco. Differences in the shape of the outflow hydrographs at the dam (Fig. 4a) and travel distance lead to minor variations in the arrival of the modelled flood waves in Nyalam and further downstream at the border with Nepal. The flood wave from Jialongco first registers after 6 min in Nyalam, with the maximum flow heights arriving 2 min later (all times relative to the initial avalanche release). In contrast, the flood wave from Galongco first registers after 10 min, with maximum flow heights arriving 4 min later. An outburst from the potential future lake has a similar arrival time of only 11 min in Nyalam, while all simulated outbursts reach the Nepalese border within a range of 28–32 min after the avalanche release. Notably, the remedial measures undertaken at Jialongco, which have lowered and armoured the lake dam (erosion set to zero in the model – see Sect. 3.2), result in a slightly larger initial peak discharge (Table 3, Fig. 4) because there is a greater splashing effect and larger overtopping volume owing to the reduced freeboard. Downstream, the simulated GLOF then attenuates at a slower rate (54 % decrease in discharge between Nyalam and Zhangmu) compared to the simulation for the original lake (81 % decrease in discharge between Nyalam and Zhangmu) (Fig. 4). Download Potential processes that could further enhance the GLOF magnitude include entrainment of large volumes of sediment along the flow path leading to additional bulking of the flow volume, blockages of a river by GLOF deposits leading to secondary outburst events, and a process chain involving more than one lake. Significant erosion of sediment from within the main river channels is considered unlikely for any of the three outburst scenarios, given that average trajectory slope angles measured along the flow paths are well below those needed to entrain sediment from within a channel (Huggel et al., 2004). However, undercutting, erosion, and destabilisation of the river banks as a result of the GLOF mean that such secondary hazards cannot be excluded, particularly in the steep-sided gorge downstream of Nyalam. Immediately below Nyalam, the valley narrows, leading to pooling of water in the simulations, and a backwash effect is produced that extends 2 km up the Poiqu river, with maximum flow depths of > 60 m under the Galongco scenario (Fig. 3a). Significant deposition of sediment can be anticipated within this backwash zone, with the potential to block the Poiqu river and form a major secondary hazard, in line with processes observed and modelled during the 2021 catastrophic mass flow in Chamoli, northern India (Shugar et al., 2021). In contrast to previous modelling results for Galongco (Shrestha et al., 2010; Zhang et al., 2021), the worst-case avalanche-triggered GLOF path is not confined to the existing river channel, overtopping the orographic right side of the valley (bounded by old moraines) and spilling over into Jialongco to form a second, larger flow path towards Nyalam (Fig. 3a). The two paths converge again about 6 km upstream from Nyalam. The hyper-elevation of the flow that enables this overtopping is consistent with observations of catastrophic mass flows of comparable magnitudes (Shugar et al., 2021). Results further indicate that an outburst event from the potential future lake could slam into, pool up, and eventually overtop the lateral moraine of Jialongco, producing a potential chain reaction where Jialongco also breaches (Fig. 5). Despite adding volume to the flow, the presence of Jialongco with its prominent lateral moraine acts as a topographic obstruction that slows and reduces the energy of the outburst event, with a 50 % reduction in discharge values measured immediately upstream and downstream of Jialongco. Although this is only one specific cascading lake interaction, this example highlights that lakes positioned downstream of another lake do not necessarily increase GLOF hazard, depending upon the downstream lake geometry and its orientation relative to the incoming GLOF path.
4.3 GLOF impact and exposure We identify from Open Street Map and Google Earth imagery the buildings in Nyalam exposed to different GLOF intensity levels according to simulated debris flow intensities (based on GAPHAZ 2017). While classification schemes vary across countries, land areas potentially affected by high flood or debris flow intensities (calculated on the basis of flow heights and/or flow velocities) are typically considered high hazard zones even for low-probability events (GAPHAZ, 2017). In Nyalam, lower flow heights associated with an outburst from Jialongco result in marginally lower levels of exposure compared to simulated events from Galongco or the potential future lake (Fig. 6). Despite the majority of buildings in Nyalam being located 10–20 m above the river channel, where they have been unaffected by past outburst events from Jialongco (Chen et al., 2013), there is clearly significant exposure within the high-intensity zone of a worst-case outburst. Furthermore, the rapid expansion of infrastructure along the river banks north of the main settlement over the past several years has significantly increased the built area exposed to potential GLOF events, with many new buildings located in the high-intensity flood zone. Overall, levels of exposure are comparable for simulated outbursts from both Galongco and the potential future lake, with both worst-case events also likely to disrupt the main highway and bridges linking to the town. Downstream from Nyalam in the reach to the border with Nepal there are few buildings located along the riverbank, and the main threat is to the 38 km stretch of the transnational highway, of which the proportion affected by high-intensity flood levels is 27 % and 40 % for modelled outbursts from Jialongco and Galongco, respectively (and 28 % for the future lake scenario). While we did not simulate beyond the border, previous events (e.g. Cook et al., 2018; Wang et al., 2018) and assessment studies (Khanal et al., 2015a; Shrestha et al., 2010) have highlighted the significant risk to Nepalese communities, hydropower stations, and other infrastructure located along the banks of the Bhote Koshi river. Download