Après Nous, le Déluge: A Human‐Triggered Jökulhlaup From a Subglacial Lake

Abstract

Glacial floods (jökulhlaups) are a phenomenon of some temperate ice masses; they are a significant natural hazard, but their complex hydrology is incompletely understood. We document a jökulhlaup from a subglacial lake in Iceland that was inadvertently triggered by a borehole drilled through the overlying ice. We propose that this borehole allowed an englacial water body to drain into the lake, inducing a transient rise in pressure that overwhelmed the lake's subglacial seal 5 days later. Runaway melting of a subglacial conduit by 4 • C lake water then initiated a flood to the outlet glacier margin. This incident suggests that draining of englacial water bodies via hydrofracturing crevasses and flooding of moulins by precipitation events are potential natural triggers of jökulhlaups and explains a correlation between surface melting and jökulhlaups. This hydraulic trigger could have wider implications for relations between meteorological conditions, drainage, and dynamics of some glaciers. Plain Language Summary Some ice caps and mountain glaciers contain water bodies that drain episodically and catastrophically as subglacial floods, sometimes threatening human life and property. Our observations suggest that we unintentionally triggered such a flood from a subglacial lake in Iceland by boring through the overlying ice. Our conduit allowed water inside the glacier to drain into the lake, causing the pressure to rise and lifting the ice shelf enough to allow the lake to rapidly drain beneath the glacier toward its edge. This event revealed a mechanism by which natural drainage events can trigger such floods and why floods from this particular lake are more likely to occur in summer. Hydrographs (flow vs. time) provide insight into causative mechanisms and dynamics of glacial floods, classifying them as either "slow rising" (1-2 weeks to peak discharge) or "fast rising" (up to 1-2 days).

Surface outburst of a subglacial flood from the Greenland Ice Sheet

“…Prior to the collapse, the basin footprint had been rising at an average rate of 46 cm a −1 between 2012 and 2013, with the outer edge of the basin domed by ∼10-15 m above the surrounding ice surface (Figure 2). We interpret this dynamic feature to be the surface signature of a subglacial lake filling, then rapidly draining, similar to events that have been previously observed in Greenland [32][33][34][35] , Antarctica 48 and Iceland 49,50 .…”

Development of a subglacial lake monitored with radio-echo sounding: Case study from the Eastern Skaftá Cauldron in the Vatnajökull ice cap, Iceland

“…This highlights the temporal variability in the englacial and supraglacial hydrology at or beneath ESC. As suggested by Gaidos et al (2020), the englacial water bodies may play an important role in the triggering of jökulhlaups from the Skaftá cauldrons. A jökulhlaup from WSC in 2015 was most likely triggered via the drilling of a borehole at the cauldron centre, which created a pressure connection between the subglacial lake and an englacial water body above it (Gaidos et al, 2020).…”

“…4). In addition to the RES the bedrock elevation has been measured directly through two boreholes (Gaidos et al, 2020), which fortunately were located within the RES data gap. From the bedrock record, including borehole measurements, a bedrock DEM (Fig.…”

Development of a subglacial lake monitored with radio-echo sounding: case study from the eastern Skaftá cauldron in the Vatnajökull ice cap, Iceland

“…In Iceland, attempts to survey water accumulation below ice cauldrons using changes in the elevation of reflective subglacial surfaces from low-frequency (5 MHz) RES data were motivated by a swift, unexpected jökulhlaup from the cauldrons of Mýrdalsjökull ice cap, southern Iceland, in July 2011 (Galeczka et al, 2014). This particular jökulhlaup destroyed the bridge over the river Múlakvísl, cutting the road connection along the south coast of Iceland for more than a week.…”