Clearly reinforcing the critical importance of glacier monitoring, an ice-dammed lake formed by the surging of Shisper Glacier breached for the second time within 2020 on 29 May, causing minor damage to property downstream according to local media reports.
Shisper Glacier, located in the Hunza Valley in northern Pakistan, has been surging since early 2018, blocking the proglacial river of Muchuhar Glacier and forming an ice-dammed lake. The latest early summer outburst was likely triggered by an anomalous temperature rise, which caused significant glacier melt and consequently engorged the lake. The Hunza meteorological station recorded a significant temperature rise in late May 2020.
The lake continues to release water, but it poses potential risk of outburst since it is prone to blockage from debris. The nearby meteorological station is monitoring developments. Using satellite imagery, we analyse the events leading up to the glacial lake outburst flood (GLOF) on 29 May.Unprecedented surge
Shisper Glacier’s surge has been unpredictable and sporadic, making it all the more risky. Beginning in January 2018, the surging continued rapidly, accelerating until November 2018. The surge continued at a stable speed through May 2019, then slowed down, and appeared to completely stop in November 2019. During its active surge phase, the glacier’s snout advanced around 1.5 km. This surge movement was the highest recorded in the Karakoram, with a flow rate of 18 m per day1 and reached up to 48 m per day2). The surge speed was highest in May and June 2018.1Formation of ice-dammed lake and first breach
The surge blocked the proglacial river of Muchuhar Glacier, and the meltwater from the glacier started collecting in mid-November 2018 (Figure 1). The water level gradually rose to form an ice-dammed lake. On 20 June 2019, the lake reached its maximum area of 0.34 sq. km. On the following day, the first breach occurred, resulting in a GLOF that partly damaged the Karakoram Highway and caused minor infrastructure downstream. The lake continued to discharge as subglacial flow till September 2019.Second breach, inundation, and potential damage to Karakoram Highway
Following the first outburst, Shisper Glacier surged at an insignificant rate (total surge of 50 m) from early July 2019 to the end of November 2019. The glacier did not surge beyond its terminus, with no significant increase in the lake extent. By October that year, the lake level had started increasing again. Then, the lake partially drained out from mid-March to early April 2020 before increasing again from 0.245 sq. km on 5 April 2020 to 0.270 sq. km by 30 April. In an image derived from Sentinel-2 data on 25 May (Figure 2), the lake’s water level had peaked at 0.340 sq. km – three days before the breach.
It is important to note that the breach occurred when the lake level reached almost the same extent as in 2019. However, the extent of debris blocking the lake had increased by more than 100 m in length compared with that during the previous surge. The discharge below the debris as observed till November 2019 indicated that the blockage of the lake was partially due to the glacier’s frontal surge.
The lake breach caused a GLOF as observed through Landsat data on 29 May 2020 (Figure 3). The total extent of inundation in the stream before it merged with the main Hunza River was 0.6 sq. km. It appears that an approximately 700 m stretch of the Karakoram Highway experienced partial damage.Meteorological anomaly during the May 2020 breach
The Hunza meteorological station recorded an average temperature of 24oC in June 2019. The temperature significantly increased to 27.7oC on average in late May. This anomalous temperature caused significant glacier melt, resulting in an increase in the lake’s area (similar in extent to the increase in June 2019) till late May 2020 – following which the outburst occurred.Constant monitoring is key
Based on data gathered during the first GLOF in 2019 and the present flood as reported in the local media, the ice-dammed lake is expected to continuously release excessive water. Close monitoring of the lake is necessary to confirm this and gather data for mitigation. Field-based studies of the lake are not currently possible as the glacier surge is blocking access. Passive satellite data acquisition is not possible during cloudy conditions, which will persist over the coming two or three months in the region. Unmanned aerial vehicles should therefore be considered as they provide an ideal means for monitoring and mapping the extent of the lake and its changes. The lake is prone to blockage and could potentially burst again, probably several times during the year. The window of greatest risk starts in April and extends through to September. It is therefore imperative to monitor the lake regularly until the surged ice melts completely.References
1 Bhambri, R., Watson, C. S., Hewitt, K., Haritashya, U. K., Kargel, J. S., Shahi, A. P., Chand, P., Kumar, A., Verma, A., & Govil, H. (2020). The hazardous 2017–2019 surge and river damming by Shispare Glacier, Karakoram. Scientific Reports, 10(1), 1–14. https://doi.org/10.1038/s41598-020-61277-8
2 Rashid, I., Majeed, U., Jan, A., & Glasser, N. F. (2020). The January 2018 to September 2019 surge of Shisper Glacier, Pakistan, detected from remote sensing observations. Geomorphology, 351, 106957.