The causes and characteristics of July 2002 Skaftarhlaup, Tungnaarjokull, Iceland

Recent research has demonstrated that some jokulhlaups exhibit a linear rise to peak discharge, the processes responsible for which are improperly understood. In July 2002, a linearly rising jokulhlaup exited Tungnaarjokull, an outlet glacier of Vatnajokull, southern Iceland. The aims of this paper are to use rare observations of the glacier surface and ice margin to reconstruct the dynamics of a complex jokulhlaup, and to explain its causes and characteristics. Aerial reconnaissance in July 2002 showed the growth of an ice-surface cauldron suggesting the rapid evacuation of subglacially stored meltwater at a site of known geothermal activity. Repeated overflights confirmed the rapid release of meltwater from the Western Skafta cauldron to the margin of Tungnaarjokull, 35km south-west, where a jokulhlaup drained into the river Skafta. Observations and photographic evidence were used to calculate ice cauldron dimensions in order to approximate the volumes of drained meltwater. Repeated observations of two main flood outlets at the margin of Tungnaarjokull, allowed flood discharges to be estimated at intervals. Peak discharge estimates from each outlet were calculated using fountain outlet and standing wave dimensions. These observations augment existing knowledge gained from downstream river gauging. Evidence suggests that the jokulhlaup outlets at Tungnaarjokull developed sequentially. The western tunnel outlet was observed during its waning stage whilst the eastern fountain outlet was still rising, resulting in a double flood peak. Frazil ice development around the eastern fountain outlet at Tungnaarjokull implies that glaciohydraulic supercooling contributed to the development of flood outlets. Discharge reconstructed at the glacier snout was approximately 2-3 times greater than the downstream gauged value, pointing to a rapid rate of peak discharge attenuation during the jokulhlaup. The July 2002 jokulhlaup from Tungnaarjokull was characterized by a double flood peak, sequential outlet development, downstream flood hydrograph attenuation and remarkable proglacial sediment reworking. This flood adds to our knowledge of linearly rising j”kulhlaups and to our understanding of jokulhlaup mechanisms and routeways. The identification of the processes at work in this type of event are applicable in other regions prone to volcanically or geothermally-induced floods.