Glacier lake outburst floods caused by glacier shrinkage: case study of Ala-Archa valley, Kyrgyz Ala Too, northern Tian Shan, Kyrgyzstan

Changes in glacier extent and runoff in Central Asia increase socio-economic stress and may result in political conflict between donors of freshwater (Kyrgyzstan, Tajikistan) and recipients of freshwater (Uzbekistan, China). Glaciers in the Pamir and Tian Shan regions have experienced an unprecedented downwasting due to regional climate changes over the past decades. This is because air temperature increases are in some areas accompanied by a decrease in precipitation. Such conditions have already resulted in a reduction of glacier runoff, especially in the northern and western Tian Shan, and an increase of the number and area of glacial lakes in Kyrgyzstan. Even though glacial lakes in the mountains are in general relatively small and located far from densely populated areas, their outbursts often produce destructive debris flows. Such debris flows are especially common in Kyrgyzstan because of its steep river channels and abundance of Holocene and Quaternary glacier deposits that can be remobilized. The glacial lake outburst flood (GLOF) in the Shakhimardan river catchment in 1999, for example, resulted in 100 fatalities in Uzbekistan, and the GLOF from the Zyndan glacial lake led to substantial economic losses in 2009. According to the latest inventory, there are more than 350 glacial lakes in Kyrgyzstan of which about 70 occur in the Kyrgyz Ala Too. The Ala-Archa valley is among the most important glacierized catchments in Kyrgyzstan. Despite the presence of a relatively small glacier-covered area of 36 km2, the Ala-Archa river is of critical importance to the Bishkek area, its agriculture, and its population which currently exceeds one million. GLOFs are therefore a threat to both numerous settlements of touristic value in the Ala-Archa headwaters and to Bishkek. The Teztor lake in the Adygene catchment of the Ala-Archa river system experienced an outburst during 1988 and 2005. On the early morning of July 31, 2012, this lake began draining through a dam composed of stagnant ice and debris. With the water discharge being merely a few m3/s, the GLOF transformed into a debris flow beyond a steep front of rock glacier, which is within 150 m downstream of the lake. After rushing through the ca. 7 km-long Adygene valley, the debris flow formed a fan in the Ala-Archa valley and transformed into a flood. The flood wave reached Bishkek located more than 40 km downstream. This led to a panic amongst local dwellers and Bishkek residents. The maximum discharge of the debris flow in the lower part of the Adygene valley was assessed as 300 m3/s and the discharge of the flood in Bishkek as 35 m3/s. The latter exceeds the standard discharge of the Ala-Archa river substantially. Although no fatalities resulted from this event, economic losses as a consequence of a destroyed mineral water factory could possibly amount to USD 200000, which is a substantial sum by Kyrgyz standards. Because of the prior history, it is expected that GLOFs from the Teztor valley will recur in the upcoming years. We conclude that installation of an early warning system in lower Adygene valley is needed to prevent further damage in the study area.