New insights into the development of slowly rising j&#246;kulhlaups from the Gr&#237;msv&#246;tn subglacial lake, Iceland, deduced from ICEYE SAR images and in-situ observations
Authors: Magnússon Eyjólfur , Drouin Vincent , Pálsson Finnur , Hannesdóttir Krista , Belart Joaquín M. C. , Wuite Jan , Sigurðsson Gunnar , Einarsson Bergur , Jóhannesson Tómas , Ófeigsson Benedikt G. , Nagler Thomas , Gudmundsson Magnús T. , Högnadóttir Thórdís , Berthier Etienne 
Published: March 2024

We present a study on two j&#246;kulhlaups from the subglacial lake Gr&#237;msv&#246;tn, beneath Vatnaj&#246;kull ice cap, SE-Iceland, giving new insights into the development of slowly rising j&#246;kulhlaups. In the first, spanning the period 14 November &#8211; 10 December, 2021, ~0.92 km3 of water was released, reaching peak discharge from the lake of ~3500 m3 s&#8211;1 from the lake on 4 December. In the latter, taking place 4&#173;&#8211;22 October, 2022, the corresponding numbers were ~0.16 km3 and ~500 m3 s&#8211;1. Both j&#246;kulhlaups were captured by ICEYE X-band radar satellites, with daily repeated SAR images, allowing construction of 3D ice motion above the ~50-km long subglacial flood route, using InSAR and amplitude offset-tracking results. During both j&#246;kulhlaups, the outflow from the lake, derived from the lake level (with GNSS), was monitored, as well as the development of the flood near the glacier margin in the river G&#237;gjukv&#237;sl. During the 2021 j&#246;kulhlaup, the ice motion above the flood path, deduced from the satellite data, was validated with data from a GNSS station operated ~30 km from the glacier margin. Surface elevation changes above the lake before, during and in between the j&#246;kulhaups were derived from Pl&#233;iades optical stereo images. Our data show that during the early phase of these j&#246;kulhaups, a flood wave propagates down glacier at pace of ~7 km d&#8211;1. The flood waves were most likely initiated at a bottleneck formed in a tunnel flow somewhere along the first 10 km of the flood path, while the discharge from the lake was still only few tens of m3 s&#8211;1. Five to seven days passed from the likely initiation of the flood wave until floodwater was detected in the river G&#237;gjukv&#237;sl. The maximum observed horizontal ice motion above the flood path in 2021 was around 3 m d-&#8211;1 or ~5 times the maximum during normal winter conditions. At many locations, the horizontal velocity is increased by an order of magnitude. After the peak discharge from Gr&#237;msv&#246;tn was reached, the glacier almost immediately started slowing down, first rapidly or by ~50% over 1&#8211;2 days, but then gradually down to normal velocities in 4&#8211;5 days. In 2021, the observed rate of uplift was up to 0.5 m d&#8211;1 during the rise of the j&#246;kulhlaup and the subsequent subsidence reached up 1.0 m d&#8211;1 during its decline. The study shows that ~0.3 km3 was stored beneath the glacier during the peak of the j&#246;kulhlaup, and it is therefore expected that the magnitude of the uplift/subsidence reached 2&#8211;3 m in some areas. The width of the flooded areas, observed from the subsidence during the early decline of the j&#246;kulhlaups, was typically 0.5&#8211;1 km the first 20 km of the flood path, while for the remaining 30 km, it was typically 2&#173;&#8211;4 km. The effect of the floods on horizontal ice motion, presumably due to a disturbance in the subglacial water pressure, are, however, observed over a much larger area.