ICE DAM FLUCTUATIONS AT THE MARGINAL LAKE GRÆNALÓN (ICELAND) BEFORE AND DURING A GLOF

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1 ICE DAM FLUCTUATIONS AT THE MARGINAL LAKE GRÆNALÓN (ICELAND) BEFORE AND DURING A GLOF Kilian Scharrer 1, Ch. Mayer 2, S. Martinis 1, U. Münzer 1, Á. Gudmundsson 3 1 Ludwig-Maximilians-University, Department of Earth and Environmental Sciences, Section Geology, Luisenstraße 37, Munich, Germany, k.scharrer@iaag.geo.uni-muenchen.de, s.martinis@iaag.geo.uni-muenchen.de, ulrich.muenzer@iaag.geo.uni-muenchen.de 2 Bavarian Academy of Sciences and Humanities, Commission for Glaciology, Alfons-Goppel Str. 11, Munich, Germany, christoph.mayer@lrz.badw-muenchen.de 3 Fjarkönnun ehf, Furugrund 46, 200 Kópavogur, Iceland, fjarkonn@simnet.is ABSTRACT We used an annual time series (August 2004 until August 2005) of ENVISAT-ASAR data, supported by two ASTER scenes to investigate ice dam fluctuations and changes of the surface area of the marginal lake Grænalón. The lake occupies a lateral valley, dammed by the westernmost flow band of Skeiðarárjökull, one of the largest outlets of Vatnajökull ice cap. During the period of investigation Skeiðarárjökull was affected by three flood events draining subglacially under this outlet: Two jökulhlaups originating from Grímsvötn (29/10 until 07/11/2004 and 04/03-14/03/2005) and a Glacial Lake Outburst Flood (GLOF) of Grænalón (07/08 14/08/2005). It was found that changes of the lake surface area and the calving front position are in phase during the two jökulhlaups originating from Grímsvötn. Therefore it seems likely, that lake Grænalón and the western flow band of Skeiðarárjökull are affected by such phenomena. This supports the new theory of sheet flow or coupled sheet and tunnel flow for the propagation of a jökulhlaup under Skeiðarárjökull. Between May and August 2005 the surface area of lake Grænalón increased by 275 ha to a maximum extent of 960 ha. During the GLOF in August 2005, the surface area decreased rapidly by 370 ha to 590 ha. Intense calving during the GLOF lead to an ice front retreat of about 400 m. The maximum extent of Grænalón measured one week before the outburst could be used as a threshold in future SAR acquisitions to estimate the risk of an imminent GLOF. 1. INTRODUCTION The western part of Vatnajökull ice cap including marginal lake Grænalón is in the focus of several ESA projects since At present ENVISAT-ASAR collects SAR data regularly in the framework of the project Hazard Assessment and Prediction Longterm Observation of Icelandic Volcanoes and Glaciers Using ENVISAT-ASAR and Other Radar Data (ESA, ID 142). In this study we used an annual sequence of ENVISAT-ASAR acquisitions supported by few Terra- ASTER scenes to investigate ice dam fluctuations and dimensional changes at marginal lake Grænalón. During that period Skeiðarárjökull, which dams lake Grænalón, was affected by three flood events draining subglacially under this outlet. Glacier generated outburst floods (jökulhlaups) are a common phenomenon in Iceland, representing a serious hazard for large areas in the surroundings of a glacier. Jökulhlaups are triggered by the discharge of meltwater stored in ice-marginal, subglacial, englacial and supraglacial reservoirs, or can be generated without water storage from volcanic eruptions or massive rainfall [1]. The first jökulhlaup during the annual period of investigation started on 29 October 2004 accompanying a subglacial eruption at the Grímsvötn volcanic system. Over a five day period 0.5 km³ of meltwater were released with a peak discharge of about 3300 km³ s -1. Conductivity measurements revealed the presence of geothermal water in river Skeiðará until early December, suggesting jökulhlaup water within the catchment until this time [2]. A second jökulhlaup from Grímsvötn caldera occurred in early March 2005, lasting for a period of 10 days (04/03-14/03/2005). The jökulhlaup was to a considerable degree smaller compared to the event in autumn 2004, whereas no exact numbers for the volume or the peak discharge are available. Further, the water level at lake Grænalón dropped about 20 m during a Glacial Lake Outburst Flood (GLOF) starting on 7 August This event lasted for one week with an estimated peak discharge of ~1200 m³ s -1 [3]. Recently several studies showed that besides the classical jökulhlaup theory of floodwater travelling in a distinct pre-existing conduit [4], a second type of glacier torrents exists. A sheet flow or coupled sheet and tunnel flow leading to a widespread basal lubrification seems more likely for the propagation of a jökulhlaup under Skeiðarárjökull [5, 6, 7]. It was found, that an extensive increase of surface velocities occurs especially during the initial phase of a jökulhlaup [7, 8]. Considering this, the question arises if variations at lake Grænalón can be associated with the two Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 jökulhlaups from Grímsvötn during the period September 2004 until September This would be another indicator for basal spreading of the jökulhlaup water. 