A conceptual model of Hiorthfjellet rock glacier, Svalbard

Field studies of the Hiorthfjellet rock glacier (78°15 N, 15°47 E) 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. Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Figure 1. Key map showing the location of the field site. frontal slope the surface gradient is again increasing to a maximum of 14°–15°. The average longitudinal surface gradient is 16.6° calculated from the lower accumulation area to the top front. There are two zones on the frontal slope where the slope gradients are 40° or steeper. The upper zone is extending down to 16–22 m below the top front of the rock glacier. The second zone is at the foot of the frontal slope. The GPR profile presented in Figure 3 was obtained using a Pulse EKKO 100 (Sensors and Software Inc., Mississauga, Canada) with an antenna frequency of 50 Mhz. The profile follows the centre flow line. The georadar measurements give no indication of any cumulative shear in the upper 20 m in the area down glacier from what is interpreted as the accumulation area. There is no significant trend in the gradient of the reflection layers relative to the surface gradient, however large anomalies are detected, believed to originate from the actual accumulation process (Figure 3). This implies that the layers, interpreted as rockslide deposits, reinforce the upper supersaturated part. If the layering of the Hiorthfjellet rock glacier are similar to the observations by Liestøl in 1954 on the other side of the fjord (Liestøl, 1962), the layers of rockslides are saturated with ice alternating with layers of ice. The radar data give few indications of the thickness of the rock glacier. Rough estimates of the thickness can be made assuming a basal shear stress of 100 kPa 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 deformation measurements in boreholes in the Eastern Alps (e.g. Heaberli et al., 1998, Hoelzle et al., 1998). The 840 Figure 2. The Hiorthfjellet rock glacier is in middle of the photo with the steep front free of snow (June 1998). 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. borehole measurements at rock glacier Murtel– Corvatsch show that the basal debris layer is about 20 m thick. This mechanism has been conceptually illustrated by Haeberli et al. (1998). In a model of pure plastic flow this mechanism will be less efficient, because the front of the glacier is simply pushed forward, and there will be no gradual increase in surface gradient at the front. The details on how a permafrost ice-rock mixture deforms is far from understood (Haeberli, 2000). However, all deformation measurements in boreholes (Haeberli et al., 1998, Hoelzle et al., 1998) and geomorphological indications strongly suggest that a pure plastic model is not applicable. This assumption is made for the Hiorthfjellet rock glacier. There are no direct field measurements of strain at depth. The steep front of a rock glacier will cause a stress field that can be modelled based on well known methods devoloped for finite slopes. The stress distribution beneath a finite slope is more complex than below an infinite slope. There are no convincing field data from the Hiorthfjellet rock glacier to make definite statements about how the stress field relates to the deformation at the front. A relative increase of the surface gradient on top of the rock glacier close to the front is consistent with an increasing submerging (vertical) velocity. Similarly, the bulge at the base of the steep frontal slope can be explained by shear strain close to the base of the basal debris layer, but other explanations like active layer block creep and a different angle of repose could be suggested as well. In the suggested model it is assumed that the stress field from the steep front can be neglected for the upper supersaturated part of the rock glacier. On the other hand, compression and damped creep close to the front in the basal debris layer could be significant for the overall morphology and dynamics of Hiorthfjellet rock glacier. Sediments in front of the rock glacier consist of lichen covered blocks interpreted as early or mid Holocene dirty snow avalanche deposits from the western flank of the Hiorthfjellet rockwall. Even close to the base of the steep rock glacier front there are very few blocks originating from the active rock glacier. This is regarded as good geomorphological evidence that very few blocks leave the front slope. The reason is probably that there are few large blocks exposed on the steep frontal slope. Debris is expected to melt out in the upper area of the front slope, and will be exposed to gravitational processes. After rearrangements in the active layer these sediments will be exposed to percolating water that refreezes in the pores. It is not known if these processes cause ice saturation or not. If air voids exist as the debris is overridden by the advancing rock glacier, the basal debris might compress due to increased normal stress. Shearing of coarse material implies movements normal to the plane of shear, which causes volume expansion (dilatancy) (Arenson and Springman, 2000). There is no reason to expect that pore water pressure influences the mechanical properties of the basal debris (permafrost). The mechanical properties of coarse grained sediments with ice content at saturation level or below are known from geotechnical investigations. Pore ice will deform even at very low stresses. The long term strength of these sediments is mainly controlled by inter-grain friction like non-permafrost sediments. Ice will add strength to cold permafrost consisting of coarse grained sediments, but for simplicity the added strength is not considered in the conceptual model. Sayles (1973) suggested that for noncohesive materials in the unfrozen state, the long term strength in the frozen state would be roughly equal to that measured in triaxial tests on the freely drained unfrozen material. In the following it is assumed that the internal shearing resistance of the basal debris is constant. This is certainly not valid when the normal stress increases as the material is overridden by the rock glacier. The assumption should be refined in future quantitative models, but the simplification is probably not critical considering the precision in the calculations. In a conceptual model of the rock glacier front processes the following assumptions are made: • The flow regime at the front is mainly controlled by the applied stress from the creeping rock glacier. • The strain caused by the steep front is neglected for the supersaturated part of the rock glacier, but is considered for the basal debris layer. • The flow of the rock glacier is caused by shear strain in supersaturated sediments. • Debris that melts out from the upper front area accumulates further down the front slopenot in front of the rock glacier. • Zero pore water pressure in basal debris (pores filled with ice or partly filled with ice in coarse debris with permafrost) • The sediments accumulating at the front are noncohesive with constant angle of shearing resistance (does not depend on the applied stress). In the initial stage of rock glacier development the front will reach the minimum angle of shearing resistance. Because of the loose state of debris in the active layer at the front of a rock glacier it is reasonable to assume that the front of an active rock glacier will be close to the minimum angle of shearing resistance. Rapp (1960) and Chandler (1973) investigated the inclination of talus and rock glacier slopes in Spitsbergen. Chandler (1973) suggested a critical value for angular rockfill material to be 39°– 40°. When a critical angle is reached the front of the rock glacier will adjust to this angle. This will cause melting of the supersaturated upper part of the rock glacier because