New insights into the development of slowly rising jkulhlaups from the Grmsvtn subglacial lake, Iceland, deduced from ICEYE SAR images and in-situ observations

We present a study on two jkulhlaups from the subglacial lake Grmsvtn, beneath Vatnajkull ice cap, SE-Iceland, giving new insights into the development of slowly rising jkulhlaups. In the first, spanning the period 14 November 10 December, 2021, ~0.92 km3 of water was released, reaching peak discharge from the lake of ~3500 m3 s1 from the lake on 4 December. In the latter, taking place 422 October, 2022, the corresponding numbers were ~0.16 km3 and ~500 m3 s1. Both jkulhlaups 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 jkulhlaups, 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 Ggjukvsl. During the 2021 jkulhlaup, 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 jkulhaups were derived from Pliades optical stereo images. Our data show that during the early phase of these jkulhaups, a flood wave propagates down glacier at pace of ~7 km d1. 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 s1. Five to seven days passed from the likely initiation of the flood wave until floodwater was detected in the river Ggjukvsl. The maximum observed horizontal ice motion above the flood path in 2021 was around 3 m d-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 Grmsvtn was reached, the glacier almost immediately started slowing down, first rapidly or by ~50% over 12 days, but then gradually down to normal velocities in 45 days. In 2021, the observed rate of uplift was up to 0.5 m d1 during the rise of the jkulhlaup and the subsequent subsidence reached up 1.0 m d1 during its decline. The study shows that ~0.3 km3 was stored beneath the glacier during the peak of the jkulhlaup, and it is therefore expected that the magnitude of the uplift/subsidence reached 23 m in some areas. The width of the flooded areas, observed from the subsidence during the early decline of the jkulhlaups, was typically 0.51 km the first 20 km of the flood path, while for the remaining 30 km, it was typically 24 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.