2. TEST SITE Vatnajökull (8100 km²) is a temperate ice cap located on the southeast coast of Iceland. In total, the glacier extends 150 km from west to east and 100 km from south to north. The southward trending Skeiðarárjökull is one of the largest outlets of the ice cap. The catchment area of Skeiðarárjökull comprises approx. 1,428 km² [9]. Its elevation ranges from 1,740 m down to 100 m a.s.l. at the terminus. Three medial moraines divide the glacier into four different flow bands (Fig. 1). Situated in a maritime climate, with relatively low summer temperatures and heavy winter precipitation, the glacier is characterized by high rates of surface mass exchange, resulting in high balance velocities [9]. central part of western Vatnajökull. Due to the constant heat flux in the caldera, jökulhlaups occur even without an eruption. Meltwater accumulates in the caldera, until the subglacial lake reaches a critical level. Then, water pressure breaks the glacial seal and a floodwave travels over a distance of 50 km subglacially under the Skeiðarárjökull outlet. Marginal lake Grænalón is sited approximately in the middle of the Grímsvötn subglacial jökulhlaup path to the Skeiðarársandur. The lake occupies a lateral valley, dammed by the westernmost flow band of Skeiðarárjökull. The ice dam has a length of about 2 km. The east-west extent of the lake is about 3.5 km on average. During the 1935 jökulhlaup the lake drained the last time entirely and the bottom topography of the lake was surveyed [10]. Depending on this data, the lake has a depth of about 100 m nowadays. The lake dimensions, the jökulhlaup cyclicty and the failure mechanism of the glacial lake outburst floods (GLOFs) are mainly influenced by the climate driven behaviour of Skeiðarárjökull. Due to the thinning of Skeiðarárjökull during the last century, the basal water pressure at the lake became periodically high enough in order to lift the glacial dam. Subsequent drainage beneath the ice then triggered a GLOF. This situation occurred several times, with the largest floods taking place in 1935 and 1939, where numerous icebergs reached the sandur plain several kilometres south of Skeiðarárjökull. During this period the lake area has been up to 18 km 2 and floods have reached peak discharges of 5000 m 3 s -1. Especially since the 1950 s, associated with the further retreat of the glacier, the failure mechanism changed and the lake drained almost annually with peak discharges of m³ s -1 and a lake level change of approx. 20 m [11]. Today, Grænalón drains via an ice tunnel about 2 km down-glacier and the length of the subglacial floodpath to the glacier margin is about 16 km. Depending on the jökulhlaup size the catchments of the rivers Súla/Núpsvötn and Gígjukvísl are potentially affected (Fig. 1). 3. DATA AND METHODS Figure 1. The southern part of the Vatnajökull ice cap with the marginal lake Grænalón As mentioned above, the glacier is affected by subglacial drainage of episodic jökulhlaups. Skeiðarárjökull encompasses the floodpath of the subglacial Grímsvötn volcanic system, one of the most famous jökulhlaup systems worldwide (Fig. 1). The about 62 km² large subglacial caldera is situated in the A time series of 16 ENVISAT-ASAR and 2 Terra- ASTER scenes covering the period from 26/08/2004 until 18/08/2005 was analysed in this study (Tab. 1). For the annual investigation period only this limited number of useful ASTER images is available, due to the Icelandic weather conditions and therefore the intense cloud coverage at many recording dates. This highlights the potentials of SAR remote sensing for monitoring purposes due to its day/night and all weather capability. The ASAR instrument is a side looking C-band SAR antenna operating at a wavelength of 5.6 cm. Due to its beam steering capability the ASAR instrument can acquire images in seven different swathes (IS1 IS7), covering an

