A Conceptual Model of Hiorthfjellet Rock Glacier, Svalbard

Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 A conceptual model of Hiorthfjellet rock glacier, Svalbard R.S. Ødegård Gjøvik University College, Gjøvik, Norway K. Isaksen Norwegian Meteorological Institute, Oslo, Norway T. Eiken and J.L. Sollid Department of Physical Geography, University of Oslo, Oslo, Norway ABSTRACT: Field studies of the Hiorthfjellet rock glacier (78°15ЈN, 15°47ЈE) near Longyearbyen, Svalbard, started with surface velocity measurements in 1994. This paper presents a conceptual model for the development of this rock glacier. There is particular focus on the complexity of the front processes. It is suggested that the surface slope is mainly controlled by the accumulation of debris at the base of the active creeping part close to the front, as the rock glacier advances. This process will depend on the flow regime, and could force the front to move upward in a more mature state, assuming a strong longitudinal stress coupling. The suggested conceptual model is discussed related to the overall morphology and dynamics of the rock glacier. The proposed model is consistent with available field measurements. Considerable additional field data will be needed to validate the model. 1 INTRODUCTION of dirty ice. This is one of the few known descriptions of the inner structure of Svalbard rock glaciers. His The Svalbard archipelago (74°N to 81°N, Figure 1) interpretation was that rockslide material accumulates has continuous permafrost with permafrost thickness on snow in early spring/summer and gradually trans- varying from less than 150 m near sea level to more forms into ice during summer. than 450 m in mountain areas (Liestøl 1977). MAAT Sollid & Sørbel (1992) suggest Holocene or even at Svalbard Airport, Longyarbyen, is Ϫ6.7°C for the pre Holocene age of some rock glaciers. André (1994) period 1961–90 (28 m a.s.l.). states that rock glacier formation started 3500 BP During the first part of the 20th century several based on dating of surface material with lichenome- interpretations existed on the origin of these features try. The surface at the top front of Hiorthfjellet rock in Svalbard. Liestøl (1962) was the first to link glacier was roughly dated to 4000 BP based on the Svalbard rock glaciers to slope processes. Talus terraces length/surface velocity ratio (LSVR) (Isaksen et al., in Svalbard were described as having a “typical flow 2000). Due to rock glacier dynamics and long-term structure some what like rock glaciers”. Liestøl inves- initial development the rock glacier probably started tigated a rock glacier in Longyearbyen in 1954 where to develop at the onset of Holocene. a slip had occurred. He found irregular layers of The Hiorthfjellet rock glacier is 400 m long with sharp-edged stones and gravel alternated with layers an estimated mean thickness of approximately 35 m (Figure 2). The first interpretations of field data from the geodetic survey, DC resistivity measurements and GPR sounding are presented in Isaksen et al. (2000). Normalized horizontal surface velocities were measured to 9.0–10.3 cm/year (Isaksen et al., 2000). This paper contains further interpretation of field data and a conceptual model is suggested for the dynamics at the front of Hiorthfjellet rock glacier. This is a contribution to understanding the long debated morphology of these features in Svalbard and to the dis- cussion of the dynamics of rock glaciers in general. 2 FIELD DATA AND INTERPRETATION The surface slope is decreasing from approx. 20° at the top to 10°–12° near the front. On the top front Figure 1. Key map showing the location of the field site. of the rock glacier 20–40 m up-glacier from the steep 839 frontal slope the surface gradient is again increasing glacier from what is interpreted as the accumulation to a maximum of 14°–15°. The average longitudinal area. There is no significant trend in the gradient of surface gradient is 16.6° calculated from the lower the reflection layers relative to the surface gradient, accumulation area to the top front. however large anomalies are detected, believed to orig- There are two zones on the frontal slope where the inate from the actual accumulation process (Figure 3). slope gradients are 40° or steeper. The upper zone is This implies that the layers, interpreted as rockslide extending down to 16–22 m below the top front of the deposits, reinforce the upper supersaturated part. If rock glacier. The second zone is at the foot of the the layering of the Hiorthfjellet rock glacier are simi- frontal slope. lar to the observations by Liestøl in 1954 on the other The GPR profile presented in Figure 3 was obtained side of the fjord (Liestøl, 1962), the layers of rockslides using a Pulse EKKO 100 (Sensors and Software Inc., are saturated with ice alternating with layers of ice. Mississauga, Canada) with an antenna frequency of The radar data give few indications of the thickness 50 Mhz. The profile follows the centre flow line. The of the rock glacier. Rough estimates of the thickness georadar measurements give no indication of any can be made assuming a basal shear stress of 100 kPa cumulative shear in the upper 20 m in the area down and an ice content of 75% giving an average thickness of the supersaturated layer of 35 m. This calculation is based on a strong longitudinal stress coupling with an average surface gradient of 16.6°. 