Flood discharge, subglacial water storage and ice movement during rapidly-rising jökulhlaups from the Skaftá cauldrons, Vatnajökull, Iceland

Results from investigations of jökulhlaups from the subglacial lakes beneath the Skaftá cauldrons in the Vatnajökull ice cap are reported. The subglacial lakes are emptied out regularly in outburst floods where the subglacial hydraulic system is flooded with manyfold the normal water input. These events therefore give an opportunity to investigate the effect of transient changes in water input to the glacier bed on the subglacial hydraulic system and on basal sliding. Several different parameters were monitored for two jökulhlaups from the western Skaftá cauldron in September 2006 and in August 2008, and for a jökulhlaup from the eastern cauldron in October 2008. Interpretation of observations of the jökulhlaup in September 2006 shows that the flood path was mainly formed by ice deformation and lifting but not by melting. The travel time of the flood front under the glacier was in the range 0.2-0.4 m/s. A development towards more efficient subglacial flow over the course of the flood can be inferred from estimates of the total volume of the subglacial floodpath and the discharge time-series. Ice movement during the 2008 floods was measured via a network of five continuously recording GPS-stations, positioned both within the cauldrons and above the flood route. The passage of both jökulhlaup was associated with enhanced down-glacier movement of the ice surface, followed by a sudden vertical uplift. We interpret these signals as evidense for increased basal sliding accompanied by basal uplift due to increasing subglacial water pressure. The jökulhlaup breached the ice surface along extensive fractures, in regions close to the glacier snout, and sediment-laden flood water temporariy flowed through the fractures up to the glacier surface during the initiation of one of the jökulhlaups. Brittle-type seismicity from the snout region of the glacier was registered by Iceland's seismic network during this event. The temperature of the jökulhlaup waters at the glacier snout was close to zero in all these events and floodwater temperatures during the 2008 jökulhlaups were measured to be within 0.02°C of freezing point. These temperature measurements indicates a very effective heat transport from the flood water to the ice walls of the subglacial flood path as essentially all initial heat of the lake water and heat formed in the flood path by potential energy dissipation has been lost from the flood water.