3 incidence angle range from 15 to 45 degrees. In this study we analysed data of the ASAR swathes IS2 (inc. angle 21.5 ), IS5 (in. angle 37.5 ) and IS6 (41 ). All ASAR images used were acquired with vertical transmit-and-receive (VV) polarisation. The ground resolution of the scenes is approx. 20 m and the swath width about 100 km. The resolution of the nadir looking ASTER images is 15 m in the visible and very near infrared spectrum (bands 1-3n). These three bands were used for the determination of the ice dam and the lake surface extent. Table 1. Overview of all utilised scenes Date Sensor Mode Track Orbit Incidence angle ASAR IS asc 37, ASTER desc Nadir ASAR IS asc 21, ASAR IS desc 21, ASAR IS asc 37, ASAR IS desc ASAR IS desc 21, ASAR IS asc 21, ASAR IS desc 21, ASAR IS asc 21, ASTER desc Nadir ASAR IS asc 21, ASAR IS asc 21, ASAR IS asc 37, ASAR IS asc 21, ASAR IS desc 21, ASAR IS asc 37, ASAR IS asc 21,5 The ASTER images were geocoded, whereas about ground control points were measured in the ice free areas around the glacier margins in the individual images. Topographic maps (1:50,000) and already geocoded satellite images were used as reference to transform the scenes into a common map projection (UTM, WGS 84 Zone 27). A horizontal referencing accuracy of less than 15 m (1 Pixel), i.e. displacement between image and reference map, was achieved. Due to the SAR inherent recording characteristics (i.e. side looking geometry) and the influence of topography (layover, foreshortening, shadow), a DEM based geocoding was applied on all ASAR images. Ten artificial corner reflectors installed in 1997 around Vatnajökull (5 oriented towards the ascending orbit, 5 towards the descending orbit) were used as ground control points. The distinct backscatter maximum of the reflectors can be located very precisely in the ENVISAT-ASAR images, resulting in position accuracy of less than one pixel (i. e. 20 m) in the terrain-corrected ASAR scenes. However, the quality of terrain geocoding depends on the quality of the utilised reference DEM. Two different DEMs were available, one based on 1: topographic maps produced in the early 1990 s, and a photogrammetric DEM developed from an aerial survey of Vatnajökull on 12/08/1997. For quality control, we compared the two DEMs in the surroundings of lake Grænalón, and found height differences between the map based DEM and the photogrammetric DEM of up to 80 m especially in the glaciated areas. Assuming a climate driven thinning of Skeidarájökull throughout recent years, the newer photogrammetric DEM was used for terrain correction of the ASAR scenes. Nevertheless, an elevation error can be expected between the date of the photogrammetric DEM production (1997) and the period of investigation ( ) of this study. Hence, a co-registration was applied, in order to avoid mismatching among the ASAR images, whereas the first scene (26/08/2004) served as the reference. Furthermore, with respect to expected elevation errors, the observed changes in surface area of lake Grænalón were not converted into volumetric estimations of the water storage/outflow. 4. RESULTS After image processing, the position of the ice front and the surface area of lake Grænalón were manually digitised on the individual satellite images and compared in a GIS environment. The ice dam fluctuations were calculated relatively to the location mapped in the first scene (26/08/2004) of the time series. In the first two month of the period of investigation, no changes could be observed at lake Grænalón (Fig. 2). The position of the calving front remained stable, as well as the lake surface area of about 590 hectares. The first significant changes coincide with the period of the Grímsvötn eruption and the accompanying jökulhlaup. Between the ASAR scenes 07/10/2004 and 24/10/2004 the glacier front protruded about 80 m and the lake area increased to approx. 605 hectares. This first impulse is probably related to an intense 4-day precipitation period between 12/10 and 15/10. During these days the nearby Skaftafell weather station recorded a total precipitation of mm. The next ASAR image (04/11/2004) shows a retreat of the ice front of about 60 m, whereas the lake area increased slightly to 610 hectares. This coincides with the maximum discharge of Grímsvötn jökulhlaup (Fig. 2). Until end of November, the calving front protrudes gradually (13/11/2004 = + 40 m, 28/11/2004 = + 20 m) and the lake area decreases to about 590 hectares. The next

