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Monitoring of the jökulhlaup


After the end of the eruption an enormous amount of water had gathered in the subglacial reservoir of Grímsvötn caldera. The water level had rosen by 110 m from 1400 m before the eruption to a height of 1510 m (Oddur Sigurðsson, pers. comm. Nov. 8, 1996).
After completing a 3-D volume model of Grímsvötn reservoir, a calculation of the volume could be done to replace the first estimation based on data of former jökulhlaups, given by Björnsson (1988) and Björnsson & Einarsson (1990).

 

3-D volume model of Grímsvötn caldera, based on data given by Björnsson (1988) and Björnsson & Einarsson (1990); look direction to ESE (240°)

The recalculation of the amount of water which possibly could be drained from the reservoir during an expected 180 m drop of the lake level due to former post jökulhlaup lake levels gave us an amount of 3.3 - 3.7 km3. In addition to that a volume of 0.5 -0.7 km3 water kept in the ice fissure in the area of the eruption has to be accounted. Therefore a volume of approx. 4 km3 had to be expected.
This is approx 2 km3 less than previously estimated on the basis of volumes of former jökulhlaups given by Björnsson (1988). Those estimations seem to be to high due to inaccurate discharge measurements in former years as well as due to changes in Grímsvötn caldera during the last 40 - 50 years, partly due to reduced geothermal activity (Magnús T. Guðmundsson, pers. comm. Nov. 19, 1996).

At 9:30 pm of Nov. 4, the long expected jökulhlaup of Grímsvötn started. A continuous high frequency tremor was registered at a nearby seismic station on Grímsfjall caused by the lifting and breaking of the ice. At about 8:30 am on Nov. 5, water reached the glacier's rim and the rivers in Skeiðarársandur started to rise from a flowrate of 50 m3/s to 6000 m3/s at 10:00 am. Three hours later the water had flooded the sandur on its entire width of 35 km and length of 20 km and had already destroyed a 380 m long bridge across Gigjukvísl, furthermore had cut the main powerline along the coast and the optical telecommunication cable which had been burried deep in the sediments of the sandur plain.

At 3:00 pm the amount of water was estimated at 25,000 m3/s; at 10:30 pm the jökulhlaup culminated. At that time an estimated amount of at least 45,000 m3/s flowed along a 50 km long subglacial path beneath the outlet glacier Skeiðarárjökull down to the alluvial plane of Skeiðarársandur, transporting ice blocks of more than 1000 tons which destroyed the bridges and damaged the foundations severely.
The figure of a runoff volume of 45,000 m3/s at the peak of the jökulhlaup has to be understood as an estimation. The accuracy of the measurements and additional estimations was affected by severe changes in the drainage pattern of the glacial rivers and nighttime (sunrise 9:23 am, sunset 4:56 pm) during the highest discharge rate.
All the day of Nov. 6, the water continued flowing to the sea on a continuously lowering level. In the afternoon of Nov. 6, a cloud of steam rose to 5000 m altitude for about 20 min caused by a small eruption in the eruption fissure which was possibly triggered by the lowering of water pressure.
On Nov. 7 around noon, the amount of water in the glacial streams diminished to a normal level and the jökulhlaup had ended.

All the areas which were flooded remained covered by ice blocks of all sizes with a weight of up to 2000 tons and a height of up to 10 - 15 m.
Two bridges and a 10 km long section of the ringroad were totally destroyed during the flood and another three bridges and 10 km of the road were severely damaged. The periglacial runoff system had been changed severely and millions of tons of sediments had been eroded and transported. For example, the former 300 m wide riverbed, where the totally destroyed 380 m long Gigjukvísl bridge was situated, has now widened to approx. 2 km.

 

Geocoded terrain corrected ERS-2 subscene (desc.), showing the area of the jökulhlaup - acquired on Nov. 7, 1996

The ERS-2 subscene of Nov. 7 shows the area affected by the jökulhlaup. The flooded areas can easily be recognized, mainly due to high reflectancy caused by the iceblocks which are spread all over the flooded sandur plain. It can be observed that all the glacial rivers draining Skeiðarárjökull were affected by the jökulhlaup and flooded most parts of Skeiðarársandur on their way to the sea, before the water gathered behind the beach sand barrier to drain into the sea through some breaches.

