An unusual jökulhlaup resulting from subglacial volcanism, Sólheimajökull, Iceland

Jökulhlaups (glacial outburst floods) are frequent in glaciated terrain. Jökulhlaups exhibiting a sudden rise to peak discharge have not been subject to detailed investigation. A volcanically-generated flood burst from Sólheimajökull, Iceland in July 1999. This paper accounts for the causes, characteristics and impacts of this flood. Pre- and post-flood fieldwork was carried out at Sólheimajökull allowing the reconstruction of mean flow velocities and peak discharge. Flood onset was rapid, rising to a peak discharge of 4.4 x 10³m³s^-1 (± 1.2 x 10³m³s^-1) within one hour. High basal water pressures resulted in floodwater bursting through the glacier surface. Ice rip-up clasts containing glacial diamict provided evidence of floodwater contact with the glacier bed. Within the glacier, jökulhlaup sedimentation occurred within hydrofractures and conduits generating complex hydrofracture fills and esker ridges. In the proglacial zone, regions of flow expansion associated with rapid reductions in sediment transport capacity controlled the locations of major jökulhlaup sedimentation. A large fan composed of material of up to boulder size was deposited at the glacier snout. Two ice-marginal basins filled and drained during the jökulhlaup. One of the basins, Jökulsárgil, emptied rapidly during the flood, via ice-dam flotation, accentuating peak jökulhlaup discharge. High rates of downstream peak discharge attenuation during the July 1999 jökulhlaup provides an analogy with flash flood hydrographs in semi-arid regions and the catastrophic failure of man-made dams. The July 1999 jökulhlaup was initially triggered by the subglacial volcanic eruption, but the characteristics of the flood were accentuated by within-event meltwater storage and release. This unusual jökulhlaup provides an important addition to our understanding of the spectrum of distinctive jökulhlaup characteristics.