4 four month, until beginning of March 2005 no changes of the lake surface area were detectable. The ice front shows only little fluctuations during this period likewise. Between the ASAR scene 24/02/2005 and the ASTER scene 13/03/2005 significant changes could be observed at lake Grænalón. The lake surface extent increased by more than 50 hectares to about 650 ha, simultaneously the ice front advanced about 80 m (Fig. 2). These variations coincide with the 10 day jökulhaup from Grímsvötn caldera between 04/03 and 14/03/2005, whereas the ASTER scene (13/03/2005) images the situation when the highest water levels are recorded at river Skeidará. Despite its relatively small size, this jökulhlaup seems to have a strong effect on lake Grænalón and the western flowband of Skeiðarárjökull. Probably the basal drainage system under Skeiðarárjökull has to redevelop during this time of the year, leading to high water pressure at the glacier base and therefore to a widespread distribution of the subglacial water. This process is although indicated by the high number of icequakes (36) recorded at Skeiðarárjökull during the jökulhlaup (Fig. 2). At the end of March, the ice front retreats again (- 80 m) due to calving, whereas the lake surface area increases to about 675 ha. Between May and August 2005 a massive increase of the surface area of lake Grænalón could be observed, finally leading to the GLOF which began on 07/08 and lasted for one week (Fig. 2). In the ASAR scene of 05/05/2005 a lake extent of 685 ha was measured, two month later (07/07/2005) 845 ha, on 14/07/ ha, and one week before the GLOF started (31/07/2005) Grænalón reached a dimension of 960 ha (Fig. 4). Especially during July the surface area increased by at least 115 ha, whereas melting of snow and glacier ice in the catchment of Grænalón seems to be the driving factor. The maximum extent of Grænalón measured one week before the outburst could be used as a threshold to estimate the risk of an imminent GLOF by analysing future ENVISAT-ASAR acquisitions. The calving front shows large fluctuations during that time likewise (Fig. 5). In the beginning of July the ice front retreats gradually due to calving (07/07/2005 = - 30 m, 28/11/2004 = - 40 m). The advance of about 90 m detectable on 31/07/2005 could be related to the floating of the glacier tongue close to lake Grænalón. During the GLOF, the surface area decreased rapidly by 370 ha compared to the dimension measured on 31/07/2005. Disintegration of the ice front lead to a retreat of about 400 m. Figure 2. Ice dam fluctuations and surface area of lake Grænalón over the annual period of investigation compared to water level of river Skeidará, precipitation data and the icequakes occurred at Skeiðarárjökull

5 An interferometric analysis of the ASAR pair 07/07/2005 and 11/08/2005 (perpendicular baseline 803 m) covering the period of the GLOF revealed no coherence in the area of Skeiðarárjökull. Therefore, potential motion of the glacier triggered by the subglacial drainage during the GLOF could not be analysed. Good coherence in the adjacent sandur plain enabled delineation of the flood area, whereas approximately 255 km² were affected by the GLOF draining via the rivers Súla/Nupsvötn and Gígjukvísl (Fig. 3). Skeiðarárjökull Flood area at least 255 km² Figure 3. Wrapped interferogram of the ASAR scenes 07/07/2005 and 11/08/2005 (perpendicular baseline 803 m) showing the flood area in the sandur plain 5. CONCLUSION An annual time series (August 2004 until August 2005) of ENVISAT-ASAR data, supported by two ASTER scenes was analysed to investigate ice dam fluctuations and changes of the surface area of the marginal lake Grænalón. The lake occupies a lateral valley, dammed by the westernmost flow band of Skeiðarárjökull, one of the largest outlets of the Vatnajökull ice cap. The ice dam has a length of about 2 km. The east-west extent of the lake is about 3.5 km on average. Skeiðarárjökull although encompasses the floodpath of the subglacial Grímsvötn volcanic system, one of the most famous jökulhlaup systems worldwide. Recent studies showed, that Skeiðarárjökull outlet is strongly affected by subglacial drainage of jökulhlaups from Grímsvötn. It was found, that an extensive increase of surface velocities occurs, which is explained by sheet flow or coupled sheet and tunnel flow of the subglacial flood leading to a widespread basal lubrification. During the period of investigation Skeiðarárjökull was affected by three flood events draining subglacially under this outlet. Two jökulhlaups originating from Grímsvötn (29/10 until 07/11/2004 and 04/0314/03/2005) and a Glacial Lake Outburst Flood (GLOF) of Grænalón (07/08 14/08/2005). The first ice dam fluctuations and changes of the lake surface area observable in time series coincides with the jökulhlaup from Grímsvötn in autumn During that period the lake surface increased