 

NOAA-AVHRR thermal channel 4 (BBT) - acquired on Nov. 6, 1996.

The amount of meltwater flowing to the sea can also be distinguished in the NOAA-AVHRR data by either the different reflection of the sediment loaden water (channel 2) and the lower temperature of the meltwater (channel 4) which reaches the sea with a temperature close to 0°C and mixes very slowely with the seawater with a temperature of approx. 10°C, due to the difference in density of freshwater and seawater.
It can be observed that the water is carried westward along the coast by a stream coming from the east.

The jökulhlaup has transported more than 100 million tons of sediments directly to the sea.

 

Geocoded terrain corrected multitemporal ERS-1/2 mosaic (desc.) - acquired on Oct. 21, Oct. 22 and Nov. 7, 1996 (BGR).

The multitemporal colour composite of ERS-1 and ERS-2 data shows the whole area which was affected by the eruption and the following jökulhlaup. In the area of the erupting fissure the ice movements caused by the lowering water level can be recognized.
In the Grímsvötn caldera the ice movements during the jökulhlaup can be observed which lowered the icelevel to 1350 m, which means a subsidence of the waterlevel of approx. 180 m accounting that the ice surface was 29 m above water level (Oddur Sigurðsson, pers. comm. Nov. 8, 1996) before the jökulhlaup.

A total amount of water which was drained to Skeiðarársandur alluvial plane was estimated close to 4 km3 (Oddur Sigurðsson, pers. comm. Nov. 8, 1996), that fits fairly good to our calculation of total expected runoff volume.

At the southeastern outlet of the Grímsvötn caldera clearly an approx. 6 km long, 500 m wide and up to 200 m deep ice canyon can be recognized. It is caused by the breaking of the roof of the subglacial channel which was created by the water forcing its way from the subglacial reservoir under the glacier over a distance of 50 km with a difference in altitude of about 1400 m down to Skeiðarársandur alluvial plain.

All together the amount of damage is estimated now at 31 Mio. US $ which means approx. 120 US $ for every Icelandic inhabitant. It will need at least two years to rebuild the ringroad completely.

At any time another subaerial or subglacial volcano may erupt in Iceland and endanger settlements and livestock as well as infastructure.


References used:

Einarsson, P., Brandsdóttir, B., Guðmundsson, M. T., Björnsson, H. (1996): A chronological account of the October eruption under the Vatnajökull ice cap. Internet document: http://www.rhi.hi.is/~mmh/gos/chrono2.html.

Björnsson, H. (1988): Hydrology of Ice Caps in Volcanic Regions. Vísindafélag Íslendinga, Societas Scientarium Islandica, Rit. XLV, Reykjavík, 133 pp.

Björnsson, H. & Einarsson, P. (1990): Volcanoes beneath Vatnajökull: Evidence from Radio Echo-Sounding, Earthquakes and Jökulhlaups. - Jökull, 40: p. 147-168; Reykjavík.

Brandsdóttir, B., Björnsson, H., Guðmundsson, M. T., Einarsson, P., Pállson, F. (1996): Jökulhlaup Update. Internet document: http://www.rhi.is/~mmh/gos/vat-update.html.



This web page is providing preliminary results as well as the former published web pages and was prepared in a very tight schedule due to the dynamic character of the event. Of course some conclusions are a hypothesis and will be investigated in more detail in further scientific work.

In the case of the Vatnajökull eruption in South Iceland it could be approved that ERS-1/2 SAR data and SAR remote sensing techniques can be applied as an operational tool for the monitoring of volcanic eruptions and their adverse effects. Even in that tight schedule results of great value could be achieved. This was only possible in a remote sensing center with an existing infrastructure, a very fast processing chain from acquisition, raw data processing, geocoding, image processing and interpretation, plus a high amount of know how and manpower.


Some of the images were derived from data supplied by the European ERS-1/2 satellites. The original data is
Copyright © 1996, ESA

This presentation is part of the doctoral work of Bettina Müschen mueschen@dfd.dlr.de and Christoph Böhm boehm@dfd.dlr.de being undertaken at DFD.


State: Jan. 14, 1997; For further technical information please contact: Achim Roth roth@dfd.dlr.de
WWW: B. Müschen & C. Böhm; A service of the German Remote Sensing Data Center
Copyright © 1996, Deutsches Zentrum für Luft- und Raumfahrt e.V.