Surge dynamics and lake outbursts of Kyagar Glacier, Karakoram

Abstract

Abstract. The recent surge cycle of Kyagar Glacier, in the Chinese Karakoram, caused formation of an ice-dammed lake and subsequent glacial lake outburst floods (GLOFs) exceeding 40 million m3 in 2015 and 2016. GLOFs from Kyagar Glacier reached double this size in 2002 and earlier, but the role of glacier surging in GLOF formation was previously unrecognised. We present an integrative analysis of the glacier surge dynamics from 2011 to 2016, assessing surge mechanisms and evaluating the surge cycle impact on GLOFs. Over 80 glacier surface velocity fields were created from TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement), Sentinel-1A, and Landsat satellite data. Changes in ice thickness distribution were revealed by a time series of TanDEM-X elevation models. The analysis shows that, during a quiescence phase lasting at least 14 years, ice mass built up in a reservoir area at the top of the glacier tongue, and the terminus thinned by up to 100 m, but in the 2 years preceding the surge onset this pattern reversed. The surge initiated with the onset of the 2014 melt season, and in the following 15 months velocity evolved in a manner consistent with a hydrologically controlled surge mechanism. Dramatic accelerations coincided with melt seasons, winter deceleration was accompanied by subglacial drainage, and rapid surge termination occurred following the 2015 GLOF. Rapid basal motion during the surge is seemingly controlled by high water pressure, caused by input of surface water into either an inefficient subglacial drainage system or unstable subglacial till. The potential lake volume increased to more than 70 million m3 by late 2016, as a result of over 60 m of thickening at the terminus. Lake formation and the evolution of the ice dam height should be carefully monitored through remote sensing to anticipate large GLOFs in the near future.

Brief communication: The Khurdopin glacier surge revisited – extreme flow velocities and formation of a dammed lake in 2017

“…Basal sliding is also most likely the dominant flow process as the cross-profiles of surface velocity indicate plug flow, characterized by flat rather than parabolic velocity profiles as was observed during the quiescent phase (Kamb et al, 1985). As previously suggested, the surge on Khurdopin is hence likely triggered by the thermal switch, but the actual surge is dominated by basal sliding (Quincey and Luckman, 2014), similar to Kyagar Glacier (Round et al, 2017). Future field observations should focus on finding possible evidence for these processes and possible feedback processes, especially related to the deformation of water-saturated granular base material that could explain these extreme acceleration rates and peak velocities (Damsgaard et al, 2015).…”

“…During the surge events, the lower tongue is pushed further into the valley and has blocked the Vijerab River on several occasions, resulting in an ice-dammed lake. In the region, a similar process has been observed and well documented for Kyagar Glacier (Round et al, 2017). Sudden drainage of the Khurdopin Lake has caused destruction to downstream villages before, which led to the development of an early warning system with bonfires along the slopes of the entire Shimshal Valley (Iturrizaga, 2005).…”

“…In combination with a gradual accumulation of mass on the upper tongue during quiescence and a resulting steepening surface gradient, the actual surge starts rapidly when a tipping point is reached. Ice deformation u d (Greve and Blatter, 2009;Round et al, 2017), with a modelled ice thickness between 120 and 350 m, results in a velocity of 1 to 60 m a −1 for the 1-4 • steep surface gradient over the whole tongue and 3 m a −1 at the steep section. This is of the same order of magnitude as the measured velocities during quiescence and the early build-up phase.…”

Brief communication: Unabated wastage of the Juneau and Stikine icefields (southeast Alaska) in the early 21st century

“…As noted in the Introduction, Melkonian et al ( , 2016 accounted for this by subtracting the C-band and Xband SRTM DEM, assuming no penetration of the X-band DEM (Gardelle et al, 2012). However, X-band penetration can reach several meters into cold snow and firn (e.g., Dehecq et al, 2016;Round et al, 2017). In the case of the SIF, Melkonian et al (2016) assumed no penetration below 1000 m a.s.l.…”

“…Melkonian et al ( , 2016 accounted for this penetration by subtracting the simultaneous C-band and X-band SRTM DEMs, assuming no penetration of the X-band DEM (Gardelle et al, 2012), the best available correction at the time of their study. However, this strategy may not be appropriate given that the X-band penetration depth has recently been recognized to reach several meters in cold and dry snow/firn (e.g., Dehecq et al, 2016;Round et al, 2017). In this context, the goal of this brief communication is to recalculate the early-21st-century geodetic mass balances of the Juneau and Stikine icefields using multi-temporal ASTER DEMs, carefully excluding the SRTM DEM to avoid a likely penetration bias.…”