EGU22-8240
https://doi.org/10.5194/egusphere-egu22-8240
EGU General Assembly 2022
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

The jökulhlaup from the subglacial lake Grímsvötn, beneath Vatnajökull ice cap, in November-December 2021, revealing new insight in to slowly rising jökulhlaups

Eyjólfur Magnússon1, Vincent Drouin2, Finnur Pálsson1, Krista Hannesdóttir1, Joaquín M. C. Belart1,3, Gunnar Sigurðsson2, Jan Wuite4, Tómas Jóhanneson2, Benedikt G. Ófeigsson2, Thomas Nagler4, Magnús T. Gudmundsson1, Thórdís Högnadóttir1, Michelle Parks2, Matthew J. Roberts2, and Etienne Berthier5
Eyjólfur Magnússon et al.
  • 1Universtity of Iceland, Institute of Earth Sciences, Reykjavík, Iceland (eyjolfm@hi.is)
  • 2Icelandic Meteorological Office, 105 Reykjavík, Iceland (vincentdr@vedur.is)
  • 3National Land Survey of Iceland, Akranes, Iceland (joaquin.m.belart@lmi.is)
  • 4ENVEO IT GmbH, Innsbruck, Austria (Jan.Wuite@enveo.at)
  • 5LEGOS, Université de Toulouse, CNRS, Toulouse, France (etienne.berthier@legos.obs-mip.fr)

The subglacial lake Grímsvötn, beneath Vatnajökull ice cap, has been an important study area since the first attempts to explain the physics of jökulhlaups. The lake, covered by an up to 300 m thick ice shelf, is situated within a caldera of an active central volcano. It collects meltwater produced by geothermal and volcanic activity, in addition to meltwater from the glacier surface. During most of the 20th century the period of water accumulation was typically 4-6 years, collecting 1-3 km3 of water. The jökulhlaups, as  observed in the river at the glacier terminus ~50 km south of the lake, typically reached peak discharge of 2,000-10,000 m3s-1 after approximately exponential increase over 2-3 weeks. In October 1996, 3.2 km3 of meltwater from an eruption north of Grímsvötn were collected in the lake. This resulted in hydrostatic uplift of the lake ice dam and sudden release of the accumulated water, reaching a peak flow of ~50,000 m3s-1 at the glacier terminus in less than a day. Due to damage to the lake ice dam during the 1996 jökulhlaup and further undermining from geothermal activity near the dam, the water accumulation and release has been different after this event. Between 1996 and 2018, smaller jökulhlaups typically occurred at 1-2 year intervals with the largest volume of ~0.6 km3 in 2004 and 2010. The jökulhlaup discharge still resembled an exponentially rising discharge, but faster, reaching a peak discharge at the glacier front in 3-5 days after detection of flood water in the river. In 2004 and 2010 the peak discharge was ~3,000 m3s-1. From autumn 2018 until November 2021, ~1 km3 of water accumulated in Grímsvötn. The lake level has been monitored since the 1990s, but now with increased accuracy using online GNSS stations located on the floating ice shelf and repeated glacier surface mapping using Pléiades stereo images. Around mid-November 2021 the GNSS instruments started subsiding, revealing that the lake had started draining. In 3 weeks, the discharge from the lake, estimated from the subsidence rate and the lake hypsometry, gradually increased from a few m3s-1 to a peak discharge of ~3500 m3s-1 on 4 December. A few days later, the lake had drained completely. We present the data showing the development at the lake prior to and during the jökulhlaup, and we report on: a) discharge measurements near the glacier front, which combined with the lake discharge allows for an estimate of the temporal subglacial floodwater storage, b) horizontal and vertical ice motion in the vicinity and above the subglacial flood route during the jökulhlaup, from ICEYE and Sentinel-1 radar images obtained with InSAR (24 hour repeat) analysis and amplitude offset tracking, showing the distribution of flood water and the widespread effect of the jökulhlaup on the horizontal ice motion, c) ice surface motion measured by a GNSS station located half-way between the lake and the glacier margin, spanning the entire jökulhlaup. All this provides new insight into the physical processes occurring during a slow, exponentially rising jökulhlaup from Grímsvötn.

How to cite: Magnússon, E., Drouin, V., Pálsson, F., Hannesdóttir, K., Belart, J. M. C., Sigurðsson, G., Wuite, J., Jóhanneson, T., Ófeigsson, B. G., Nagler, T., Gudmundsson, M. T., Högnadóttir, T., Parks, M., Roberts, M. J., and Berthier, E.: The jökulhlaup from the subglacial lake Grímsvötn, beneath Vatnajökull ice cap, in November-December 2021, revealing new insight in to slowly rising jökulhlaups, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8240, https://doi.org/10.5194/egusphere-egu22-8240, 2022.

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