Development of a Potentially Hazardous Pro-Glacial Lake in Aksay Valley, Kyrgyz Range, Northern Tien Shan

The lake formed between 1988 and 1994 on the site of a retreated glacier in the upper part of the valley. The study aimed to assess the possibility of an outburst of this pro-glacial lake in the future, which could cause powerful debris flows posing a threat to permanent residents living downstream in the Ala-Archa Valley. The Aksay Valley is a debris flow-dominated watershed, with a high Melton index of 0.6 and an average stream gradient of 0.17. The valley is characterized by steep slopes, loose material accumulation, and a large source area of debris flow deposits. The Aksay moraine-glacier complex is among the most dynamically developing glacier complexes in Kyrgyzstan, providing all the natural conditions for the formation of GLOF-induced debris flows in the future. The study involved a combination of field work and remote sensing methods to evaluate possible impacts of lake outburst floods and debris flows in populated areas. The researchers conducted bathymetry mapping of the lake, detailed profile sections of the valley, flow rate measurements on the Aksay river, and monitoring of the lake development using satellite images. Modelling of secondary debris flow inundation heights and hazard footprints was also undertaken. The results of the study suggest that the pro-glacial lake has undergone certain changes in runoff to the subsurface, and an increase in lake depth is observed. Glacial lakes with subsurface drainage are considered to be the most hazardous type, as the knowledge of drainage channels functioning is still very limited and the timing of an outburst is hard to predict. Development of monitoring approaches to support forecasting of these hazards is of paramount importance to safety in mountain territories globally. The Aksay Valley has a long history of GLOFs and secondary debris flow events, with repeated large-magnitude events leading to the formation of a large alluvial fan system at the mouth of the valley. The major events on record were in 1968 and 1969, triggered by an outburst of an englacial water pocket. During these events, local authorities recorded peak discharge values of almost 900 and 800 m3 s−1, respectively. The study site is located in the upper part of the Ala-Archa River basin, which is part of the large endorheic Chu basin. The Aksay catchment size is 28.3 km2, and its elevation ranges from 4895 m a.s.l. (Semenov Tianshanskiy peak) to 2200 m a.s.l. (lowest point of the fan). The pro-glacial lake is at an elevation of 3617 m a.s.l., and its approximate volume is 27 × 103 m3 (as of 2016). The Aksay moraine-glacier complex provides all the natural conditions for the formation of GLOF-induced debris flows in the future, including glacier retreat, development of a lake, steep valley, and large source area of loose material accumulated in the valley floor. In contrast to other complexes in the Ala-Archa Valley, the Aksay complex creates a direct threat to infrastructure and people associated with rest areas and the road network which connects Bishkek to the national park.