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Surface Displacement and Volume Changes of a Debris Covered Glacier and a Moving Debris Body at the Tröllaskagi Peninsula, Northern Iceland, by the use of Digital Photogrammetry B. Wangensteen¹, Á. Guðmundsson2 , T. Eiken¹, A. Kääb3, H. Farbrot1 and B. Etzelmüller11.Department of Geo Sciences, University of Oslo, P.O.Box 1047 Blindern, 0316 Oslo, Norway2. Jarðfræðistofán Geological Services, Raudðagerði 31, 108 Reykjavik, Iceland3. Department of Geography, University of Zürich-Irchel, Wintherthurerstrasse 190, CH-8057 Zürich, SwitzerlandE-mail:email@example.com
The Tröllaskagi peninsula is located between Skagafjörður and Eyafjörður on Northern Iceland between 65°20' - 66°10'N and 18°- 19°30' W. In this study, measurements of the displacement field and volume change of a moving debris body and a debris covered glacier are presented. The displacment fields are obtained by cross correlation matching of multi-temporal orthophotos and the volume is found by differencing sets of digital terrain models (DTM). Both orthophotos and DTMs are generated by the use of digital photogrammtry. (Z/I-Imaging). Cross correlation matching is done with the CIAS-software (Kääb & Vollmer, 2000).
Moving debris body in Siglufjörður
The moving debris body is located at the road along the coast to Siglufjörður at the northern tip of the Tröllaskagi peninsula (66°10'N, 19°W). At the meteorological station at Sauðanes (20 m.a.s.l), just 5 km north of the debris body, the MAAT is 3.6°C. The moving debris body is situated between 0 and 300 m.a.s.l. and one should not expect a MAAT under +1.8°C based on simple extrapolation and, hence, no permafrost conditions. Geodetic displacement measurements along the road that crosses the moving debris body undertaken since 1977 by the Icelandic road authorities, have revealed displacements of up to 1 m/year. The measurements were initiated due to
Figure 1: a) Location of the sites on Norhtern Iceland b) Location of the debris covered glacier at Hólar.
Debris covered glacier at Hólar
The debris covered glacier is located at the central part of the Tröllaskagi peninsula (65°40'N, 19°W) close to Hólar at an altitude of 900-1200 m.a.s.l. Geophysical and modelled results by other researchers in the project show that this is at and above what seems to be the lower limit of sporadic permafrost. Simple extrapolation from the meteorological station in Hólar (140 m.a.s.l.), 7 km away from the glaciers, gives a MAAT of about -2.5°C at the front of the debris covered glaciers. The debris covered area of the glacier is in the range of 70-80% and up to several metres thick. The debris layer is therefore protecting the glacier surface from melting and produces a much greater glaciated area than would otherwise be the case under the same conditions
Figure 5: Location and aerial photo of the moving debris body in Siglufjörður .
observed damage on the road. The displacement field based on air photos from 1985 and 1994 are shown in figure 4. The greatest velocities are found in the SW part with an average around 60 cm/y. In the mid section the velocities are around 20 cm/y, while theyare around 10 cm/y at in the upper part of the body. At some places the movements show rather a discrete than continuous pattern with zones where the velocity changes dramatically over small areas. In figure 6 a the velocity at both sides of a crevasse-looking feature is shown, with displacements of 50-60 cm/y on one side and 20 cm/y on the other. The overall pattern and magnitude of the displacement is also confirmed by the cross-correlation matching of all sets of orthophotos from 1977, 1985 and 1994. These measurements are also in agreement with the GPS measurements along the road. The cause of the movement is unsure, as glacier and permafrost factors can be ruled out due to the location. The initiation of the movement on the other hand could have started in times where the conditions were different. Soil and/or hydrological factors can have supported further movement. Length and average velocity implies an age of over 7.000 years.
Figure 3: Yearly displacement from 1985 to 1994 of the debris covered glacier at Hólar.
Figure 2: a) Oblique view of the debris covered glacier at Hólar b) Photo from the surface of the same glacier (location marked in a).
The results from the cross-correlation matching of orthophotos from 1985 and 1994 are shown in figure 3. The movement of the debris cover show signs of both glacier movement and secondary creep related to permafrost processes. The displacements are in the range from less than 10 cm/y to a maximum of 84 cm/y at the front. The accuracy of the method is believed to be about 10 cm/y. The measured displacements are greater than the 25 cm/y measured by Whalley and Martin (1994) at the Nautárdalur glacier further to the west at the same peninsula. The magnitude of the surface displacements indicates that permafrost creep rather than glacier flow dominates the cause of the movements. The displacement vectors are mainly oriented perpendicular to the contour lines. The difference between the DTMs of the two years are also calculated. The glacier show an average surface lowering of 1.6 m. This lowering is at the scale of the accuracy of the DTMs and is not siginificant. The length of the feature and average velocity implies an age of 4.000 years.
Figure 5: Close up of the crevasse-looking feature with a dramatically change in velocity at Siglufjörður.
Kääb, A. & Vollmer, M. 2000. Surface Geometry, Thickness Changes and Flow Fields on Creeping Mountain Permafrost: Automatic Extraction by Digital Image Analysis. Permafrost and Periglac. Process. 11: 315-326.
Whalley, W.B & Martin, H.E. 1994. Rock Glaciers in Tröllaskagi in Environmental Change in Iceland. Münchener Geographische Abhandlungen, Reihe B, Band B12, 289-308.
Figure 4: Displacment of the mooving debris body at Siglufjörðurbased on orthophotos from 1985 and 1994