What Are Jökulhlaups? Glacial Flash Floods Release Meltwater

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Eyjafjallajokull eruption

Subglacial eruptions such as this at Eyjafjallajokull can cause dramatic floods. Image credit: Halldór Björnsson, IMO

The word jökulhlaup is Icelandic in origin and describes the sudden, violent and short-lived increase in discharge of glacial meltwater.

Relatively common, most jökulhlaups occur when meltwater on a glacier or icecap is contained behind an ice dam which, inevitably, bursts, causing a brief but significant flood event and are commonly associated with subglacial volcanic eruptions. Such jökulhlaups were a feature of the decay of the ice sheets during the last ice age, when many of them occurred on a very large scale.

These massive floods were the product of melting over a long period of time; but jökulhlaups can also be triggered by the existence of volcanic activity. Where geothermal heat sources underlie an ice sheet, the melting is rapid and the flooding, though on a relatively small scale, is nonetheless spectacular.

Jökulhlaups occur wherever glacier ice melts, sometimes seasonally. The best documentation of such events, however, is in Iceland where the combination of a large ice cap and an active volcanic environment makes them relatively common.

The current (September 2014) eruption of Iceland’s Bárðarbunga volcano has potential to cause such a flash flood.

Jökulhlaups in Iceland

With considerable current interest in subglacial eruptions in Iceland, it’s worth considering the potential for flood damage. The floods are extremely common: Bjornsson amassed statistics in 2003 which suggest that 15 subglacial eruptions in the twentieth century had glacial outburst flooding associated with them and that such floods might last from days to weeks; while in 2008, Gudmunsson, et al. put the number of actual jökulhlaups at 40-45.

Volcanically-triggered jökulhlaups can, through rapid heating, cause significant and spectacular events, the largest and most noteworthy of which occur in association with eruptions at Katla. Their scale is astonishing: “The greatest jökulhlaups … are among the largest floods that humans have witnessed. At their maximum, the discharge may be larger than the average discharge of the River Amazon” according to Sigurðsson et al.

Tomasson described the 1918 jökulhlaup as ‘the greatest even in the glaciological history of Iceland,’ and calculated the total discharge rate as probably greater than 30,000 cubic metres per second.

The flood deposits left by the Eyjafjallajokull eruption. Photo: Jennifer Young

The flood deposits left by the Eyjafjallajokull eruption. Copyright image by Jennifer Young, all rights reserved.

Being relatively accessible and widely predicted, the jökulhlaup at Gjalp, part of the Grímsvötn system, in 1996 was monitored in detail. A spectacular event, it was exceptionally rapid, draining a volume of 3.2 cubic km in 40 hours with flow as high as 5000 cubic metres per second. The level of the glacial lake fell by 170m: Scientists who reported on this event described it as, ‘extraordinary.’

Obviously, such a force of nature had a considerable impact on the local infrastructure. Although the event was predicted and monitored, the flood damaged or destroyed several roads and bridges, along with power and telephone lines.

Risks to Humans From Icelandic Jökulhlaups: Planning vs. Complacency

Because jökulhlaups are more easily predicted than, say earthquakes, it is easier to manage the risks involved and to evacuate the population – though not, as is clear from the Grimsvötn experience, the infrastructure.

The authorities had evacuation plans in place in place prior to the 1996 event and no lives were lost. It is interesting to note, however, that a survey in the local area indicated that only around half of local residents believed that there was a risk to them and their community, according to the 2009 research of Deanne Bird.

Similarly, a survey Bird conducted in a popular tourist area near the Katla volcano found a level of complacency among the tourist population. While all of those surveyed knew that the area was volcanically active, there was a significant lack of awareness of the jökulhlaup hazard and of emergency procedures in place .

Bárðarbunga Fissure Eruption

The current fissure eruption at Bárðarbunga is moving towards the ice and may become subglacial. If it does so, then Iceland’s Institute of Earth Sciences bulletin of 6 September indicates that it “would most likely produce a flood in Jökulsá á Fjöllum.” The scale of such a flood remains to be seen; but it’s certain that it won’t be the last event of its kind.

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