by about 25 hectares and the ice front advanced by about 80 m. The next four month, until beginning of March 2005 no changes of the lake surface area were detectable. The ice front shows only little fluctuations during this period likewise. Between the ASAR scene 24/02/2005 and the ASTER scene 13/03/2005 covering the second jökulhlaup from Grímsvötn significant changes could be observed at lake Grænalón. The lake surface extent increased by more than 50 ha to about 650 ha; simultaneously the ice front advanced about 80 m. These strong effects on lake Grænalón and the western flowband of Skeiðarárjökull can probably be explained by the undeveloped basal drainage system under Skeiðarárjökull during this time of the year. This would lead to high water pressure at the glacier base and therefore to a widespread distribution of the subglacial water. This process is although indicated by the high number of icequakes (36) recorded at Skeiðarárjökull during the jökulhlaup. Between May and August 2005 the surface area of lake Grænalón increased by 275 ha to a maximum extent of 960 ha. During the GLOF, the surface area decreased rapidly by 370 ha to 590 ha. Intense calving during the GLOF lead to an ice front retreat of about 400 m. Generally changes of the lake surface area and the calving front position are in phase during the two jökulhlaups originating from Grímsvötn. Therefore it seems likely, that lake Grænalón and the western flowband of Skeiðarárjökull are affected by such phenomena. This supports the new theory of sheet flow or coupled sheet and tunnel flow for the propagation of a jökulhlaup under Skeiðarárjökull. Further, the maximum extent of Grænalón measured one week before the outburst could be used as a threshold to estimate the risk of an imminent GLOF. By using this threshold, continuous ENVISAT-ASAR monitoring has the potential of a reliable forecasting system for future outbursts of lake Grænalón. ACKNOWLEDGEMENTS ENVISAT-ASAR data were kindly provided by the European Space Agency (ESA). Thanks to the Icelandic Meteorological Office (Vedurstofa), Reykjavík, for the meteorological and earthquake data. The research was made possible by the Bavarian Research Foundation (DPA 37/04). Sincere thanks are given to this institution.

6 REFERENCES 1. Roberts, M.J. (2005). Jökulhlaups: A reassessment of floodwater flow through glaciers. Reviews of Geophysics 43, Hardardóttir, J., Jónsson, P., Sigurðsson, G., Elefsen, S.O., Sigfússon, B. & Gíslason, S.R. (2005). Discharge and Sediment Monitoring of the 2004 glacial Outburst Flood Event (Jökulhlaup) on Skeiðará Sandur Plain, South Iceland. Geophysical Research Abstracts 7, Roberts, M.J. (2005): Aerial and seismic observations of the August 2005 jökulhlaup from Grænalón. Icelandic Meteorological Office. Grænalón summary, 12/2005, Internal Report. 4. Nye, J.F. (1976). Water flow in glaciers: jökulhlaups, tunnels and veins. Journal of Glaciology 17(76), Jóhannesson, T. (2002). Propagation of a subglacial flood wave during the initiation of a jökulhlaup. Hydrological Sciences Journal 47(3), Flowers, G., Björnsson, H., Pálsson, F. & Clarke, G. (2004). A coupled sheet-conduit mechanism for jökulhlaup propagation. Geophysical Research Letters 31(5), doi: /2003gl issn: Magnússon, E., Rott, H., Björnsson, H., Roberts, M.J., Berthier, E. & Pálsson, F. (2006). The effects of basal water beneath Vatnajökull, Iceland, observed by SAR interferometry. In Proc. of Fringe 2005 Workshop (Ed. H. Lacoste), ESA SP-610 (CD-ROM), ESA Publications Division, European Space Agency, Noordwijk, The Netherlands. 8. Roberts, M.J., Sturkell, E., Geirsson, H., Gudmundsson, M.T., Pálsson, F., Björnsson, H., Gudmundsson, G.B., Elefsen, S.O., Gíslason, S., Sígfusson, B. & Jónsson, P. (2005): Large increase in glacier sliding during subglacial flooding. Geophysical Research Abstracts 7, Aðalsgeirsdóttir, G. (2003): Flow dynamics of Vatnajökull ice cap, Iceland. Ph.D thesis, 178 pp., ETH-VAW, Zurich, Switzerland. 10. Áskelsson, J On the last eruptions at Vatnajökull. Reykjavík, Societas Scientiarum Islandica (Vísindafélag Íslendinga). 11. Björnsson, H., Pálsson, F. & Mahlmann, A. (2004): Inventory and database of Icelandic jökulhlaups. In Glaciorisk - Survey and prevention of extreme glaciological hazards in European mountainous regions (Eds. D. Richard & M. Gay), EVGI , Grenoble, France, pp Figure 5. Ice dam fluctuations at lake Grænalón measured in the ASAR time series before and during the GLOF Figure 4. Examples of the ASAR time series showing the variations of the lake surface area before and during the GLOF