3 A CONCEPTUAL MODEL FOR THE FRONT PROCESSES Heaberli (1985) addressed processes at the front of a rock glacier, stating that “coarse debris, which falls off the top of active rockglacier fronts, is subsequently overridden, and may then form a relatively stiff basal layer”. The hypothesis was later confirmed by defor- Figure 2. The Hiorthfjellet rock glacier is in middle of the mation measurements in boreholes in the Eastern Alps photo with the steep front free of snow (June 1998). (e.g. Heaberli et al., 1998, Hoelzle et al., 1998). The Figure 3. Radar profile and interpretation. In the lower plot the white dots are depths down to 9 m, grey dots 10–15 m depth and black dots 16–20 m depth. 840 borehole measurements at rock glacier Murtel– to the plane of shear, which causes volume expansion Corvatsch show that the basal debris layer is about 20 m (dilatancy) (Arenson and Springman, 2000). There is no thick. This mechanism has been conceptually illustrated reason to expect that pore water pressure influences the by Haeberli et al. (1998). In a model of pure plastic flow mechanical properties of the basal debris (permafrost). this mechanism will be less efficient, because the front The mechanical properties of coarse grained sedi- of the glacier is simply pushed forward, and there will ments with ice content at saturation level or below are be no gradual increase in surface gradient at the front. known from geotechnical investigations. Pore ice will The details on how a permafrost ice-rock mixture deform even at very low stresses. The long term strength deforms is far from understood (Haeberli, 2000). of these sediments is mainly controlled by inter-grain However, all deformation measurements in boreholes friction like non-permafrost sediments. Ice will add (Haeberli et al., 1998, Hoelzle et al., 1998) and geo- strength to cold permafrost consisting of coarse grained morphological indications strongly suggest that a pure sediments, but for simplicity the added strength is not plastic model is not applicable. This assumption is considered in the conceptual model. Sayles (1973) sug- made for the Hiorthfjellet rock glacier. There are no gested that for noncohesive materials in the unfrozen direct field measurements of strain at depth. state, the long term strength in the frozen state would The steep front of a rock glacier will cause a stress be roughly equal to that measured in triaxial tests on field that can be modelled based on well known meth- the freely drained unfrozen material. In the following ods devoloped for finite slopes. The stress distribution it is assumed that the internal shearing resistance of beneath a finite slope is more complex than below an the basal debris is constant. This is certainly not valid infinite slope. There are no convincing field data from when the normal stress increases as the material is the Hiorthfjellet rock glacier to make definite state- overridden by the rock glacier. The assumption should ments about how the stress field relates to the defor- be refined in future quantitative models, but the sim- mation at the front. A relative increase of the surface plification is probably not critical considering the gradient on top of the rock glacier close to the front is precision in the calculations. consistent with an increasing submerging (vertical) In a conceptual model of the rock glacier front velocity. Similarly, the bulge at the base of the steep processes the following assumptions are made: frontal slope can be explained by shear strain close to The flow regime at the front is mainly controlled by the base of the basal debris layer, but other explana- • the applied stress from the creeping rock glacier. tions like active layer block creep and a different angle The strain caused by the steep front is neglected for of repose could be suggested as well. • the supersaturated part of the rock glacier, but is In the suggested model it is assumed that the stress considered for the basal debris layer. field from the steep front can be neglected for the upper The flow of the rock glacier is caused by shear strain supersaturated part of the rock glacier. On the other • in supersaturated sediments.

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