Post-Eruptive Lahar and Floods Resulting from the 2010 Eruption of Eyjafjallajökull: Observations, Mapping, and Modelling Esther Hlíðar Jensen1(
[email protected]), Jón Kristinn Helgason1 , Ármann Höskuldsson2, Guðrún Sverrisdóttir2, Björn Oddsson2 , Rósa Ólafsdóttir2, Matthew Roberts1, og Sigurjón Einarsson3 1
= Icelandic Meteorological Office
2
= Institute of Earth Science, University of Iceland
Inngangur
3
= Soil Conservation Service of Iceland
Airborne synthetic-aperture radar
Historic, post-eruptive debris flows of remobilised volcanic ash are rare in Iceland, being restricted to explosive eruptions. A slurry of volcanic ash from the southern slopes of the ice-capped Eyjafjallajökull volcano on 19 May 2010 is the first lahar observed in Iceland since the 1947 Hekla eruption. An explosive eruption in Eyjafjallajökull 2010 started on April 14th.
Direct measurements of ash fallout
The origin of the flow was first observed on a radar image taken by the Icelandic coast guard aircraft TF-Sif, on May 19th. TF-SIF is a Dash8 equipped with a Side looking radar (SLAR). The image shows several sliding areas on the south slopes of the glacier. The red lines show contours of these areas, but the blue one delineates the area later verified to have fed the Svaðbælisá lahar flow.
On April 17th the explosive activity was intense and extremely fine ash settled in a large quantity on the south flanks of the volcano, partly by base surges. The ashfall continued for a month in varying wind directions, but far the most accumulation of tephra was on the south and east flanks of the volcano. Estimated total volume falling in Iceland was around 150 million cubic meters. One third of that fell within basins of the volcano and about 70% is above ablation zone of the glacier. About 14 million cubic meters is expected to be below the ablation zone.
Airborne LIDAR survey Icelandic Meteorological Office and Institute of Earth Sciences has since 2008 worked on the mapping of the Icelandic glaciers by laser measurement (LiDAR). Detailed models have been made of many Icelandic glaciers. The mapping is part of an extension of the International Polar Year between 2007 and 2009 (IPY). Detailed models of glaciers are useful for a variety of research and have also great practical significance.
Direct measurements of the 19 May lahar The night before May 19th, the first considerable rain for weeks occurred in the area south of the volcano. The rain was moderate in the inhabited area, but was presumably intense at higher elevation. All rivers draining the southern slopes of the glacier were overflowed by muddy water. In Svaðbælisá, the river which drained a jökulhlaup in the beginning of the eruption, the flow was more concentrated or similar to lahar debris flow. The flow occurred in the morning and was described to have the consistency of wet cement. It reached peak discharge within an hour and was soon diluted by the river and several tributary streams. Fig. 2 is taken after the flow had receded and diluted to muddy streamflow. The photo is overlain by a schematic drawing of the lahar deposit.
Flow modelling using LAHARZ As the sediment volume measured roughly 120.000 m3 and it was still wet, 200.000 m3 of saturated flow seem reasonable to use as an input volume for the LAHARZ program (Shilling 1998, Iverson et al., 1998). That conforms well with the measured sediment area (Fig. 4, Fig. 10). However, the volume of the flow was somewhat larger as diluted sediments were carried all the way to the coast or at least 7 km from the onset of the accumulative area. Inundation areas of larger volumes up to 1 million m3 were also calculated in LAHARZ.
Aerial and ground-based imagery
Hydrological behaviour of watersheds and erosion
Tómas Jóhannesson, 2010
Measurements done on the ash availability last summer reveal that the catchment area of Svaðbælisá alone was loaded with about 5 million m3 of ash, half of the quantity on the ablation zone on the lower slope of the glacier. However, according to field observations last summer and autumn the conditions leading to “ash avalanche” has changed drastically. Rainy weather in the autumn has formed a network of channels in the ash layers and gradually transports ash by muddy streams. Nevertheless, by intensive ablation of the glacier in the springtime, concentrated tephra-snow-water lahar can not be ruled out (Manville et al., 2000).
Tómas Jóhannesson, 2010
18.09.2003
Veðustofa Íslands
Bustadavegur 9 150 Reykjavik, Iceland Sími. +354 5226000 / FAX +354 5226001 www.vedur.is
12.07.2010
18.09.2010
References: Vallance, J.W. Beverage, J.P., Culbertson, J.K., 1964. Hyperconcentrations of suspended sediment. Journal of the Hydraulic Division, America Society of Civil engineering 90, 117-128. Cronin S.J., Lecointre J.A., Palmer A.S., Neall V. E., 2000. Transformation, internal stratification, and depositional processes within a channelised, multi-peaked lahar flow. New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 117-128. Castruccio A., Clavero J., Rivera A., 2009. Comparative study of lahars generated by the 1961 and 1971 eruptions of Calbuco and Villarica volcanoes, Southern Andes of Chile. Journal of Volcanology and Geothermal research 190 (2010): 297-311. Manville V., Hodgson K.A., Houghton B.F., Keys J.R.(H.),White J.D.L., 2000. Tephra, snow and water: complex sedimentary responses at an active snow-capped stratovolcano, Ruapehu, New Zealand. Bull. Volcanol (2000) 62: 278-293. Schilling, S.P., 1998. LAHARZ; Gis programs for automated mapping of lahar-inundation hazard zones. U.S. Geological Survey Open-file Report, pp. 98-638. Iverson, R.M., Schilling, S.P., Vallance, J.W., 1998. Objective delineation of lahar-indundation hazard zones. GSA Bulletin 100, 972-984.
