Deformation measurements on the rock glacer ‘Foura da l’amd Ursina’, above Pontresina using terrestrial laser scanning

Figure 1: Laserscanning at Schafberg
Figure 2: Vertical changes on the rock glacier snout. Losses in volume (blue) and deposits (red) can be seen, caused by small and medium sized rocks moving along clearly visible tracks. The large light blue area on the right is a harmless zone of subsidence.
Figure 3: Creep rates of the rock glacier between 2009 and 2010. Two tongues in the centre of the image displayed the highest creep rates (up to 20 cm). Please click on the picture to enlarge it (Figure from: Kenner and Phillips, Bündner Wald, 6/2010).

Rock glaciers are a typical landform in Alpine permafrost. They consist of a mixture of rocks and ice and creep downslope slowly. A typical characteristic of rock glaciers is their steep front or snout. This is where the rock material is freed from its ice matrix and forms potentially unstable slopes. The SLF permafrost research group monitors this type of terrain, including the rock glacier ‘Foura da l’amd Ursina’ above Pontresina using terrestrial laser scanning.

Rock glacier Foura da l’amd Ursina

The rock glacier Foura da l’amd Ursina is located in a cirque to the West of Piz Muragl. Its steep snout fills the upper end of the Val Giandains gully and lies 900 metres above an avalanche and debris-flow retention dam which was built in 2003, in Pontresina. Two highly frequented hiking trails cross the rock glacier snout. Borehole temperature measurements in the snout show that the active layer is around 5 m thick in summer. This is where rapid mass movements such as debris flows and rock fall could be initiated.

Laser scanner records small changes

The SLF has been monitoring the rock glacier with a laser scanner since 2009 for the Amt für Wald und Naturgefahren Graubünden. The method is based on the measurement of the run-time measurement of short laser impulses which are reflected from the terrain surface and registered by the laser scanner. This allows to obtain a highly precise, three-dimensional image of the ground surface. The scientists can register changes of around 2 cm in the course of measurement campaigns which are repeated yearly. This allows them to recognize potentially hazardous movements and to improve their understanding of the overall dynamics of the rock glacier.

No hazard at the moment

The measurements indicate that the snout of the rock glacier is currently stable. Small slides and rock falls with a total volume of around 4 m³ were observed between 2009 and 2011. These probably occurred during snowmelt (Fig. 2). Potentially critical changes such as extensive slumps or a steepening of the entire rock glacier snout proved to be absent.

The rest of the rock glacier consists of various tongues displaying variable creep rates. Two tongues in the centre of the cirque moved fastest, with creep rates of up to 20 cm between 2009 and 2010. In the remaining sectors and just above the snout, the rates of movement are below the level of significance of the measurements (Fig. 3). There is therefore no short- to mid-term risk of an increase in the supply of material in the surroundings of the rock glacier snout.

Conclusions

Terrestrial laser scanning has proved to be useful here and at other sites in permafrost terrain. With their high temporal and spatial resolution, the measurement results is useful for both researchers and for risk management. This monitoring method is an important tool to improve the understanding of the dynamics of rock glaciers and contributes towards an efficient protection against natural hazards.

Contact

• Marcia Phillips