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

We present a study on two jökulhlaups from the subglacial lake Grímsvötn, beneath Vatnajökull ice cap, SE-Iceland, giving new insights into the development of slowly rising jökulhlaups. 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 s^–1 from the lake on 4 December. In the latter, taking place 4­–22 October, 2022, the corresponding numbers were ~0.16 km³ and ~500 m³ s^–1. Both jökulhlaups 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 jökulhlaups, 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 Gígjukvísl. During the 2021 jökulhlaup, 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 jökulhaups were derived from Pléiades optical stereo images. Our data show that during the early phase of these jökulhaups, a flood wave propagates down glacier at pace of ~7 km d^–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^–1. Five to seven days passed from the likely initiation of the flood wave until floodwater was detected in the river Gígjukvísl. 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 Grímsvötn was reached, the glacier almost immediately started slowing down, first rapidly or by ~50% over 1–2 days, but then gradually down to normal velocities in 4–5 days. In 2021, the observed rate of uplift was up to 0.5 m d^–1 during the rise of the jökulhlaup and the subsequent subsidence reached up 1.0 m d^–1 during its decline. The study shows that ~0.3 km³ was stored beneath the glacier during the peak of the jökulhlaup, and it is therefore expected that the magnitude of the uplift/subsidence reached 2–3 m in some areas. The width of the flooded areas, observed from the subsidence during the early decline of the jökulhlaups, was typically 0.5–1 km the first 20 km of the flood path, while for the remaining 30 km, it was typically 2­–4 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.